TELEPHONY 


A    COMPREHENSIVE    AND    DETAILED    EXPOSITION    OF    THE    THEORY    AND 
PRACTICE    OF    THE    TELEPHONE    ART 


By  SAMUEL  G.  McMEEN 

Member,  American  Institute  of  Electrical  Engineers,  and  Western  Society  of  Engineers 

and 
KEMPSTER  B.  MILLER 

Member,  American  Institute  of  Electrical  Engineers,  and  Western  Society  of  Engineers 
Author  ol  "American  Telephone  Practice" 


ILLUSTRATED 


CHICAGO 

AMERICAN  SCHOOL  OF  CORRESPONDENCE 
1912 


t 


Engineering 
library 


I 

M 
COPYRIGHT  1912  BY 

AMERICAN  SCHOOL  OP  CORRESPONDENCE 


Entered  at  Stationers'  Hall,  London 
All  Rights  Reserved 


Preface 


A'  the  very  opening  of  this  work  we  have  endeavored  to 
give  a  general  view,  a  mere  sketch,  of  the  telephone  art 
as  a  whole,  so  that  the  reader,  if  a  beginner,  may  be 
disabused  of  the  idea  that  telephony  is  fairly  represented  by 
the  instrument  on  his  wall  or  desk.  Also,  before  plunging  into 
the  maze  of  details,  we  have  endeavored  to  set  forth  the  gen- 
eral principles  involved  in  the  transmission  of  speech  and  of 
signals,  and  to  discuss  the  electrical  properties  with  which  the 
art  has  to  deal.  It  has  been  our  hope  that  this  would  give  the 
student  a  greater  interest  and  a  broad  enough  view  to  enable 
him  to  correlate  individual  facts  with  other  phases  of  the  art. 

C.  A  similar  course  has  been  followed  in  the  preparation  of  the 
circuit  diagrams.  The  telephone  man  has  a  sign  language  of 
his  own,  of  which  there  are  many  dialects.  We  have,  in  the 
preliminary  discussion  of  each  piece  of  apparatus,  shown  one 
or  more  appropriate  conventional  symbols  representing  it,  and 
these  symbols  have  been  adhered  to  in  the  preparation  of  all 
the  drawings.  Where,  as  in  automatic  systems,  intricate 
mechanical  functions  are  involved,  we  have  made  the  circuit 
drawings  as  suggestive  as  possible  of  the  mechanical  as  well 
as  the  electrical  working.  Thus,  the  drawings  in  large  measure 
tell  their  own  stories,  always  in  the  same  language,  avoiding 
the  necessity  of  employing  the  bewildering  mass  of  reference 
letters  and  figures  so  often  found.  Every  drawing  used  has 
been  prepared  especially  for  this  work.  The  photographs  have 
been  collected  with  care,  and,  like  the  drawings,  they  have 
been  used  to  illustrate  the  text  rather  than  as  a  medium  around 
which  to  build  the  text. 

238200 


C.  No  attempt  has  been  made  to  make  the  work  historically 
complete,  and  except  in  rare  occasions,  where  a  distinct  lesson 
could  be  drawn  from  obsolete  types  or  methods,  we  have  con- 
fined the  discussion  to  modern  practice — to  the  surviving  works 
of  the  first  generation  of  telephone  men. 

C.  It  is  fitting  that  we  should  express  our  appreciation  of  the 
intelligent,  painstaking,  and  faithful  work  of  those  who  have 
aided  us.  Our  first  acknowledgment  is  tc  Mr.  Leigh  S.  Keith, 
whose  fund  of  information,  thoughtful  criticism,  and  unlimited 
patience  have  contributed  in  no  small  degree  to  whatever  merit 
and  accuracy  this  work  may  possess.  It  also  gives  us  pleasure 
to  commend  the  work,  and  the  spirit  in  which  it  has  been  done, 
of  Miss  Leona  Ekstrom,  of  Chicago,  and  Miss  Gertrude  Cohen, 
of  San  Francisco.  These  young  ladies  have  carefully  refrained 
from  mutilating  our  thoughts,  always  returning  them  to  us  in 
form  as  good  or  better  than  when  received. 

C.  Acknowledgment  is  due  to  the  various  manufacturing  com- 
panies, who  have  freely  placed  at  our  disposal  illustrations  and 
information  concerning  their  products.  When  compared  with 
the  attitude  existing  some  years  ago,  the  willingness  on  the 
part  of  some  to  make  such  matters  public  is,  we  think,  one  of 
the  hopeful  signs  of  the  times. 

C,  Nor  must  the  American  School  of  Correspondence,  the  pub- 
lisher of  this  work,  be  forgotten  in  our  acknowledgment.  It 
has  co-operated  in  every  way  in  which  a  publisher  may  co- 
operate with  authors.  The  physical  form  in  which  the  work 
is  to  appear  will  bear  evidence  of  this. 

SAMUEL  G.  MCMEEN 
KEMPSTER  B.  MILLER 
January  1,  1912. 


TABLE  OF  CONTENTS 


INTRODUCTION  Page 

HISTORY  AND  DEVELOPMENT 1 

The  First  Telephone— The  Work  of  Alexander  Graham  Bell,  Sir  William  Thom- 
son, and  Lord  Kelvin — Public  Use  of  the  Telephone — Telephone  System — Tele- 
phone Exchange — Operation  of  the  System — Switchboards — Manual  Apparatus 
— Automatic  Telephony— Progress  in  the  Telephone  Field — Long- Distance  Trans- 
mission— The  Bell  Organization — Independent  Companies 

CHAPTER  I 
ACOUSTICS  9 

The  Art  of  Telephony— Propagation  of  Sound— Characteristics  of  Sound— Loud- 
ness — Pitch — Timbre — Harmonics — Human  Voice — Human  Ear 

CHAPTER  II 
ELECTRICAL  REPRODUCTION  OF  SPEECH  ....         .11 

Limitations  of  the  Present  Art  of  Telephony — Complete  Telephonic  Cycle — Mag- 
neto Telephones  —  Early  Conceptions  — Electrostatic  Telephone — Variation  of 
Electrical  Pressure— Variation  of  Resistance — Carbon — Carbon  Transmitter — In- 
duction Coil— Capacity — The  Condenser — Measurement  of  Telephone  Currents 

CHAPTER  III 
ELECTRICAL  SIGNALS 29 

Audible  Signals— Telephone  Sounder — Vibrating  Bell— Magneto  Bell — Polarized 
Ringer — Telephone  Receiver— Visible  Signals — Electromagnetic-Signal — Electric 
Lamp  Signal— Signal  Circuits 

CHAPTER  IV 
TELEPHONE  LINES 37 

Telephone  Line  Circuit  —  Underground,  Submarine,  and  Aerial  Cables  — 
Materials  —  Conductivity  —  Ohm's  Law — Electrostatic  Capacity — Units — Induc- 
tance—Reactance— Insulation  of  Conductors — Inductance  vs.  Capacity — Trans- 
mission Distances— Pupin  or  Loading  Coils — Improving  Transmission 

CHAPTER  V 
TRANSMITTERS 53 

Variable- Resistance  Transmitter  —  Single-Contact  Transmitter — Multiple-Con- 
tact Transmitter — Granular  Carbon — Western  Electric  Solid-Back  Transmitter — 
New  Western  Electric  Transmitter — Kellogg  Transmitter — Automatic  Electric 
Company  Transmitter — Monarch  Transmitter — Electrodes — Packing — Acousticon 
Transmitter— Switchboard  Transmitter 

CHAPTER  VI 
RECEIVERS  70 

Early  Types — Modern  Receivers — Western  Electric  Receiver — Kellogg  Receiver 
—Direct-Current  Receiver — Automatic  Electric  Company  Receiver— Monarch 
Receiver — Dean  Receiver — Operator's  Receiver 


vi  TABLE  OF  CONTENTS 

CHAPTER  VII 
PRIMARY  CELLS .        82 

Work  of  Galvani  and  Volta— Simple  Voltaic  Cell— Potential  Differences — Electro- 
lytes— Polarization — Local  Action — Amalgamation — Series  and  Multiple  Connec- 
tions—Types of  Primary  Cells— Open-Ci  rcuit  Cells— Wet  Cells— Dry  Cells— Closed- 
Circuit  Cells — Types  and  Construction — Standard  Cell 

CHAPTER  VIII 
MAGNETO  SIGNALING  APPARATUS 105 

Method  of  Signaling — Battery  Bell — Magneto  Bell — Magneto  Generator — Arma- 
ture— Automatic  Shunt — Pulsating  Current— Polarized  Ringer — Types — Biased 
Bell 

CHAPTER  IX 
THE  HOOK  SWITCH 122 

Automatic  Operation — Design — Kellogg,  Western  Electric,  and  Dean  Wall-Tele- 
phone Hooks — Western  Electric  and  Kellogg  Desk-Stand  Hooks 

CHAPTER  X 
ELECTROMAGNETS  AND  INDUCTIVE  COILS 133 

The  Electromagnet — Permeability  —  Magnetization  Curves  —  Reluctance — Low- 
Reluctance  Circuits — Bar,  Horseshoe,  and  Ironclad  Electromagnets — Armature 
Motion — Differential  Electromagnet — Magnet  Wire — Wire  Table — Insulation — 
Winding  Impedance  Coils — Types — Induction  Coil — Design — Current  and  Voltage 
Ratios— Repeating  Coil— Types 

CHAPTER  XI 

NON-INDUCTIVE  RESISTANCE   DEVICES 162 

Temperature  Coefficient — Wire  Tables— Inductive  Neutrality— Heating— Types — 
Mica-Card  Unit— Differentially-Wound  Unit— Lamp  Filament— Iron  Wire  Ballast 

CHAPTER  XII 
CONDENSERS 168 

Charge — Capacity — Units —  Dielectrics  —  Inductive  Capacity — Condenser  Sizes — 
Functions— Assorting  Currents 

CHAPTER  XIII 
CURRENT  SUPPLY  TO  TRANSMITTERS 176 

Local  Battery — Common  Battery — Advantages  of  the  Common  Battery — Series 
Battery— Bridging  Battery  with  Repeating  Coil — Bell  Substation  Arrangement- 
Bridging  Battery  with  Impedance  Coils — Double  Battery  with  Impedance  Coils — 
Kellogg,  Dean,  Stromberg-Carlson,  and  North  Substation  Arrangements  — 
Common  Source  Supply — Repeating  Coils — Retardation  Coils 

CHAPTER  XIV 
THE  TELEPHONE  SET  197 

Classification — Magneto,  Common- Battery,  Wall,  and  Desk  Telephones — Circuits 
of  each  Class — Micro-Telephone  Set 

CHAPTER  XV 
NON-SELECTIVE  PARTY-LINE  SYSTEMS 217 

Party-Lines—Private  Lines — Party-Line  Signaling — Series  Systems — Bridging 
Systems— Signal  Code 


TABLE  OF  CONTENTS  vii 

CHAPTER  XVI 
SELECTIVE  PARTY-LINE  SYSTEMS 228 

Problem  of  Selection  —  Classification:  Polarity,  Harmonic,  Step-by-Step,  and 
Broken-Line  Systems — Polarity  Method — Two  and  Four-Party  Circuits — Har- 
monic Method  —  Tuning  —  Under-Tuned  System  —  Under-Tuned  and  In-Tuned 
Ringers — Harmonic  System  Circuits  —  Step-by-Step  Method  —  Broken-Line 
Method 

CHAPTER  XVII 
LOCK-OUT  PARTY-LINE  SYSTEMS 253 

Rural  Party-Line  Problems— Methods— Poole  System — Step-by-Step  System— 
Broken-Line  System — Operation 

CHAPTER  XVIII 
ELECTRICAL  HAZARDS 277 

Hazards,  Exposed  and  Unexposed  —  Lightning  Discharges  —  Power  Circuits — 
Classification  —  Electrical  Actions  —  Design  of  Apparatus  to  Resist  Electrical 
Hazards — Self -Protecting  Devices — Electrolysis 

CHAPTER  XIX 
PROTECTIVE  MEANS  284 

Protection  Against  High  Potentials— Air-Gap  Arrester— Gap  Discharges— Carbon 
— Commercial  Types  of  Arresters — Arcs — Protection  Against  Strong  Currents — 
Fuses — Types — Fuse  Banks — Protection  Against  Sneak  Currents — Sneak-Cur- 
rent Arresters  —  Heat  Coil  —  Complete  Line  Protection  —  Exposed  and  Unex- 
posed Wiring  — Location  of  Fuses  —  Central  Office  Protectors — Self -Soldering 
Heat  Coils — Cook  Arrester — Subscribers'  Station  Protectors — City  Exchange 
Requirements — Drainage  Coils 

CHAPTER  XX 
GENERAL  FEATURES  OF  THE  TELEPHONE  EXCHANGE          .        .        .307 

Subscribers'  Lines — Trunk  Lines — Toll  Lines — Districts — Switchboards — Manual 
Boards — Magneto  and  Common  Battery  Switchboards — Multiple  and  Non-Mul- 
tiple Switchboards— Transfer  Systems — Trunking  Systems— Call  Circuits— Toll 
Boards 

CHAPTER  XXI 
THE  SIMPLE  MAGNETO  SWITCHBOARD 314 

Operation — Component  Parts— Line  Signals — Jacks  and  Plugs — Keys — Line  and 
Cord  Equipments — Operation  of  the  Magneto  Switchboard — Circuits — Commer- 
cial Types  of  Drops  and  Jacks  —  Night  Alarm — Restoration — Code  Signaling — 
Switchboard  Plugs — Cord  Attachment  —  Switchboard  Cords  —  Conductors- 
Operator's  Telephone  Equipment — Complete  Switchboard  Circuits — Lines — Mag- 
neto Cord-Circuit  Connections — Convertible  Cord  Circuits — Through-Ringing- 
Cabinets—Target  Signals— Sectional  Switchboards 

CHAPTER  XXII 
THE  SIMPLE  COMMON-BATTERY  SWITCHBOARD  .        .        .        .377 

Advantage  of  Common- Battery  Operation  —  Line  Lamps  —  Pilot  Lamp — Cord 
Circuits— Supervisory  Signals— Common- Battery  Switchboard  Cycle  of  Opera- 
tions— Mechanical  Signals — Western  Electric,  Kellogg,  and  Monarch  Types — 
Relays — Jacks 

CHAPTER  XXIII 
TRANSFER  SWITCHBOARD 400 

Limitations  of  Simple  Switchboard—Transfer  Lines— Types  of  Trunks— Plug- 
Seat  Switch — Handling  Transfers — Limitations  of  Transfer  System — Obsolete 
Systems — Modern  Systems 


viii  TABLE  OF  CONTENTS 

CHAPTER  XXIV 
PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD 409 

The  Multiple  Feature— Line  Signals— Cord  Circuits— Double  Connections — Busy 
Test— Field  of  Each  Operator— Influence  of  Traffic 

CHAPTER  XXV 
THE  MAGNETO  MULTIPLE  SWITCHBOARD 419 

Series- Multiple  Board— Circuits — Operation — Branch-Terminal  Multiple  Board— 
Operation—Modern  Magneto  Multiple  Boards— Tests— Construction 

CHAPTER  XXVI 

THE  COMMON-BATTERY  MULTIPLE  SWITCHBOARD       ;  .        .435 

Types — Western  Electric  Number  1  Relay  Board — Circuits  and  Operation— Test- 
ing—Distributing  Frames— Signals— Relays— The  Western  Electric  Number  10 
Board — Operation  —  Kellogg  Two- Wire  Multiple  Board  —  Circuits  —  Signals  — 
Battery  Feed— Busy  Test— Wiring  of  Line  Circuit— Dean  Multiple  Board— Keys 
—Tests  — Stromberg-Carlson  Multiple  Board— Signals— Tests— Multiple  Switch- 
board Apparatus— Jacks — Lamp  Jacks— Relays— Key  and  Jack  Arrangement 

CHAPTER  XXVII 
TRUNKING  IN  MULTI-OFFICE  SYSTEMS 475 

Multi-Office  Exchanges— Trunk:ng  between  Exchanges  — Two- Way  Trunks— 
One-Way  Trunks— Trunk  Operation — Western  Electric  Trunk  Circuits— Trunk 
Relays — Kellogg  Trunk  Circuits 

CHAPTER  XXVIII 
FUNDAMENTAL  CONSIDERATIONS  OF  AUTOMATIC  SYSTEMS          .        .      501 

Complexity— Expense— Flexibility— Attitude  of  the  Public— Subscriber's  Station 
Equipment — Comparative  Costs — Operation  —  Strowger  or  Automatic  Electric 
Company  System— Lorimer  System— Switches— Grouping  of  Subscribers— Trunk- 
ing— Testing 

CHAPTER  XXIX 

THE  AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM          .        .        .        .515 

Selecting  Switch— Up-and- Around  Movement— Line  Switch— Trunking— Selector 
— Connector  —  Systems  —  Subscriber's  Station  Apparatus  —  Operation  —  Line- 
Switch  Units— Master  Switch— Locking— Cut-Off—Guarding— Selecting  Switches 
— Side  Switch — First  Selector — Busy  Signals— Battery  Supply— Release— Multi- 
Office  System— Automatic  Sub-Offices — Party  Lines 

CHAPTER  XXX 
THE  LORIMER  AUTOMATIC  SYSTEM      .        .        .        .        .  .571 

Subscriber's  Station  Equipment — Central-Office  Apparatus — The  Section — The 
Connective  Division  —  Switches  —  Signal-Transmitter  Controller  —  Selector — 
Connectors 

CHAPTER  XXXI 
THE  AUTOMANUAL  SYSTEM 584 

Operation — Subscriber's  Apparatus — Operator's  Equipment — Automatic  Switch- 
ing Equipment — Distribution  of  Calls — Connecting— Speed 


TABLE  OF  CONTENTS  ix 

CHAPTER  XXXII 
POWER  PLANTS  593 

Direct  Current— Alternating  Current— Types  of  Power  Plants — Operator's  Trans- 
mitter Supply — Ringing-Current  Supply — Pole  Changers — Ringing  Dynamos — 
Auxiliary  Signaling  Currents— Primary  Sources  of  Power— Rotary  Converters — 
Mercury-Arc  Rectifiers — Provision  Against  Breakdown — Storage  Battery — Care 
and  Management — Power  Switchboard— Circuits 

CHAPTER  XXXIII 
HOUSING  CENTRAL-OFFICE  EQUIPMENT 615 

FireHazard — Building — Provisions  for  Employes  »nd  Apparatus — Arrangement 
of  Apparatus  in  Manual  Offices— Automatic  Offices 

CHAPTER  XXXIV 
PRIVATE  BRANCH  EXCHANGES 637 

Functions  of  the  Private  Branch- Exchange  Operator — Switchboards — Types — 
Supervision— Connection  with  Automatic  Offices— Battery  Supply — Ringing  Cur- 
rent—Desirable Features 

CHAPTER  XXXV 
INTERCOMMUNICATING  SYSTEMS  648 

Limitations  of  the  System — Types — Simple  Magneto  System — Common -Battery 
Systems— Kellogg,  Western  Electric,  and  Monarch  Systems — Intercommunica- 
ting System  for  Private  Branch  Exchanges 

CHAPTER  XXXVI 

LONG-DISTANCE   SWITCHING 659 

Repeating  Coils — Switching  through  Local  Board — Operator's  Orders — Two- 
Number  Calls— Particular-Party-Calls — Trunking — Ticket  Passing — Way  Stations 
— Center  Checking 

CHAPTER  XXXVI. 
TELEPHONE  TRAFFIC  664 

Unit  of  Traffic— Traffic  Variations— Rates— Traffic  Study— Data— Loads— Trunk- 
ing  Factor — Trunk  Efficiency — Toll  Traffic — Quality  of  Service — Time  Consumed — 
Busy  and  Don't  Answer  Calls — Checks 

CHAPTER  XXXVIII 
MEASURED  SERVICE  676 

Paying— Rates— Units— ^oll  Service— Long  Haul— Short  Haul— Timing  Toll  Col- 
lections—Types of  Local  Service  Methods— Ticket  Method— Meter  Method- 
Prepayment  Method 

CHAPTER  XXXIX 
PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS  ....      687 

Phantom  Circuits — Transpositions — Transmission  over  Phantom  Circuits — Sim- 
plex Circuits  —  Composite  Circuits  —  Ringing  —  Phantoms  from  Simplex  and 
Composite  Circuits— Railway  Composite  Circuit  and  Equipment 


x  TABLE  OF  CONTENTS 

CHAPTER  XL 
TELEPHONE  TRAIN  DISPATCHING 699 

Growth — Its  Introduction — Transmitting  Orders — Western  Electric  Selector — 
Gill  Selector— Cummings-Wray  Selector — Telephone  Equipment— Dispatcher's 
Transmitter — Waystation  and  Siding  Telephones — Circuits — Voltage — Simulta- 
neous Talking  and  Signaling— Test  Boards— Locking  Sets— Dispatching  on  Elec- 
tric Railways 

CHAPTER  XLI 
TYPES  OF  TELEPHONE  LINES 725 

Underground  and  Overhead  Lines— Insulation  Poles— Resistance— Cables— Gen- 
eral Practice 

CHAPTER  XLII 
OPEN  WIRES ...      728 

Materials — Iron— Galvanizing— Copper— Copper-Clad  Steel— Insulated  Open  Wire 
—Wire  Tables 

CHAPTER  XLIII 
CABLES 736 

Types— Insulating  Material— Pairs— Core— Lead  Sheath — Capacity— Corrective 
Factors— Diameters  and  Weights— Submarine  Cable— Loading 

CHAPTER    XLIV 
POLES  AND  POLE  FITTINGS 745 

Equipment — Timber — Cutting—  Sizes  —  Trimming  —  Treating  —  Roofing  —  Cross 
Arms — Pins — Steps — Hardware  Requirements — Pole  Setting — Toll  Line  Construc- 
tion— Guys — Braces — Stringing  Wire — Transpositions — Tying — Sag — Dead-End- 
ing— Test  Points— Rural  Lines— City  Exchange  Lines— Locating— Crossing- 
Grading — Strain — Splices— Cable  Erection— Terminals— Pole  Seats 

CHAPTER  XLV 
UNDERGROUND  CONSTRUCTION 799 

Early  Practice— Buried  Cable— Underground  Conduit— Materials— Types  of  Con- 
duit—Laying— Manholes— Grading— Curves— Lateral  Runs— Risers—  Cable  Sup- 
ports— Installing  Underground  Cables — Drawing  In — Vaults 

CHAPTER  XLVI 
CABLE  SPLICING  .        . 828 

Dryness— Methods— Straight  Splice— Tap  Splice — Y-Splices  —  Lead  Sleeves — 
Pot-  Heads— Types 

CHAPTER    XLVII 
OFFICE  TERMINAL  CABLES 838 

Aerial  Cable  Entrance— Underground  Cable  Entrance  for  Small  Plants — Under- 
ground Entrances  for  Large  Plants— Treatment  of  Cable  Ends— Cable  Runs  and 
Shafts 

CHAPTER  XLVIII 

SERVICE  CONNECTIONS         ...  .848 

Connections  from  Bare  Wire  Lines— Connections  from  Cable  Lines — Drop  Wire 
Distribution— Ties— Rear  Wall  or  Fence  Distribution— Distribution  from  Under- 
ground Terminals  in  Buildings 


TABLE  OF  CONTENTS  xi 

CHAPTER  XLIX 

SUBSCRIBER'S  STATION  WIRING  862 


CHAPTER  L 
ELECTROLYSIS  OF  UNDERGROUND  CABLES 878 

Results — Early  Controversy — Court  Decisions — McCluer  System — Metallic  Cir- 
cuits and  Cables — Troubles — Causes— Conditions — Remedies — Insulation  Joints — 
Moisture- Proof  Conduits — Bonding 

CHAPTER  LI 
DEVELOPMENT  STUDIES 887 

Long-Distance  or  Toll  Line — Exchange  Development  Study — Ratio  of  Telephones 
to  Population — House  Count — Ratio  of  Telephones  to  Buildings — Comparison  of 
Estimates— Office  Districts — Central-Office  Locations— Conduit  and  Pole-Line 
Routes— Ultimate  Sizes — Subdivision 

CHAPTER  LII 
CA*E  OF  PLANT 895 

Maintenance  and  Depreciation — The  Outside  Plant — Supports,  Conduits,  Open 
Wire  and  Cables — Subscriber's  Equipment — Manual  Office  Equipment — Troubles 
— Remedies — Soldering 

CHAPTER  LIII 
TESTING 904 

Implements— Faults — Wire  Chief's  Tests — Galvanometer  Tests — Identification — 
Cable  Testing — -Instruments — Testing  Sets — Loss-of-Charge  Tests — Varley  Loop 
Tests — Murray  Loop  Tests — Capacity  Tests  for  Opens — Listening  Tests 


ALEXANDER  GRAHAM  BELL 
The  Inventor  of  the  Telephone, 


TELEPHONY 


INTRODUCTION 


The  telephone  was  invented  in  1875  by  Alexander  Graham  Bell, 
a  resident  of  the  United  States,  a  native  of  Scotland,  and  by  pro- 
fession a  teacher  of  deaf  mutes  in  the  art  of  vocal  speech.  In  that 
year,  Professor  Bell  was  engaged  in  the  experimental  development 
of  a  system  of  multiplex  telegraphy,  based  on  the  use  of  rapidly 
varying  currents.  During  those  experiments,  he  observed  an  iron 
reed  to  vibrate  before  an  electromagnet  as  a  result  of  another  iron 
reed  vibrating  before  a  distant  electromagnet  connected  to  the 
nearer  one  by  wires. 

The  telephone  resulted  from  this  observation  with  great  prompt- 
ness. In  the  instrument  first  made,  sound  vibrated  a  membrane 
diaphragm  supporting  a  bit  of  iron  near  an  electromagnet;  a  line 
joined  this  simple  device  of  three  elements  to  another  like  it;  a  bat- 
tery in  the  line  magnetized  both  electromagnet  cores;  the  vibration 
of  the  iron  in  the  sending  device  caused  the  current  in  the  line  to 
undulate  and  to  vary  the  magnetism  of  the  receiving  device.  The 
diaphragm  of  the  latter  was  vibrated  in  consequence  of  the  varying 
pull  upon  its  bit  of  iron,  and  these  vibrations  reproduced  the  sound 
that  vibrated  the  sending  diaphragm. 

The  first  public  use  of  the  electric  telephone  was  at  the  Centen- 
nial Exposition  in  Philadelphia  in  1876.  It  was  there  tested  by 
many  interested  observers,  among  them  Sir  William  Thomson,  later 
Lord  Kelvin,  the  eminent  Scotch  authority  on  matters  of  electrical 
communication.  It  was  he  who  contributed  so  largely  to  the  suc- 
cess of  the  early  telegraph  cable  system  between  England  and 


2  TELEPHONY 

America.      Two  of  his  comments  which  are  characteristic  are  as  fol- 
lows: 

To-day  I  have  seen  that  which  yesterday  I  should  have  deemed  impos- 
sible.    Soon  lovers  will  whisper  their  secrets  over  an  electric  wire. 


Who  can  but  admire  the  hardihood  of  invention  which  devised  such  slight 
means  to  realize  the  mathematical  conception  that  if  electricity  is  to  convey  all 
the  delicacies  of  sound  which  distinguish  articulate  speech,  the  strength  of  its 
current  must  vary  continuously  as  nearly  as  may  be  in  simple  proportion  to 
the  velocity  of  a  particle  of  the  air  engaged  in  constituting  the  sound. 

Contrary  to  usual  methods  of  improving  a  new  art,  the  ear- 
liest improvement  of  the  telephone  simplified  it.  The  diaphragms 
became  thin  iron  disks,  instead  of  membranes  carrying  iron;  the 
electromagnet  cores  were  made  of  permanently  magnetized  steel 
instead  of  temporarily  magnetized  soft  iron,  and  the  battery  was 
omitted  from  the  line.  The  undulatory  current  in  a  system  of  two 
such  telephones  joined  by  a  line  is  produced  in  the  sending  telephone 
by  the  vibration  of  the  'iron  diaphragm.  The  vibration  of  the  dia- 
phragm in  the  receiving  telephone  is  produced  by  the  undulatory 
current.  Sound  is  produced  by  the  vibration  of  the  diaphragm  of 
the  receiving  telephone. 

Such  a  telephone  is  at  once  the  simplest  known  form  of  electric 
generator  or  motor  for  alternating  currents.  It  is  capable  of  trans- 
lating motion  into  current  or  current  into  motion  through  a  wide 
range  of  frequencies.  It  is  not  known  that  there  is  any  frequency  of 
alternating  current  which  it  is  not  capable  of  producing  and  trans- 
lating. It  can  produce  and  translate  currents  of  greater  complexity 
than  any  other  existing  electrical  machine. 

Though  possessing  these  admirable  qualities  as  an  electrical 
machine,  the  simple  electromagnetic  telephone  had  not  the  ability 
to  transmit  speech  loudly  enough  for  all  practical  uses.  Trans- 
mitters producing  stronger  telephonic  currents  were  developed  soon 
after  the  fundamental  invention.  Some  forms  of  these  were  invented 
by  Professor  Bell  himself.  Other  inventors  contributed  devices 
embodying  the  use  of  carbon  as  a  resistance  to  be  varied  by  the 
motions  of  the  diaphragm.  This  general  form  of  transmitting 
telephone  has  prevailed  and  at  present  is  the  standard  type. 

It  is  interesting  to  note  that  the  earliest  incandescent  lamps,  as 


INTRODUCTION  3 

invented  by  Mr.  Edison,  had  a  resistance  material  composed  of 
carbon,  and  that  such  a  lamp  retained  its  position  as  the  most  effi- 
cient small  electric  illuminant  until  the  recent  introduction  of  metal 
filament  lamps.  It  is  possible  that  some  form  of  metal  may  be 
introduced  as  the  resistance  medium  for  telephone  transmitters,  and 
that  such  a  change  as  has  taken  place  in  incandescent  lamps  may 
increase  the  efficiency  of  telephone  transmitting  devices. 

At  the  time  of  the  invention  of  the  telephone,  there  were  in 
existence  two  distinct  types  of  telegraph,  working  in  regular  com- 
mercial service.  In  the  more  general  type,  many  telegraph  stations 
were  connected  to  a  line  and  whatever  was  telegraphed  between 
two  stations  could  be  read  by  all  the  stations  of  that  line.  In  the  other 
and  less  general  type,  many  lines,  each  having  a  single  telegraph 
station,  were  centered  in  an  office  or  "exchange,"  and  at  the  desire 
of  a  user  his  line  could  be  connected  to  another  and  later  dis- 
connected from  it. 

Both  of  these  types  of  telegraph  service  were  imitated  at  once 
in  telephone  practice.  Lines  carrying  many  telephones  each,  were 
established  with  great  rapidity.  Telephones  actually  displaced  tele- 
graphic apparatus  in  the  exchange  method  of  working  in  America. 
The  fundamental  principle  on  which  telegraph  or  telephone  ex- 
changes operate,  being  that  of  placing  any  line  in  communication 
with  any  other  in  the  system,  gave  to  each  line  an  ultimate  scope  so 
great  as  to  make  this  form  of  communication  more  popular  than 
any  arrangement  of  telephones  on  a  single  line.  Beginning  in  1877, 
telephone  exchanges  were  developed  with  great  rapidity  in  all  of  the 
larger  communities  of  the  United  States.  Telegraph  switching 
devices  were  utilized  at  the  outset  or  were  modified  in  such  minor 
particulars  as  were  necessary  to  fit  them  to  the  new  task. 

In  its  simplest  form,  a  telephone  system  is,  of  course,  a  single 
line  permanently  joining  two  telephones.  In  its  next  simplest  form, 
it  is  a  line  permanently  joining  more  than  two  telephones.  In  its 
most  useful  form,  it  is  a  line  joining  a  telephone  to  some  means  of 
connecting  it  at  will  to  another. 

A  telephone  exchange  central  office  contains  means  for  con- 
necting lines  at  will  in  that  useful  way.  The  least  complicated  ma- 
chine for  that  purpose  is  a  switchboard  to  be  operated  by  hand, 
having  some  way  of  letting  the  operator  know  that  a  connection  is 


4  TELEPHONY 

wished  and  a  way  of  making  it.  The  customary  way  of  connecting 
the  lines  always  has  been  by  means  of  flexible  conductors  fitted 
with  plugs  to  be  inserted  in  sockets.  If  the  switchboard  be  small 
enough  so  that  all  the  lines  are  within  arm's  reach  of  the  operator, 
the  whole  process  is  individual,  and  may  be  said  to  be  at  its  best  and 
simplest.  There  are  but  few  communities,  however,  in  .which  the 
number  of  lines  to  be  served  and  calls  to  be  answered  is  small  enough 
so  that  the  entire  traffic  of  the  exchange  can  be  handled  by  a  single 
person.  An  obvious  way,  therefore,  is  to  provide  as  many  operators 
in  a  central  office  as  may  be  required  by  the  number  of  calls  to  be 
answered,  and  to  terminate  before  each  of  the  operators  enough  of 
the  lines  to  bring  enough  work  to  keep  that  operator  economically 
occupied.  This  presents  the  additional  problem,  how  to  connect  a 
line  terminating  before  one  operator  to  a  line  normally  terminating 
before  another  operator.  The  obvious  answer  is  to  provide  lines 
from  each  operator's  place  of  work  to  each  other  operator's  place, 
connecting  a  calling  line  to  some  one  of  these  lines  which  are 
local  within  the  central  office,  and,  in  turn,  connecting  that  chosen 
local  line  to  the  line  which  is  called. 

Such  lines  between  operators  have  come  to  be  known  as  trunk 
lines,  because  of  the  obvious  analogy  to  trunk  lines  of  railways  be- 
tween common  centers,  and  such  a  system  of  telephone  lines  may  be 
called  a  trunking  system.  Very  good  service  has  been  given  and  can 
be  given  by  such  an  arrangement  of  local  trunks,  but  the  growth 
in  lines  and  in  traffic  has  developed  in  most  instances  certain  weak- 
nesses which  make  it  advisable  to  find  speedier,  more  accurate,  and 
more  reliable  means. 

For  the  serving  of  a  large  traffic  from  a  large  number  of  lines, 
as  is  required  in  practically  every  city  of  the  world,  a  very  great 
contribution  to  the  practical  art  was  made  by  the  development  of  the 
multiple  switchboard.  Such  a  switchboard  is  merely  such  a  device 
as  has  been  described  for  the  simpler  cases,  with  the  further  refine- 
ment that  within  reach  of  each  operator  in  the  central  office  appears 
every  line  which  enters  that  office,  and  this  without  regard  to  what 
point  in  the  switchboard  the  lines  may  terminate  for  the  answering 
of  calls.  In  other  words,  while  each  operator  answers  a  certain 
subordinate  group  of  the  total  number  of  lines,  each  operator  may 
reach,  for  calling  purposes,  every  line  which  enters  that  office.  It 


INTRODUCTION  5 

is  probable  that  the  invention  and  development  of  the  multiple 
switchboard  was  the  first  great  impetus  toward  the  wide-spread 
use  of  telephone  service. 

Coincident  with  the  development  of  the  multiple  switchboard 
for  manually  operated,  central -office  mechanisms  was  the  beginning 
of  the  development  of  automatic  apparatus  under  the  control  of  the 
calling  subscriber  for  finding  and  connecting  with  a  called  line.  It 
is  interesting  to  note  the  general  trend  of  the  early  development  of 
automatic  apparatus  in  comparison  with  the  development,  to  that 
time,  of  manual  telephone  apparatus. 

While  the  manual  apparatus  on  the  one  hand  attempted  to  meet 
its  problem  by  providing  local  trunks  between  the  various  operators 
of  a  central  office,  and  failing  of  success  in  that,  finally  developed 
a  means  which  placed  all  the  lines  of  a  central  office  within  con- 
necting reach  of  each  operator,  automatic  telephony,  beginning  at 
that  point,  failed  of  success  in  attempting  to  bring  each  line  in  the 
central  office  within  connecting  reach  of  each  connecting  mechanism. 

In  other  terms,  the  first  automatic  switching  equipment  con- 
sisted of  a  machine  for  each  line,  which  machine  was  so  organized 
as  to  be  able  to  find  and  connect  its  calling  line  with  any  called  line 
of  the  entire  central-office  group.  It  may  be  said  that  an  attempt 
to  develop  this  plan  was  the  fundamental  reason  for  the  repeated 
failure  of  automatic  apparatus  to  solve  the  problem  it  attacked. 
All  that  the  earlier  automatic  system  did  was  to  prove  more  or  less 
successfully  that  automatic  apparatus  had  a  right  to  exist,  and  that 
to  demand  of  the  subscriber  that  he  manipulate  from  his  station  a 
distant  machine  to  make  the  connection  without  human  aid  was  not 
fallacious.  When  it  had  been  recognized  that  the  entire  multiple 
switchboard  idea  could  not  be  carried  into  automatic  telephony  with 
success,  the  first  dawn  of  hope  in  that  art  may  be  said  to  have  come. 

Success  in  automatic  telephony  did  come  by  the  re-adoption  of 
the  trunking  method.  As  adopted  for  automatic  telephony,  the 
method  contemplates  that  the  calling  line  shall  be  extended,  link 
by  link,  until  it  finds  itself  lengthened  and  directed  so  as  to  be  able 
to  seize  the  called  line  in  a  very  much  smaller  multiple  than  the  total 
group  of  one  office  of  the  exchange. 

A  similar  curious  reversion  has  taken  place  in  the  development 
of  telephone  lines.  The  earliest  telephone  lines  were  merely  tele- 


6  TELEPHONY 

graph  lines  equippeJ  with  telephone  instruments,  and  the  earliest 
telegraph  lines  were  planned  by  Professor  Morse  to  be  insulated 
wires  laid  in  the  earth.  A  lack  of  skill  in  preparing  the  wires  for 
putting  in  the  earth  caused  these  early  underground  lines  to  be 
failures.  At  the  urging  of  one  of  his  associates,  Professor  Morse 
consented  to  place  his  earliest  telegraph  lines  on  poles  in  the  air. 
Each  such  line  originally  consisted  of  two  wires,  one  for  the  going  and 
one  for  the  returning  current,  as  was  then  considered  the  action.  Upon 
its  being  discovered  that,  a  single  wire,  using  the  earth  as  a  return, 
would  serve  as  a  satisfactory  telegraph  line,  such  practice  became 
universal.  Upon  the  arrival  of  the  telephone,  all  lines  obviously 
were  built  in  the  same  way,  and  until  force  of  newer  circumstances 
compelled  it,  the  present  metallic  circuit  without  an  earth  connec- 
tion did  not  come  into  general  use. 

The  extraordinary  growth  of  the  number  of  telephone  lines  in 
a  community  and  the  development  of  other  methods  of  electrical 
utilization,  as  well  as  the  growth  in  the  knowledge  of  telephony 
itself,  ultimately  forced  the  wires  underground  again.  At  the  same 
time  and  for  the  same  causes,  a  telephone  line  became  one  of  two 
wires,  so  that  it  becomes  again  the  counterpart  of  the  earliest  tele- 
graph line  of  Professor  Morse. 

Another  curious  and  interesting  example  of  this  reversion  to 
type  exists  in  the  simple  telephone  receiver.  An  early  improve- 
ment in  telephone  receivers  after  Professor  Bell's  original  inven- 
tion was  to  provide  the  necessary  magnetism  of  the  receiver  core  by 
making  it  of  steel  and  permanently  magnetizing  it,  whereas  Pro- 
fessor Bell's  instrument  provided  its  magnetism  by  means  of  direct 
current  flowing  in  the  line.  In  later  days 'the  telephone  receiver  has 
returned  almost  to  the  original  form  in  which  Professor  Bell  pro- 
duced it  and  this  change  has  simplified  other  elements  of  telephone- 
exchange  apparatus  in  a  very  interesting  and  gratifying  way. 

By  reason  of  improvements  in  methods  of  line  construction 
and  apparatus  arrangement,  the  radius  of  communication  steadily 
has  increased.  Commercial  speech  now  is  possible  between  points 
several  thousand  miles  apart,  and  there  is  no  theoretical  reason  why 
communication  might  not  be  established  between  any  two  points  on 
the  earth's  surface.  The  practical  reasons  of  demand  and  cost  may 
prevent  so  great  an  accomplishment  as  talking  half  around  the  earth. 


INTRODUCTION  7 

So  far  as  science  is  concerned  there  would  seem  to  be  no  reason  why 
this  might  not  be  done  today,  by  the  careful  application  of  what 
already  is  known. 

In  the  United  States,  telephone  service  from  its  beginning  has 
been  supplied  to  users  by  private  enterprise.  In  other  countries, 
it  is  supplied  by  means  of  governmentally-owned  equipment.  In 
general,  it  may  be  said  that  the  adequacy  and  the  amount,  as  well 
as  the  quality  of  telephone  service,  is  best  in  countries  where  the 
service  is  provided  by  private  enterprise. 

Telephone  systems  in  the  United  States  were  under  the  control 
of  the  Bell  Telephone  Company  from  the  invention  of  the  device 
in  1876  until  1893.  The  fundamental  telephone  patent  expired 
in  1893.  This  opened  the  telephone  art  to  the  general  public,  be- 
cause it  no  longer  was  necessary  to  secure  telephones  solely  from  the 
patent-holding  company  nor  to  pay  royalty  for  the  right  to  use  them, 
if  secured  at  all.  Manufacturers  of  electrical  apparatus  generally 
then  began  to  make  and  sell  telephones  and  telephone  apparatus, 
and  operating  companies,  also  independent  of  the  Bell  organization, 
began  to  install  and  use  telephones.  At  the  end  of  seventeen  years 
of  patent  monopoly  in  the  United  States,  there  were  in  operation  a 
little  over  250,000  telephones.  In  the  seventeen  years  since  the  ex- 
piration of  the  fundamental  patent,  independent  telephone  com- 
panies throughout  the  United  States  have  installed  and  now  have  in 
daily  successful  use  over  3,911,400  telephones.  In  other  words, 
since  its  first  beginnings,  independent  telephony  has  brought  into 
continuous  daily  use  nearly  sixteen  times  as  many  telephones  as 
were  brought  into  use  in  the  equal  time  of  the  complete  monopoly 
of  the  Bell  organization. 

At  the  beginning  of  1910,  there  were  in  service  by  the  Bell  or- 
ganization about  3,633,900  telephones.  These  with  the  3,911,400 
independent  telephones,  make  a  total  of  7,545,300,  or  about 
one-twelfth  as  many  telephones  as  there  are  inhabitants  of  the 
United  States.  The  influence  of  this  development  upon  the  lives 
of  the  people  has  been  profound.  Whether  the  influence  has  been 
wholly  for  good  may  not  be  so  conclusively  apparent.  Lord 
Bacon  has  declared  that,  excepting  only  the  alphabet  and  the 
art  of  printing,  those  inventions  abridging  distance  are  of  the 
greatest  service  to  mankind.  If  this  be  true,  it  may  be.  said  that 


8  TELEPHONY 

the  invention  of  telephony  deserves  high  place  winong  the  civilizing 
influences. 

There  is  no  industrial  art  in  which  the  advancement  of  the 
times  has  been  followed  more  closely  by  practical  application  than 
in  telephony.  Commercial  speech  by  telephone  is  possible  by  means 
of  currents  which  so  far  are  practically  unmeasurable.  In  other 
words,  it  is  possible  to  speak  clearly  and  satisfactorily  over  a  line 
by  means  of  currents  which  cannot  be  read,  with  certainty  as  to  their 
amount,  by  any  electrical  measuring  device  so  far  known.  In  this 
regard,  telephony  is  less  well  fortified  than  are  any  of  the  arts  utiliz- 
ing electrical  power  in  larger  quantities.  The  real  wonder  is  that 
with  so  little  knowledge  of  what  takes  place,  particularly  as  to 
amount,  those  working  in  the  art  have  been  able  to  do  as  well  as 
they  have.  When  an  exact  knowledge  of  quantity  is  easily  ob- 
tainable, very  striking  advances  may  be  looked  for. 

The  student  of  these  phases  of  physical  science  and  industrial 
art  will  do  well  to  combine  three  processes:  study  of  the  words  of 
others;  personal  experimentation;  and  digestive  thought.  The  last 
mentioned  is  the  process  of  profoundest  value.  On  it  finally  depends 
mastery.  It  is  not  of  so  much  importance  how  soon  the  concept  shall 
finally  be  gained  as  that  it  is  gained.  A  statement  by  another  may 
seem  lifeless  and  inert  and  the  meaning  of  an  observation  may  be 
obscure.  Digestive  thought  is  the  only  assimilative  process.  The 
whole  art  of  telephony  hangs  on  taking  thought  of  things.  Judge 
R.  F.  Taylor  of  Indiana  said  of  Professor  Bell,  "It  has  been  said 
that  no  man  by  taking  thought  may  add  a  cubit  to  his  stature,  yet 
here  is  a  man  who,  by  taking  thought,  has  added  not  cubits  but 
miles  to  the  lengths  of  men's  tongues  and  ears." 

In  observations  of  many  students,  it  is  found  that  the  notion  of 
each  must  pass  through  a  certain  period  of  incubation  before  his 
private  and  personal  knowledge  of  Ohm's  law  is  hatched.  Once 
hatched,  however,  it  is  his.  By  just  such  a  process  must  come  each 
principal  addition  to  his  stock  of  concepts.  The  periods  may  vary 
and  practice  in  the  uses  of  the  mind  may  train  it  in  alertness  in 
its  work.  If  time  is  required,  time  should  be  given,  the  object  al- 
ways being  to  keep  thinking  or  re-reading  or  re-trying  until  the 
thought  is  wholly  encompassed  and  possessed. 


CHAPTER  I 
ACOUSTICS 

Telephony  is  the  art  of  reproducing  at  a  distant  point,  usually 
by  the  agency  of  electricity,  sounds  produced  at  a  sending  point.  In 
this  art  the  elements  of  two  general  divisions  of  physical  science  are 
concerned,  sound  and  electricity. 

Sound  is  the  effect  of  vibrations  of  matter  upon  the  ear.  The 
vibrations  may  be  .those  of  air  or  other  matter.  Various  forms  of 
matter  transmit  sound  vibrations  in  varying  degrees,  at  different 
specific  speeds,  and  with  different  effects  upon  the  vibrations.  Any 
form  of  matter  may  serve  as  a  transmitting  medium  for  sound  vibra- 
tions. Sound  itself  is  an  effect  of  .sound  vibrations  upon  the  ear. 

Propagation  of  Sound.  Since  human  beings  communicate  with 
each  other  by  means  of  speech  and  hearing  through  the  air,  it  is  with 
air  that  the  acoustics  of  telephony  principally  is  concerned.  In 
air>  sound  vibrations  consist  of  successive  condensations  and  rare- 
factions tending  to  proceed  outwardly  from  the  source  in  all  direc- 
tions. The  source  is  the  center  of  a  sphere  of  sound  vibrations. 
Whatever  may  be  the  nature  of  the  sounds  or  of  the  medium  trans- 
mitting them,  they  consist  of  waves  emitted  by  the  source  and  ob- 
served by  the  ear.  A  sound  wave  is  one  complete  condensation  and 
rarefaction  of  the  transmitting  medium.  It  is  produced  by  one 
complete  vibration  of  the  sound-producing  thing. 

Sound  waves  in  air  travel  at  a  rate  of  about  1,090  feet  per  second. 
The  rate  of  propagation  of  sound  waves  in  other  materials  varies  with 
the  density  of  the  material.  For  example,  the  speed  of  transmis- 
sion is  much  greater  in  water  than  in  air,  and  is  much  less  in  highly 
rarefied  air  than  in  air  at  ordinary  density.  The  propagation  of 
sound  waves  in  a  vacuum  may  be  said  not  to  take  place  at  all. 

Characteristics  of  Sound.  Three  qualities  distinguish  sound- 
ioudness,  pitch,  and  timbre. 


10  TELEPHONY 

Loudness.  Loudness  depends  upon  the  violence  of  the  effect 
upon  the  ear;  sounds  may  be  alike  in  their  other  qualities  and  differ 
in  loudness,  the  louder  sounds  being  produced  by  the  stronger  vi- 
brations of  the  air  or  other  medium  at  the  ear.  Other  things  being 
equal,  the  louder  sound  is  produced  by  the  source  radiating  the 
greater  energy  and  so  producing  the  greater  degree  of  condensation 
and  rarefaction  of  the  medium. 

Pitch.  Pitch  depends  upon  the  frequency  at  which  the  sound 
waves  strike  the  ear.  Pitches  are  referred  to  as  high  or  low  as  the 
frequency  of  waves  reaching  the  ear  are  greater  or  fewer.  Familiar 
low  pitches  are  the  left-hand  strings  of  a  piano;  the  larger  ones  of 
stringed  instruments  generally;  bass  voices;  and  large  bells.  Fa- 
miliar high  pitches  are  right-hand  piano  strings;  smaller  ones  of  other 
stringed  instruments;  soprano  voices;  small  bells;  and  the  voices  of 
most  birds  and  insects. 

Doppler's  Principle: — As  pitch  depends  upon  the  frequency  at 
which  sound  waves  strike  the  ear,  an  object  may  emit  sound  waves 
at  a  constant  frequency,  yet  may  produce  different  pitches  in  ears 
differently  situated.  Such  a  case  is  not  usual,  but  an  example  of 
it  will  serve  a  useful  purpose  in  fixing  certain  facts  as  to  pitch. 
Conceive  two  railroad  trains  to  pass  each  other,  running  in  opposite 
directions,  the  engine  bells  of  both  trains  ringing.  Passengers  on 
each  train  will  hear  the  bell  of  the  other,  first  as  a  rising  pitch, 
then  as  a  falling  one.  Passengers  on  each  train  will  hear  the  bell 
of  their  own  train  at  a  constant  pitch. 

The  difference  in  the  observations  in  such  a  case  is  due  to 
relative  positions  between  the  ear  and  the  source  of  the  sound.  As 
to  the  bell  of  their  own  train,  the  passengers  are  a  fixed  distance  from 
it,  whether  the  train  moves  or  stands;  as  to  the  bell  of  the  other  train, 
the  passengers  first  rapidly  approach  it,  then  pass  it,  then  recede 
from  it.  The  distances  at  which  it  is  heard  vary  as  the  secants  of  a 
circle,  the  radius  in  this  case  being  a  length  which  is  the  closest  ap- 
proach of  the  ear  to  the  bell. 

If  the  bell  have  a  constant  intrinsic  fundamental  pitch  of  200 
waves  per  second  (a  wave-length  of  about  5.5  feet),  it  first  will  be 
heard  at  a  ~>itch  of  about  200  waves  per  second.  But  this  pitch 
rises  rapidly,  as  if  the  bell  were  changing  its  own  pitch,  which  bells 
do  not  do.  The  rising  pitch  is  heard  because  the  ear  is  rushing 


ACOUSTICS  11 

down  the  wave-train,  every  instant  nearer  to  the  source.  At  a  speed 
of  45  miles  an  hour,  the  pitch  rises  rapidly,  about  12  vibrations  per 
second.  If  the  rate  of  approach  between  the  ear  and  the  bell  were 
constant,  the  pitch  of  the  bell  would  be  heard  at  212  waves  per  second. 
But  suddenly  the  ear  passes  the  bell,  hears  the  pitch  stop  rising  and 
begin  to  fall;  and  the  tone  drops  12  waves  per  second  as  it  had  risen. 
Such  a  circumflex  is  an  excellent  example  of  the  bearing  of  wave- 
lengths and  frequencies  upon  pitch. 

Vibration  of  Diaphragms: — Sound  waves  in  air  have  the 
power  to  move  other  diaphragms  than  that  of  the  ear.  Sound  waves 
constantly  vibrate  such  diaphragms  as  panes  of  windows  and  the 
walls  of  houses.  The  recording  diaphragm  of  a  phonograph  is  a 
window  pane  bearing  a  stylus  adapted  to  engrave  a  groove  in  a 
record  blank.  In  the  cylinder  form  of  record,  the  groove  varies  in 
depth  with  the  vibrations  of  the  diaphragm.  In  the  disk  type  of 
phonograph,  the  groove  varies  sidewise  from  its  normal  true  spiral. 

If  the  disk  record  be  dusted  with  talcum  powder,  wiped,  and 
examined  with  a  magnifying  glass,  the  waving  spiral  line  may  be  seen. 
Its  variations  are  the  result  of  the  blows  struck  upon  the  diaphragm 
by  a  train  of  sound  waves. 

In  reproducing  a  phonograph  record,  increasing  the  speed  of 
the  record  rotation  causes  the  pitch  to  rise,  because  the  blows  upon 
the  air  are  increased  in  frequency  and  the  wave-lengths  shortened. 
A  transitory  decrease  in  speed  in  recording  will  cause  a  transitory 
rise  in  pitch  when  that  record  is  reproduced  at  uniform  speed. 

Timbre.  Character  of  sound  denotes  that  difference  of  effect 
produced  upon  the  ear  by  sounds  otherwise  alike  in  pitch  and  loud- 
ness.  This  characteristic  is  called  timbre.  It  is  extraordinarily 
useful  in  human  affairs,  human  voices  being  distinguished  from  each 
other  by  it,  and  a  great  part  of  the  joy  of  music  lying  in  it. 

A  bell,  a  stretched  string,  a  reed,  or  other  sound-producing  body, 
emits  a  certain  lowest  possible  tone  when  vibrated.  This  is  called  its 
fundamental  tone.  The  pitch,  loudness,  and  timbre  of  this  tone  de- 
pend upon  various  controlling  causes.  Usually  this  fundamental 
tone  is  accompanied  by  a  number  of  others  of  higher  pitch,  blend- 
ing with  it  to  form  the  general  tone  of  that  object.  These  higher 
tones  are  called  harmonics.  The  Germans  call  them  overtones. 
They  are  always  of  a  frequency  which  is  some  multiple  of  the  funda- 


12  TELEPHONY 

mental  frequency.  That  is,  the  rate  of  vibration  of  a  harmonic  is 
2,  3,  4,  5,  or  some  other  integral  number,  times  as  great  as  the 
fundamental  itself.  A  tone  having  no  harmonics  is  rare  in  nature 
and  is  not  an  attractive  one.  The  tones  of  the  human  voice  are 
rich  in  harmonics. 

In  any  tone  having  a  fundamental  and  harmonics  (multiples),  the 
wave-train  consists  of  a  complex  series  of  condensations  and  rarefac- 
tions of  the  air  or  other  transmitting  medium.  In  the  case  of  mere 
noises  the  train  of  vibrations  is  irregular  and  follows  no  definite  order. 
This  is  the  difference  between  vowel  sounds  and  other  musical  tones 
on  the  one  hand  and  all  unmusical  sounds  (or  noises)  on  the  other. 

Human  Voice.  Human  beings  communicate  with  each  other 
in  various  ways.  The  chief  method  is  by  speech.  Voice  is  sound 
vibration  produced  by  the  vocal  cords,  these  being  two  ligaments 
in  the  larynx.  The  vocal  cords  in  man  are  actuated  by  the  air  from 
the  lungs.  The  size  and  tension  of  the  vocal  cords  and  the  volume 
and  the  velocity  of  the  air  from  the  lungs  control  the  tones  of  the 
voice.  The  more  tightly  the  vocal  cords  be  drawn,  other  things 
being  equal,  the  higher  will  be  the  pitch  of  the  sound;  that  is,  the 
higher  the  frequency  of  vibration  produced  by  the  voice.  The 
pitches  of  the  human  voice  lie,  in  general,  between  the  frequencies 
of  87  and  768  per  second.  These  are  the  extremes  of  pitch,  and  it 
is  not  to  be  understood  that  any  such  range  of  pitch  is  utilized  in 
ordinary  speech.  An  average  man  speaks  mostly  between  the 
fundamental  frequencies  of  85  and  160  per  second.  Many  female 
speaking  voices  use  fundamental  frequencies  between  150  and  320 
vibrations  per  second.  It  is  obvious  from  what  has  been  said  that 
in  all  cases  these  speaking  fundamentals  are  accompanied  by  their 
multiples,  giving  complexity  to  the  resulting  wave-trains  and  char- 
acter to  the  speaking  voice. 

Speech-sounds  result  from  shocks  given  to  the  air  by  the  organs 
of  speech;  these  organs  are  principally  the  mouth  cavity,  the  tongue, 
and  the  teeth.  The  vocal  cords  are  voice-organs;  that  is,  man  only 
truly  speaks,  yet  the  lower  animals  have  voice.  Speech  may  be 
whispered,  using  no  voice.  Note  the  distinction  between  speech 
and  voice,  and  the  organs  of  both. 

The  speech  of  adults  has  a  mean  pitch  lower  than  that  of  chil- 
dren; of  adult  males,  lower  than  that  of  females. 


ACOUSTICS  13 

There  is  no  close  analogue  for  the  voice-organ  in  artificial 
mechanism,  but  the  use  of  the  lips  in  playing  a  bugle,  trumpet,  cornet, 
or  trombone  is  a  fairly  close  one.  Here  the  lips,  in  contact  with 
each  other,  are  stretched  across  one  end  of  a  tube  (the  mouthpiece) 
while  the  air  is  blown  between  the  lips  by  the  lungs.  A  musical  tone 
results;  if  the  instrument  be  a  bugle  or  a  trumpet  of  fixed  tube  length, 
the  pitch  will  be  some  one  of  several  certain  tones,  depending  on  the 
tension  on  the  lips.  The  loudness  depends  on  the  force  of  the  blast 
of  air;  the  character  depends  largely  on  the  bugle. 

Human  Ear.  The  human  ear,  the  organ  of  hearing  in  man,  is  a 
complex  mechanism  of  three  general  parts,  relative  to  sound  waves: 
a  wave-collecting  part;  a  wave-observing  part,  and  a  wave-interpre- 
ting part. 

The  outer  ear  collects  and  reflects  the  waves  inwardly  to  beat 
upon  the  tympanum,  or  ear  drum,  a  membrane  diaphragm.  The 
uses  of  the  rolls  or  convolutions  of  the  outer  ear  are  not  conclusively 
known,  but  it  is  observed  that  when  they  are  filled  up  evenly  with  a 
wax  or  its  equivalent,  the  sense  of  direction  of  sound  is  impaired,  and 
usually  of  loudness  also. 

The  diaphragm  of  the  ear  vibrates  when  struck  by  sound  waves, 
as  does  any  other  diaphragm.  By  means  of  bone  and  nerve 
mechanism,  the  vibration  of  the  diaphragm  finally  is  made  known  to 
the  brain  and  is  interpretable  therein. 

The  human  ear  can  appreciate  and  interpret  sound  waves  at 
frequencies  from  32  to  about  32,000  vibrations  per  second.  Below 
the  lesser  number,  the  tendency  is  to  appreciate  the  separate  vibra- 
tions as  separate  sounds.  Above  the  higher  number,  the  vibra- 
tions are  inaudible  to  the  human  ear.  The  most  acute  perception  of 
sound  differences  lies  at  about  3,000  vibrations  per  second.  It  may 
be  that  the  range  of  hearing  of  organisms  other  than  man  lies  far 
above  the  range  with  which  human  beings  are  familiar.  Some  trained 
musicians  are  able  to  discriminate  between  two  sounds  as  differing 
one  from  the  other  when  the  difference  in  frequency  is  less  than  one- 
thousandth  of  either  number.  Other  ears  are  unable  to  detect  a 
difference  in  two  sounds  when  they  differ  by  as  much  as  one  full  step 
of  the  chromatic  scale.  Whatever  faculty  an  individual  may  pos- 
sess as  to  tone  discrimination,  it  can  be  improved  by  training  and 
practice. 


CHAPTER  II 
ELECTRICAL  REPRODUCTION  OF  SPEECH 

The  art  of  telephony  in  its  present  form  has  for  its  problem  so 
to  relate  two  diaphragms  and  an  electrical  system  that  one  diaphragm 
will  respond  to  all  the  fundamental  and  harmonic  vibrations  beating 
upon  it  and  cause  the  other  to  vibrate  in  exact  consonance,  produc- 
ing just  such  vibrations,  which  beat  upon  an  ear. 

The  art  does  not  do  all  this  today;  it  falls  short  of  it  in  every 
phase.  Many  of  the  harmonics  are  lost  in  one  or  another  stage  of 
the  process;  new  harmonics  are  inserted  by  the  operations  of  the  sys- 
tem itself  and  much  of  the  volume  originally  available  fails  to  reap- 
pear. The  art,  however,  has  been  able  to  change  commercial  and 
social  affairs  in  a  profound  degree. 

Conversion  from  Sound  Waves  to  Vibration  of  Diaphragm. 
However  produced,  by  the  voice  or  otherwise,  sounds  to  be  transmit- 
ted by  telephone  consist  of  vibrations  of  the  air.  These  vibrations, 
upon  reaching  a  diaphragm,  cause  it  to  move.  The  greatest  ampli- 
tude of  motion  of  a  diaphragm  is,  or  is  wished  to  be,  at  its  center, 
and  its  edge  ordinarily  is  fixed.  The  diaphragm  thus  serves  as  a 
translating  device,  changing  the  energy  carried  by  the  molecules  of 
the  air  into  localized  oscillations  of  the  matter  of  the  diaphragm. 
The  waves  of  sound  in  the  air  advance;  the  vibrations  of  the  mole- 
cules are  localized.  The  agency  of  the  air  as  a  medium  for  sound 
transmission  should  be  understood  to  be  one  in  which  its  general 
volume  has  no  need  to  move  from  place  to  place.  What  occurs  is  that 
the  vibrations  of  the  somid-producer  cause  alternate  condensations 
and  rarefactions  of  the  air.  Each  molecule  of  the  air  concerned  mere- 
ly oscillates  through  a  small  amplitude,  producing,  by  joint  action, 
shells  of  waves,  each  traveling  outward  from  the  sound-producing 
center  like  rapidly  growing  coverings  of  a  ball. 

Conversion  from  Vibration  to  Voice  Currents.  Fig.  1  illustrates 
a  simple  machine  adapted  to  translate  motion  of  a  diaphragm  into 


ELECTRICAL  REPRODUCTION  OF  SPEECH 


15 


an  alternating  electrical  current.  The  device  is  merely  one  form 
of  magneto  telephone  chosen  to  illustrate  the  point  of  immediate 
conversion.  1  is  a  diaphragm  adapted  to  vibrate  in  response  to  the 
sounds  reaching  it.  2  is  a  permanent  magnet  and  3  is  its  armature. 
The  armature  is  in  contact  with  one  pole  of  the  permanent  magnet 
and  nearly  in  contact  with  the  other.  The  effort  of  the  armature  to 
touch  the  pole  it  nearly  touches  places  the  diaphragm  under  tension. 
The  free  arm  of  the  magnet  is  surrounded  by  a  coil  4>  whose  ends 
extend  to  form  the  line. 

When  sound  vibrates  the  diaphragm,  it  vibrates  the  armature 
also,  increasing  and  decreasing  the  distance  from  the  free  pole  of  the 
magnet.  The  lines  of  force  thread- 
ing the  coil  4  are  varied  as  the  gap 
between  the  magnet  and  the  armature 
is  varied. 

The  result  of  varying  the  lines  of 
force  through  the  turns  of  the  coil  is 
to  produce  an  electromotive  force  in 
them,  and  if  a  closed  path  is  provided 
by  the  line,  a  current  will  flow.  This 
current  is  an  alternating  one  having  a 
frequency  the  same  as  the  sound  caus- 
ing it.  As  in  speech  the  frequencies 

vary  constantly,  many  pitches  consti- 

.  ".,  Fig.  1.    Type  of  Magneto  Telephone 

tutmg  even  a  single  spoken  word,  so 

the  alternating  voice  currents  are  of  great  varying  complexity,  and 
every  fundamental  frequency  has  its  harmonics  superposed. 

Conversion  from  Voice  Currents  to  Vibration.  The  best  knowl- 
edge of  the  action  of  such  a  telephone  as  is  shown  in  Fig.  1  leads  to  the 
conclusion  that  a  half-cycle  of  alternating  current  is  produced  by  an 
inward  stroke  of  the  diaphragm  and  a  second  half -cycle  of  alternating 
current  by  the  succeeding  outward  stroke,  these  half-cycles  flowing 
in  opposite  directions.  Assume  one  complete  cycle  of  current  to 
pass  through  the  line  and  also  through  another  such  device  as  in  Fig.  1 
and  that  the  first  half-cycle  is  of  such  direction  as  to  increase  the  per- 
manent magnetism  of  the  core.  The  effort  of  this  increase  is  to  nar- 
row the  gap  between  the  armature  and  pole  piece.  The  diaphragm 
will  throb  inward  during  the  half-cycle  of  current.  The  succeeding 


16  TELEPHONY 

• 

half-cycle  being  of  opposite  direction  will  tend  to  oppose  the  magnetism 
of  the  core.  In  practice,  the  flow  of  opposing  current  never  would 
be  great  enough  wholly  to  nullify  and  reverse  the  magnetism  of  the 
core,  so  that  the  opposition  results  in  a  mere  decrease,  causing  the 
armature's  gap  to  increase  and  the  diaphragm  to  respond  by  an 
outward  blow. 

Complete  Cycle  of  Conversion.  The  cycle  of  actions  thus  is  com- 
plete; one  complete  sound-wave  in  air  has  produced  a  cycle  of  motion 
in  a  diaphragm,  a  cycle  of  current  in  a  line,  a  cycle  of  magnetic  change 
in  a  core,  a  cycle  of  motion  in  another  diaphragm,  and  a  resulting 
wave  of  sound.  It  is  to  be  observed  that  the  chain  of  operation  involves 
the  expenditure  of  energy  only  by  the  speaker,  the  only  function 
of  any  of  the  parts  being  that  of  translating  this  energy  from  one 
form  to  another.  In  every  stage  of  these  translations,  there  are 
losses;  the  devising  of  means  of  limiting  these  losses  as  greatly  as 
possible  is  a  problem  of  telephone  engineering. 

Magneto  Telephones.  The  device  in  Fig.  1  is  a  practical  magneto 
receiver  and  transmitter.  It  is  chosen  as  best  picturing  the  idea  to 
be  proposed.  Fig.  2  illustrates  a  pair  of  magneto  telephones  of  the 
early  Bell  type;  1-1  are  diaphragms;  2-2  are  permanent  magnets  with 


rft 

,/c                                                                                           /A, 

/- 

Fig.  2.     Magneto  Telephones  and  Line 

a  free  end  of  each  brought  as  near  as  possible,  without  touching,  to 
the  diaphragm.  Each  magnet  bears  on  its  end  nearest  the  diaphragm 
a  winding  of  fine  wire,  the  two  ends  of  each  of  these  windings  being 
joined  by  means  of  a  two-wire  line.  All  that  has  been  said  concern- 
ing Fig.  1  is  true  also  of  the  electrical  and  magnetic  actions  of  the 
devices  of  Fig.  2.  In  the  latter,  the  flux  which  threads  the  fine  wire 
winding  is  disturbed  by  motions  of  the  transmitting  diaphragm. 
This  disturbance  of  the  flux  creates  electromotive  forces  in  those 
windings.  Similarly,  a  variation  of  the  electromotive  forces  in  the 
windings  varies  the  pull  of  the  permanent  magnet  of  the  receiving 
instrument  upon  its  diaphragm. 


ELECTRICAL  REPRODUCTION  OF  SPEECH  IV 

Fig.  3  illustrates  a  similar  arrangement,  but  it  is  to  be  understood 
that  the  cores  about  which  the  windings  are  carried  in  this  case  are 
of  soft  iron  and  not  of  hard  magnetized  steel.  The  necessary  mag- 
netism which  constantly  enables  the  cores  to  exert  a  pull  upon  the 
diaphragm  is  provided  by  the  battery  which  is  inserted  serially  in  the 


Fig.  3.     Magneto  Telephones  without  Permanent  Magnets 

line.  Such  an  arrangement  in  action  differs  in  no  particular  from 
that  of  Fig.  2,  for  the  reason  that  it  matters  not  at  all  whether 
the  magnetism  of  the  core  be  produced  by  electromagnetic  or  by 
permanently  magnetic  conditions.  The  arrangement  of  Fig.  3  is  a 
fundamental  counterpart  of  the  original  telephone  of  Professor  Bell, 
and  it  is  of  particular  interest  in  the  present  stage  of  the  art  for  the 
reason  that  a  tendency  lately  is  shown  to  revert  to  the  early  type, 
abandoning  the  use  of  the  permanent  magnet. 

The  modifications  which  have  been  made  in  the  original  magneto 
telephone,  practically  as  shown  in  Fig.  2,  have  been  many.  Thirty- 
five  years'  experimentation  upon  and  daily  use  of  the  instrument 
has  resulted  in  its  refinement  to  a  point  where  it  is  a  most  successful 
receiver  and  a  most  unsuccessful  transmitter.  Its  use  for  the  latter 
purpose  may  be  said  to  be  nothing.  As  a  receiver,  it  is  not  only 
wholly  satisfactory  for  commercial  use  in  its  regular  function,  but 
it  is,  in  addition,  one  of  the  most  sensitive  electrical  detecting  devices 
known  to  the  art. 

Loose  Contact  Principle.  Early  experimenters  upon  Bell's 
device,  all  using  in  their  first  work  the  arrangement  utilizing  current 
from  a  battery  in  series  with  the  line,  noticed  that  sound  was  given 
out  by  disturbing  loose  contacts  in  the  line  circuit.  This  observa- 
tion led  to  the  arrangement  of  circuits  in  such  a  way  that  some  im- 
perfect contacts  could  be  shaken  by  means  of  the  diaphragm,  and 
the  resistance  of  the  line  circuit  varied  in  this  manner.  An  early  and 


18  TELEPHONY 

interesting  form  of  such  imperfect  contact  transmitter  device  con- 
sisted merely  of  metal  conductors  laid  loosely  in  contact.  A  simple 
example  is  that  of  three  wire  nails,  the  third  lying  across  the  other 
two,  the  two  loose  contacts  thus  formed  being  arranged  in  series 
with  a  battery,  the  line,  and  the  receiving  instrument.  Such  a  device 
when  slightly  jarred,  by  the  voice  or  other  means,  causes  abrupt 
variation  in  the  resistance  of  the  line,  and  will  transmit  speech. 

Early  Conceptions.  The  conception  of  the  possibility  and  desir- 
ability of  transmitting  speech  by  electricity  may  have  occurred  to 
many,  long  prior  to  its  accomplishment.  It  is  certain  that  one  person, 
at  least,  had  a  clear  idea  of  the  general  problem.  In  1854,  Charles 
Bourseul,  a  Frenchman,  wrote:  "I  have  asked  myself,  for  example, 
if  the  spoken  word  itself  could  not  be  transmitted  by  electricity;  in  a 
word,  if  what  was  spoken  in  Vienna  might  not  be  heard  in  Paris? 
The  thing  is  practicable  in  this  way: 

"Suppose  that  a  man  speaks  near  a  movable  disk  sufficiently 
flexible  to  lose  none  of  the  vibrations  of  the  voice;  that  this  disk  al- 


Fig.  4.     Reis  Transmitter 

ternately  makes  and  breaks  the  connection  from  a  battery;  you  may 
have  at  a  distance  another  disk  which  will  simultaneously  execute 
the  same  vibrations."  The  idea  so  expressed  is  weak  in  only  one 
particular.  This  particular  is  shown  by  the  words  italicized  by  our- 
selves. It  is  impossible  to  transmit  a  complex  series  of  waves  by  any 
simple  series  of  makes  and  breaks.  Philipp  Reis,  a  German,  devised 
the  arrangement  shown  in  Fig.  4  for  the  transmission  of  sound,  let- 
ting the  make  and  break  of  the  contact  between  the  diaphragm  1 
and  the  point  2  interrupt  the  line  circuit.  His  receiver  took  several 
forms,  all  electromagnetic.  His  success  was  limited  to  the  trans- 
mission of  musical  sounds,  and  it  is  not  believed  that  articulate 
speech  ever  was  transmitted  by  such  an  arrangement. 

It  must  be  remembered  that  the  art  of  telegraphy,  particularly 
in  America,  was  well  established  long  before  the  invention  of  the 


ELECTRICAL  REPRODUCTION  OF  SPEECH 


19 


telephone,  and  that  an  arrangement  of  keys,  relays,  and  a  battery, 
as  shown  in  Fig.  5,  was  well  known  to  a  great  many  persons.  At- 
taching the  armatures  of  the  relays  of  such  a  line  to  diaphragms, 


I 

Fig.   5.     Typical  Telegraph  Line 

as  in  Fig.  6,  at  any  time  after  1838,  would  have  produced  the  tele- 
phone. "The  hardihood  of  invention"  to  conceive  such  a  change 
was  the  quality  required. 

Limitations  of  Magneto  Transmitter.  For  reasons  not  finally 
established,  the  ability  of  the  magneto  telephone  to  produce  large 
currents  from  large  sounds  is  not  equal  to  its  ability  to  produce 
large  sounds  from  large  currents.  As  a  receiving  device,  it  is  unex- 
celled, and  but  slight  improvement  has  been  made  since  its  first 
invention.  It  is  inadequate  as  a  transmitter,  and  as  early  as  1876, 
Professor  Bell  exhibited  other  means  than  electromagnetic  action  for 
producing  the  varying  currents  as  a  consequence  of  diaphragm 


Fig.  6.     Telegraph  Equipment  Converted  into  Telephone  Equipment 

motion.     Much  other  inventive  effort  was  addressed  to  this  problem, 
the  aim  of  all  being  to  send  out  more  robust  voice  currents. 

Other  Methods  of  Producing  Voice  Currents.  Some  of  these 
means  are  the  variation  of  resistance  in  the  path  of  direct  current, 
variation  in  the  pressure  of  the  source  of  that  current,  and  variation 
in  the  electrostatic  capacity  of  some  part  of  the  circuit. 


20 


TELEPHONY 


Electrostatic  Telephone.  The  latter  method  is  principally  that 
of  Dolbear  and  Edison.  Dolbear's  thought  is  illustrated  in  Fig.  7. 
Two  conducting  plates  are  brought  close  together.  One  is  free  to 
vibrate  as  a  diaphragm,  while  the  other  is  fixed.  The  element  1  in 
Fig.  7  is  merely  a  stud  to  hold  rigid  the  plate  it  bears  against.  Each 
of  two  instruments  connected  by  a  line  contains  such  a  pair  of  plates, 
and  a  battery  in  the  line  keeps  them  charged  to  its  potential.  The 
two  diaphragms  of  each  instrument  are  kept  drawn  towards  each  other 
because  their  unlike  charges  attract  each  other.  The  vibration  of  one 
of  the  diaphragms  changes  the  potential  of  the  other  pair;  the  degree 
of  attraction  thus  is  varied,  so  that  vibration  of  the  diaphragm  and 
sound  waves  result. 

Examples  of  this  method  of  telephone  transmission  are  more 
familiar  to  later  practice  in  the  form  of  condenser  receivers.  A  con- 
denser, in  usual  present  practice,  being  a  pair  of  closely  adjacent 
conductors  of  considerable  surface  insulated  from  each  other,  a  rap- 


Fig.   7.     Electrostatic  Telephone 

idly  varying  current  actually  may  move  one  or  both  of  the  conductors. 
Ordinarily  these  are  of  thin  sheet  metal  (foil)  interleaved  with  an  in- 
sulating material,  such  as  paper  or  mica.  Voice  currents  can  vibrate 
the  metal  sheets  in  a  degree  to  cause  the  condenser  to  speak.  These 
condenser  methods  of  telephony  have  not  become  commercial. 

Variation  of  Electrical  Pressure.  Variation  of  the  pressure  of 
the  source  is  a  conceivable  way  of  transmitting  speech-  To  utilize  it, 
would  require  that  the  vibrations  of  the  diaphragm  cause  the  electro- 
motive force  of  a  battery  or  machine  to  vary  in  harmony  with  the 
sound  waves.  So  far  as  we  are  informed  this  method  never  has  corne 
into  practical  use. 

Variation  of  Resistance.  Variation  of  resistance  proportional 
to  the  vibrations  of  the  diaphragm  is  the  method  which  has  produced 
the  present  prevailing  form  of  transmission.  Professor  Bell's  Centen- 
nial exhibit  contained  a  water-resistance  transmitter.  Dr.  Elisha  Gray 


ELECTRICAL  REPRODUCTION  OF  SPEECH  21 

also  devised  one.  In  both,  the  diaphragm  acted  to  increase  and  di- 
minish the  distance  between  two  conductors  immersed  in  water, 
lowering  and  raising  the  resistance  of  the  line.  It  later  was  dis- 
covered by  Edison  that  carbon  possesses  a  peculiarly  great  property 
of  varying  its  resistance  under  pressure.  Professor  David  E.  Hughes 
discovered  that  two  conducting  bodies,  preferably  of  rather  poor 
conductivity,  when  laid  together  so  as  to  form  a  loose  contact  between 
them,  possessed,  in  remarkable  degree,  the  ability  to  vary  the  resist- 
ance of  the  path  through  them  when  subject  to  such  vibrations  as 
would  alter  the  intimacy  of  contact.  He  thus  discovered  and  formu- 
lated the  principles  of  loose  contact  upon  which  the  operation  of  all 
modern  transmitters  rests.  Hughes'  device  was  named  by  him  a 
"microphone,"  indicating  a  magnification  of  sound  or  an  ability  to 
respond  to  and  make  audible  minute  sounds.  It  is  shown  in  Fig.  8. 
Firmly  attached  to  a  board  are  two  carbon  blocks,  shown  in  section 
in  the  figure.  A  rod  of 
carbon  with  cone-shaped 
ends  is  supported  loosely 
between  the  two  blocks, 
conical  depressions  in 
the  blocks  receiving  the 
ends  of  the  rod.  A  bat- 
tery and  magneto  receiver 
are  connected  in  series  Fig  8  Hughes' Microphone 

with  the  device.     Under 

certain  conditions  of  contact,  the  arrangement  is  extraordinarily  sen- 
sitive to  small  sounds  and  approaches  an  ability  indicated  by  its 
name.  Its  practical  usefulness  has  been  not  as  a  serviceable  speech 
transmitter,  but  as  a  stimulus  to  the  devising  of  transmitters  using 
carbon  in  other  ways.  Variation  of  the  resistance  of  metal  conduc- 
tors and  of  contact  between  metals  has  served  to  transmit  voice 
currents,  but  no  material  approaches  carbon  in  this  property. 

Carbon.  Adaptability.  The  application  of  carbon  to  use  in 
transmitters  has  taken  many  forms.  They  may  be  classified  as 
those  having  a  single  contact  and  those  having  a  plurality  of  con- 
tacts; in  all  cases,  the  intimacy  of  contact  is  varied  by  the  diaphragm 
excursions.  An  example  of  the  single-contact  type  is  the  Blake 
transmitter,  long  familiar  in  America.  An  example  of  the  multi- 


TELEPHONY 

pie-contact  type  is  the  loose-carbon  type  universal  now.  Other 
types  popular  at  other  times  and  in  particular  places  use  solid  rods 
or  blocks  of  carbon  having  many  points  of  contact,  though  not  in  a 
powdered  or  granular  form.  Fig.  9  shows  an  example  of  each  of 
the  general  forms  of  transmitters. 

The  use  of  granular  carbon  as  a  transmitter  material  has  ex- 
tended greatly  the  radius  of  speech,  and  has  been  a  principal  con- 
tributing cause  for  the  great  spread  of  the  telephone  industry. 


S£fi/ES    CONTACT 


MULT/PLE    CONTACT 

Fig.  9.     General  Types  of  Transmitters 

Superiority.  The  superiority  of  carbon  over  other  resistance- 
varying  materials  for  transmitters  is  well  recognized,  but  the 
reason  for  it  is  not  well  known.  Various  theories  have  been  pro- 
posed to  explain  why,  for  example,  the  resistance  of  a  mass  of  carbon 
granules  varies  with  the  vibrations  or  compressions  to  which  they  are 
subjected. 

Four  principal  theories  respectively  allege: 

First,  that  change  in  pressure  actually  changes  the  specific  resistance  of 
carbon. 

Second,  that  upon  the  surface  of  carbon  bodies  exists  some  gas  in  some 
form  of  attachment  or  combination,  variations  of  pressure  causing  variations 
of  resistance  merely  by  reducing  the  thickness  of  this  intervening  gas 

Third,  that  the  change  of  resistance  is  caused  by  variations  in  the  length 
of  electrical  arcs  between  the  particles. 

Fourth,  that  change  of  pressure  changes  the  area  of  contact,  as  is  true  of 
solids  generally- 


ELECTRICAL  REPRODUCTION  OF  SPEECH  23 

One  may  take  his  choice.  A  solid  carbon  block  or  rod  is  not 
found  to  decrease  its  resistance  by  being  subjected  to  pressure.  The 
gas  theory  lacks  experimental  proof  also.  The  existence  of  arcs 
between  the  granules  never  has  been  seen  or  otherwise  observed 
under  normal  working  conditions  of  a  transmitter;  when  arcs  surely 
are  experimentally  established  between  the  granules  the  usefulness 
of  the  transmitter  ceases.  The  final  theory,  that  change  of  pressure 
changes  area  of  surface  contact,  does  not  explain  why  other  conduc- 
tors than  carbon  are  not  good  materials  for  transmitters.  This,  it 
may  be  noticed,  is  just  what  the  theories  set  out  to  make  clear. 

There  are  many  who  feel  that  more  experimental  data  is  re- 
quired before  a  conclusive  and  satisfactory  theory  can  be  set  up. 
There  is  need  of  one,  for  a  proper  theory  often  points  the  way  for 
effective  advance  in  practice. 

Carbon  and  magneto  transmitters  differ  wholly  in  their  methods 
of  action.  The  magneto  transmitter  produces  current;  the  carbon 
transmitter  controls  current.  The  former  is  an  alternating-current 
generator;  the  latter  is  a  rheostat.  The  magneto  transmitter  pro- 
duces alternating  current  without  input  of  any  electricity  at  all;  the 
carbon  transmitter  merely  controls  a  direct  current,  supplied  by  an 
external  source,  and  varies  its  amount  without  changing  its  direction. 

The  carbon  transmitter,  however,  may  be  associated  with  other 
devices  in  a  circuit  in  such  a  way  as  to  transform  direct  currents  into 


Fig.  10.     Battery  in  Line  Circuit 

alternating  ones,  or  it  may  be  used  merely  to  change  constant  direct 
currents  into  undulating  ones,  which  never  reverse  direction,  as 
alternating  currents  always  do.  These  distinctions  are  important. 

Limitations.  A  carbon  transmitter  being  merely  a  resistance- 
varying  device,  its  usefulness  depends  on  how  much  its  resistance 
can  vary  in  response  to  motions  of  air  molecules.  A  granular-car- 
bon transmitter  may  vary  between  resistances  of  5  to  50  ohms  while 
transmitting  a  particular  tone,  having  the  lower  resistance  when  its 


24 


TELEPHONY 


diaphragm  is  driven  inward.  Conceive  this  transmitter  to  be  in  a 
line  as  shown  in  Fig.  10,  the  line,  distant  receiver,  and  battery  together 
having  a  resistance  of  1,000  ohms.  The  minimum  resistance  then  is 
1,005  ohms  and  the  maximum  1,050  ohms.  The  variation  is  limi- 
ted to  about  4.5  per  cent.  Trffe  greater  the  resistance  of  the  line  and 
other  elements  than  the  transmitter,  the  less  relative  change  the 
transmitter  can  produce,  and  the  less  loudly  the  distant  receiver  can 
speak. 


Fig.   11.     Ba,ttery  in   Local   Circuit 

Induction  Coil.  Mr.  Edison  realized  this  limitation  to  the  us*? 
of  the  carbon  transmitter  direct  in  the  line,  and  contributed  the 
means  of  removing  it.  His  method  is  to  introduce  an  induction  coil 
between  the  line  and  the  transmitter,  its  function  being  to  translate 
the  variation  of  the  direct  current  controlled  by  the  transmitter  into 
true  alternating  currents. 

An  induction  coil  is  merely  a  transformer,  and  for  the  use  under 
discussion  consists  of  two  insulated  wires  wound  around  an  iron  core. 
Change  in  the  current  carried  by  one  of  the  windings  produces  a 
current  in  the  other.  If  direct  current  be  flowing  in  one  of  the 
windings,  and  remains  constant,  no  current  whatever  is  produced  in 
the  other.  It  is  important  to  note  that  it  is  change,  and  change  only, 
which  produces  that  alternating  current. 

Fig.  11  shows  an  induction  coil  related  to  a  carbon  transmitter, 
a  battery,  and  a  receiver.  Fig.  12  shows  exactly  the  same  arrange- 
ment, using  conventional  signs.  The  winding  of  the  induction  coil 
which  is  in  series  with  the  transmitter  and  the  battery  is  called  the 
primary  winding;  the  other  is  called  the  secondary  winding.  In  the 
arrangement  of  Figs.  11  and  12  the  battery  has  no  metallic  connec- 
tion with  the  line,  so  that  it  is  called  a  local  battery.  The  circuit 
containing  the  battery,  transmitter,  and  primary  winding  of  the  in- 
duction coil  is  called  the  local  circuit 


ELECTRICAL  REPRODUCTION  OF  SPEECH  25 

Let  us  observe  what  is  the  advantage  of  this  arrangement  over 
the  case  of  Fig.  10.  Using  the  same  values  of  resistance  in  the  trans- 
mitter and  line,  assume  the  local  circuit  apart  from  the  transmitter 
to  have  a  fixed  resistance  of  5  ohms.  The  limits  of  variations  in 
the  local  circuit,  therefore,  are  10  and  55  ohms,  thus  making  the 
maximum  5.5  times  the  minimum,  or  an  increase  of  450  per  cent  as 
against  4.5  per  cent  in  the  case  of  Fig.  10.  The  changes,  therefore, 
are  100  times  as  great. 

The  relation  between  the  windings  of  the  induction  coil  in  this 
practice  are  such  that  the  secondary  winding  contains  many  more  turns 
than  the  primary  winding.  Changes  in  the  circuit  of  the  primary 
winding  produce  potentials  in  the  secondary  winding  correspond- 
ingly higher  than  the  potentials  producing  them.  These  secondary 
potentials  depend  upon  the  ratio  of  turns  in  the  two  windings  and 
therefore,  within  close  limits,  may  be  chosen  as  wished.  High  poten- 
tials in  the  secondary  winding  are  admirably  adapted  to  transmit 
currents  in  a  high-resistance  line,  for  exactly  the  same  reason  that 
long-distance  power  transmission  meets  with  but  one-quarter  of  one 
kind  of  loss  when  the  sending  potential  is  doubled,  one-hundredth 
of  that  loss  when  it  is  raised  tenfold,  and  similarly.  The  induction 


Fig.  12.     Conventional  Diagram  of  Talking  Circuit 

coil,  therefore,  serves  the  double  purpose  of  a  step-up  transformer  to 
limit  line  losses  and  a  device  for  vastly  increasing  the  range  of  change 
in  the  transmitter  circuit. 

Fig.  13  is  offered  to  remind  the  student  of  the  action  of  an  induc- 
tion coil  or  transformer  in  whose  primary  circuit  a  direct  current  is 
increased  and  decreased.  An  increase  of  current  in  the  local  wind- 
ing produces  an  impulse  of  opposite  direction  in  the  turns  of  the 
secondary  winding;  a  decrease  of  current  in  the  local  winding  pro- 
duces an  impulse  of  the  same  direction  in  the  turns  of  the  secondary 


26  TELEPHONY 

winding.  The  key  of  Fig.  13  being  closed,  current  flows  upward  in 
the  primary  winding  as  drawn  in  the  figure,  inducing  a  downward 
impulse  of  current  in  the  secondary  winding  and  its  circuit  as  noted 
at  the  right  of  the  figure.  On  the  key  being  opened,  current  ceases 
in  the  primary  circuit,  inducing  an  upward  impulse  of  current  in 
the  secondary  winding  and  circuit  as  shown.  During  other  than 
instants  of  opening  and  closing  (changing)  the  local  circuit,  no  cur- 
rent whatever  flows  in  the  secondary  circuit. 


Wrien 
c/osect 


W/?en 

I      0/76/76  tf 


Fig.   13.     Induction-Coil  Action 


It  is  by  these  means  that  telephone  transmitters  draw  direct 
current  from  primary  batteries  and  send  high-potential  alternating 
currents  over  lines;  the  same  process  produces  what  in  Therapeutics 
are  called  "Faradic  currents,"  and  enables  also  a  simple  vibrating  con- 
tact-maker to  produce  alternating  currents  for  operating  polarized 
ringers  of  telephone  sets. 

Detrimental  Effects  of  Capacity.  Electrostatic  capacity  plays 
an  important  part  in  the  transmission  of  speech.  Its  presence  be- 
tween the  wires  of  a  line  and  between  them  and  the  earth  causes 
one  of  the  losses  from  which  long-distance  telephony  suffers.  Its 
presence  in  condensers  assists  in  the  solution  of  many  circuit  and 
apparatus  problems. 

A  condenser  is  a  device  composed  of  two  or  more  conductors 
insulated  from  each  other  by  a  medium  called  the  dielectric.  A  pair 
of  metal  plates  separated  by  glass,  a  pair  of  wires  separated  by  air,  or 
a  pair  of  sheets  of  foil  separated  by  paper  or  mica  may  constitute  a 
condenser.  The  use  of  condensers  as  pieces  of  apparatus  and  the 
problems  presented  by  electrostatic  capacity  in  lines  are  discussed  in 
other  chapters. 

Measurements  of  Telephone  Currents.  It  has  been  recognized 
in  all  branches  of  engineering  that  a  definite  advance  is  possible  only 
when  quantitative  data  exists.  The  lack  of  reliable  means  of  meas- 


ELECTRICAL  REPRODUCTION  OF  SPEECH  27 

uring  telephone  currents  has  been  a  principal  cause  of  the  difficulty 
in  solving  many  of  its  problems.  It  is  only  in  very  recent  times  that 
accurate  and  reliable  means  have  been  worked  out  for  measuring  the 
small  currents  which  flow  in  telephone  lines.  These  ways  are  of 
two  general  kinds:  by  thermal  and  by  electromagnetic  means. 

Thermal  Method.  The  thermal  methods  simply  measure,  in 
some  way,  the  amount  of  heat  which  is  produced  by  a  received  tele- 
phone current.  When  this  current  is  allowed  to  pass  through  a  con- 
ductor the  effect  of  the  heat  generated  in  that  conductor  is  observed 
in  one  of  three  ways  :  by  the  expansion  of  the  conductor,  by  its  change 
in  resistance,  or  by  the  production  of  an  electromotive  force  in  a  ther- 
mo-electric couple  heated  by  the  conductor.  Any  one  of  these  three 
ways  can  be  used  to  get  some  idea  of  the  amount  of  current  which  is 
received.  None  of  them  gives  an  accurate  knowledge  of  the  forms  of 
the  waves  which  cause  the  reproduction  of  speech  in  the  telephone 
receiver. 

Electromagnetic  Method.     An 
electromagnetic  device  adapted  to 


tell  something  of   the   magnitude    '  ~' 

of  the  telephone  current  and  also    ^^^^^^  e 

something  of  its  form,  i.  e.,  some- 

thing of  its  various  increases  and 

decreases  and  also  of  its  changes 

in    direction    is    the    oscillograph. 

An  oscillograph  is  composed  of  a 

magnetic    field   formed   by  direct 

currents  or  by  a  permanent  mag- 

net,    a   turn  of    wire    under   me- 

chanical  tension  in  that  field,  and 

a  mirror  borne  by  the  turn  of  wire, 

adapted  to  reflect  a  beam  of  light 

to   a   photographic     film    or    to    a  Pig.  14.  Oscillogram  of  Telephone 

°  Currents 

rotating  mirror. 

When  a  current  is  to  be  measured  by  the  oscillograph,  it  is  passed 
through  the  turn  of  wire  in  the  magnetic  field.  While  no  current 
is  passing,  the  wire  does  not  move  in  the  magnetic  field  and  its  mirror 
reflects  a  stationary  beam  of  light.  A  photographic  film  moved  in  a 
direction  normal  to  the  axis  of  the  turn  of  wire  will  have  drawn 


28  TELEPHONY 

upon  it  a  straight  line  by  the  beam  of  light.  If  the  beam  of  light, 
however,  is  moved  by  a  current,  from  side  to  side  at  right  angles  to 
this  axis,  it  will  draw  a  wavy  line  on  the  photographic  film  and  this 
wavy  line  will  picture  the  alternations  of  that  current  and  the  oscilla- 
tions of  the  molecules  of  air  which  carried  the  originating  sound. 
Fig.  14  is  a  photograph  of  nine  different  vowel  sounds  which  have 
caused  the  oscillograph  to  take  their  pictures.  They  are  copies  of 
records  made  by  Mr.  Bela  Gati,  assisted  by  Mr.  Tolnai.  The  meas- 
uring instrument  consisted  of  an  oscillograph  of  the  type  described, 
the  transmitter  being  of  the  carbon  type  actuated  by  a  2-volt  bat- 
tery. The  primary  current  was  transformed  by  an  induction  coil  of 
the  ordinary  type  and  the  transformed  current  was  sent  through  a 
non-inductive  resistance  of  3,000  ohms.  No  condensers  were 
placed  in  the  circuit.  It  will  be  seen  that  the  integral  values  of  the 
curves,  starting  from  zero,  are  variable.  The  positive  and  the  negative 
portions  of  the  curves  are  not  equal,  so  that  the  solution  of  the  in- 
dividual harmonic  motion  is  difficult  and  laborious. 

These  photographs  point  out  several  facts  very  clearly.  One  is 
that  the  alternations  of  currents  in  the  telephone  line,  like  the  motions 
of  the  molecules  of  air  of  the  original  sound,  are  highly  complex  and 
are  not,  as  musical  tones  are,  regular  .recurrences  of  equal  vibrations. 
They  show  also  that  any  vowel  sound  may  be  considered  to  be  a 
regular  recurrence  of  certain  groups  of  vibrations  of  different  ampli- 
tudes and  of  different  frequencies. 


CHAPTER  III 
ELECTRICAL  SIGNALS 

Electric  calls  or  signals  are  of  two  kinds :  audible  and  visible. 

Audible  Signals.  Telegraph  Sounder.  The  earliest  electric 
signal  was  an  audible  one,  being  the  telegraph  sounder,  or  the  Morse 
register  considered  apart  from  its  registering  function.  Each  tele- 


Fig.  15.     Telegraph  Sounder  and  Key 

graph  sounder  serves  as  an  audible  electric  signal  and  is  capable  of 
signifying  more  than  that  the  call  is  being  made.  Such  a  signal  is 
operated  by  the  making  and  breaking  of  current  from  a  battery.  An 


/* 


Fig.  16.     Vibrating  Bell 


arrangement  of  this  kind  is  shown  in  Fig.  15,  in  which  pressure  upon 
the  key  causes  the  current  from  the  battery  to  energize  the  sounder 
and  give  one  sharp  audible  rap  of  the  lever  upon  the  striking  post. 


30  TELEPHONY 

Vibrating  Bell.  The  vibrating  belly  so  widely  used  as  a  door 
bell,  is  a  device  consequent  to  the  telegraph.  Its  action  is  to  give  a 
series  of  blows  on  its  gong  when  its  key  or  push  button  closes  the  bat- 
tery circuit.  At  the  risk  of  describing  a  trite  though  not  trivial  thing, 
it  may  be  said  that  when  the  contact  1  of  Fig.  16  is  closed,  current 
from  the  battery  energizes  the  armature  2,  causing  the  latter  to  strike 
a  blow  on  the  gong  and  to  break  the  line  circuit  as  well,  by  opening  the 
contact  back  of  the  armature.  So  de-energized,  the  armature  falls 
back  and  the  cycle  is  repeated  until  the  button  contact  is  released. 
A  comparison  of  this  action  with  that  of  the  polarized  ringer  (to  be 
described  later)  will  be  found  of  interest. 

Magneto-Bell.  The  magneto-bell  came  into  wide  use  with  the 
spread  of  telephone  service.  Its  two  fundamental  parts  are  an 
alternating-current  generator  and  a  polarized  bell-ringing1  device. 


Fig.  17.     Elemental  Magneto-Generator 

Each  had  its  counterpart  long  before  the  invention  of  the  telephone, 
though  made  familiar  by  the  latter.  The  alternating-current  gen- 
erator of  the  magneto-bell  consists  of  a  rotatable  armature  composed 
of  a  coil  of  insulated  wire  and  usually  a  core  of  soft  iron,  its  rotation 
taking  place  in  a  magnetic  field.  This  field  is  usually  provided  by 
a  permanent  magnet,  hence  the  name  "magneto-generator."  The 
purist  in  terms  may  well  say,  however,  that  every  form  whatever  of 
the  dynamo-electric  generator  is  a  magneto-generator,  as  magnetism 
is  one  link  in  every  such  conversion  of  mechanical  power  into  electric- 
ity. The  terms  magneto-electric,  magneto-generator,  etc.,  involving 
the  term  "magneto,"  have  come  to  imply  the  presence  of  permanently 
magnetized  steel  as  an  element  of  the  construction. 

In  its  early  form,  the  magneto-generator  consisted  of  the  arrange- 
ment shown  in  Fig.  17,  wherein  a  permanent  magnet  can  rotate  on  an 
axis  before  an  electromagnet  having  soft  iron  cores  and  a  winding. 
Reversals  of  magnetism  produce  current  in  alternately  reversing  half- 
cycles,  one  complete  rotation  of  the  magnet  producing  one  such  cy- 
cle. Obviouslv  the  result  would  be  the  same  if  the  magnet  were 


ELECTRICAL  SIGNALS  31 

stationary  and  the  coils  should  rotate,  which  is  the  construction  of 

more  modern  devices.     The  turning  of  the  crank  of  a  magneto-bel! 

rotates  the  armature  in  the  magnetic  field  by  some  form  of  gearing  at 

a  rate  usually  of  the  order  of  twenty  turns  per  second,  producing  an 

alternating  current  of  that  frequency.     This  current  is  caused  by  an 

effective  electromotive  force  which 

may  be  as  great  as  100  volts,  pro- 

duced immediately  by  the  energy 

of   the   user.      In  an  equipment 

using    a    magneto-telephone     as 

both  receiver  and  transmitter  and 

a  magneto-bell  as  its  signal-send-    Fjg  18  Extension  of  a  Permanent  Magnet 

ing  machine,  as  was  usual  in  1877, 

it  is  interesting  to  note  that  the  entire  motive  power  for  signals  and 

speech  transmission  was  supplied  by   the  muscular   tissues  of   the 

user  —  a  case  of  working  one's  passage. 

The  alternating  current  from  the  generator  is  received  and  con- 
verted into  sound  by  means  of  the  polarized  ringer,  a  device  which 
is  interesting  as  depending  upon  several  of  the  electrical,  mechan- 
ical, and  magnetic  actions  which  are  the  foundations  of  telephone 
engineering. 

"Why  the  ringer  rings"  may  be  gathered  from  a  study  of  Figs. 
18  to  21.  A  permanent  magnet  will  impart  temporary  magnetism 
to  pieces  of  iron  near  it.  In  Fig.  18  two  pieces  of  iron  are  so  ener- 
gized. The  ends  of  these  pieces  which  are  nearest  to  the  permanent 
magnet  1  are  of  the  opposite  po- 
larity to  the  end  they  approach, 
the  free  ends  being  of  opposite 


/y 


_    _ 

polarity.    In  the  figure,  these  free   '  -  - 
ends  are  marked  N,  meaning  they 
are  of  a  polarity  to  point  north  if 

,  ..  Fig.  19.    Extension  of  a  Permanent  Magnet 

free   to    point    at    all.     English- 

speaking  persons  call  this  north  polarity.  Similarly,  as  in  Fig.  19, 
any  arrangement  of  iron  near  a  permanent  magnet  always  will  have 
free  poles  of  the  same  polarity  as  the  end  of  the  permanent  magnet 
nearest  them. 

A  permanent  magnet  so  related  to  iron  forms  part  of  a  polar- 
ized ringer.     So  does  an  electromagnet  composed  of  windings  and 


32 


TELEPHONY 


iron  cores.  Fig.  20  reminds  us  of  the  ,law  of  electromagnets.  If 
current  flows  from  the  plus  towards  the  minus  side,  with  the  wind- 
ings as  drawn,  polarities  will  be  induced  as  marked. 

If,  now,  such  an  electromag- 
net, a  permanent  magnet,  and  a 
pivoted  armature  be  related  to  a 
pair  of  gongs  as  shown  in  Fig. 
21,  a  polarized  ringer  results.  It 
should  be  noted  that  a  permanent 
magnet  has  both  its  poles  pre- 
sented (though  one  of  the  poles 
is  not  actually  attached)  to  two 
parts  of  the  iron  of  the  electro- 
magnet.  The  result  is  that  the 
ends  of  the  armature  are  of  south 
Fig.  20.  Electromagnet  polarity  and  those  of  the  core  are 

of  north  polarity.  All  the  markings 

of  Fig.  21  relate  to  the  polarity  produced  by  the  permanent  magnet. 
If,  now,  a  current  flow  in  the  ringer  winding  from  plus  to  minus, 
obviously  the  right-hand  pole  will  be  additively  magnetized,  the  cur- 
rent tending  to  produce  north  mag- 
netism there;  also  the  left-hand  pole 
will  be  subtractively  magnetized,  the 
current  tending  to  produce  south 
magnetism  there.  If  the  current  be 
of  a  certain  strength,  relative  to  the 
certain  ringer  under  study,  mag- 
netism in  the  left  pole  will  be  neu- 
tralized and  that  in  the  right  pole 
doubled.  Hence  the  armature  will 
be  attracted  more  by  the  right  pole 
than  by  the  left  and  will  strike  the 
right-hand  gong.  A  reversal  of  cur- 
rent produces  an  opposite  action, 

r  Fig.  21.     Polarized  Rmger 

the   left-hand   gong    being    struck. 

The  current  ceasing,  the  armature  remains  where  last  thrown. 

It  is  important  to  note  that  the  strength  of  action  depends  upon 
the  strength  of  the  current  up  to  a  certain  point  only.     That  depends 


ELECTRICAL  SIGNALS  33 

upon  the  strength  of  the  permanent  magnet.  Whenever  the  current 
is  great  enough  just  to  neutralize  the  normal  magnetism  of  one  pole 
and  to  double  that  of  the  other,  no  increase  in  current  will  cause  the 
device  to  ring  any  louder.  This  makes  obvious  the  importance  of 
a  proper  permanent  magnetism  and  displays  the  fallacy  of  some  effort 
to  increase  the  output  of  various  devices  depending  upon  these 
principles.  This  discussion  of  magneto-electric  signaling  is  intro- 
duced here  because  of  a  belief  in  its  being  fundamental.  Chapter 
VIII  treats  of  such  a  signaling  in  further  detail. 

Telephone  Receiver.  The  telephone  receiver  itself  serves  a  use- 
ful purpose  as  an  audible  signal.  An  interrupted  or  alternating  cur- 
rent of  proper  frequency  and  amount  will  produce  in  it  a  musical 
tone  which  can  be  heard  throughout  a  large  room.  This  fact  enables 
a  telephone  central  office  to  signal  a  subscriber  who  has  left  his  re- 
ceiver off  the  switch  hook,  so  that  normal  conditions  may  be  restored. 

Visible  Signals.  Electromag- 
netic Signal.  Practical  visual  sig- 
nals are  of  two  general  kinds: 
electromagnetic  devices  for  mov- 
ing a  target  or  pointer,  and  in- 
candescent lamps.  The  earliest 
and  most  widely  used  visible 
signal  in  telephone  practice  was  Fte-  22-  Gravity-Drop 

the  annunciator,  having  a  shutter 

adapted  to  fall  when  the  magnet  is  energized.  Fig.  22  is  such  a 
signal.  Shutter  1  is  held  by  the  catch  2  from  dropping  to  the  right 
by  its  own  gravity.  The  name  "gravity-drop"  is  thus  obvious. 
Current  energizing  the  core  attracts  the  armature  3,  lifts  the  catch 
2,  and  the  shutter  falls.  A  simple  modification  of  the  gravity-drop 
produces  the  visible  signal  shown  in  Fig.  23.  Energizing  the  core 
lifts  a  target  so  as  to  render  it  visible  through  an  opening  in  the 
plate  1.  A  contrast  of  color  between  the  plate  and  the  target  height- 
ens the  effect. 

The  gravity-drop  is  principally  adapted  to  the  magneto-bell 
system  of  signaling,  where  an  alternating  current  is  sent  over  the 
line  to  a  central  office  by  the  operation  of  a  bell  crank  at  the  subscrib- 
er's station,  this  current,  lasting  only  as  long  as  the  crank  is  turned, 
energizes  the  drop,  which  may  be  restored  by  hand  or  otherwise  and 


34 


TELEPHONY 


Pig.  23.     Electromagnetic  Visible  Signal 


will  remain  latched.  The  visible  signal  is  better  adapted  to  lines  in 
which  the  signaling  is  done  by  means  of  direct  current,  as,  for  ex- 
ample, in  systems  where  the  removal  of  the  receiver  from  the  hook 
at  the  subscriber's  station  closes  the  line  circuit,  causing  current  to  flow 

through  the  winding  of  the  visible 
signal  and  so  displaying  it  until  the 
receiver  has  been  hung  upon  the 
hook  or  the  circuit  opened  by  some 
operation  at  the  central  office.  Visi- 
ble signals  of  the  magnetic  type  of 
Fig.  23  have  been  widely  used  in 
connection  with  common-battery 
systems,  both  for  line  signals  and 
for  supervisory  purposes,  indicat- 
ing the  state  and  the  progress  of 
the  connection  and  conversation. 

Electric-Lamp  Signal.     Incandescent  electric  lamps  appeared 
in  telephony  as  a  considerable  element  about  1890.     They  are  better 
than  either  form  of  mechanical  visible  signals  because  of  three  prin- 
cipal qualities:  simplicity  and  ease  of  restoring  them  to  normal  as 
compared  with  drops;  their  compactness;  and  their  greater  promi- 
nence when  displayed.     Of  the  latter  quality,  one 
may  say  that  they  are  more  insistent,  as  they  give 
out  light  instead  of  reflecting  it,  as  do  all  other 
visible  signals.     In  its  best  form,  the  lamp  signal  is 
mounted  behind  a  hemispherical  lens,  either  slightly 
clouded  or  cut  in  facets.     This  lens  serves  to  dis- 
tribute the  rays  of  light  from  the  lamp,  with  the 
result  that  the  signal  may  be  seen  from  a  wide 
angle  with  the  axis  of  the  lens,  as  shown  in  Fig.  24. 
This  is  of  particular  advantage  in  connection  with 
manual-switchboard  connecting  cords,  as  it  enables 
the  signals  to  be  mounted  close  to  and  even  among 
the   cords,    their   great  visible  prominence   when 
shining  saving  them  from  being  hidden. 

The  influence  of  the  lamp  signal  was  one  of  the  potent  ones  in 
the  development  of  the  type  of  multiple  switchboard  which  is  now 
universal  as  the  mechanism  of  large  manual  exchanges.  The 


Fig.  24.  Lamp  Signal 
and  Lens 


ELECTRICAL  SIGNALS  35 

first  large  trial  of  such  an  equipment  was  in  1896  in  Worcester, 
Mass.  No  large  and  successful  multiple  switchboard  with  any  other 
type  of  signal  has  been  built  since  that  time. 

Any  electric  signal  has  upper  and  lower  limits  of  current  between 
which  it  is  to  be  actuated.  It  must  receive  current  enough  to  operate* 
but  not  enough  to  become  damaged  by  overheating.  The  magnetic 
types  of  visible  signals  have  a  wider  range  between  these  limits  than 
have  lamp  signals.  If  current  in  a  lamp  is  too  little,  its  filament 
either  will  not  glow  at  all  or  merely  at  a  dull  red,  insufficient  for  a 
proper  signal.  If  the  current  is  too  great,  the  filament  is  heated  be- 
yond its  strength  and  parts  at  the  weakest  place. 

This  range  between  current  limits  in  magnetic  visible  signals 
is  great  enough  to  enable  them  to  be  used  direct  in  telephone  lines, 
the  operating  current  through  the  line  and  signal  in  series  with  a 
fixed  voltage  at  the  central  office  being  not  harmfully  great  when  the 
entire  line  resistance  is  shunted  out  at  or  near  the  central  office.  The 
increase  of  current  may  be  as  great  as  ten  times  without  damage  to  the 
winding  of  such  a  signal.  In  lamps,  the  safe  margin  is  much  less. 
The  current  which  just  gives  a  sufficient  lighting  of  the  signal  may 
be  about  doubled  with  safety  to  the  filament  of  the  lamp.  Con- 
sequently it  is  not  feasible  to  place  the  lamp  directly  in  series  with 
long  exposed  lines.  A  short  circuit  of  such  a  line  near  the  central 
office  will  burn  it  out. 

The  qualities  of  electromag- 
nets and  lamps  in  these  respects 
are  used  to  advantage  by  the  lamp 
signal  arrangement  shown  in  Fig. 
25.  A  relay  is  in  series  with  the  Fig  2g  Lamp  Signal  Controlled  by  Relay 
line  and  provides  a  large  range  of 

sensibility.  It  is  able  to  carry  any  current  the  central -office  current 
source  can  pass  through  it.  The  local  circuit  of  the  relay  includes 
the  lamp.  Energizing  the  relay  lights  the  lamp,  and  the  reverse; 
the  lamp  is  thus  isolated  from  danger  and  receives  the  current  best 
adapted  to  its  needs. 

All  lines  are  not  long  and  when  enclosed  in  cable  or  in  well- 
insulated  interior  wire,  may  be  only  remotely  in  danger  of  being 
short-circuited.  Such  conditions  exist  in  private-branch  exchanges, 
which  are  groups  of  telephones,  usually  local  to  limited  premises,  con- 


36 


TELEPHONY 


Fig.   26.     Lamp  Signal  Directly  in  Line 


nected  to  a  switchboard  on  those  premises.  Such  a  situation  per- 
mits the  omission  of  the  line  relay,  the  lamp  being  directly  in  the  line. 
Fig.  26  shows  the  extreme  simplicity  of  the  arrangement,  containing 
no  moving  parts  or  costly  elements.  Lamps  for  such  service  have 

improved  greatly  since  the  demand 
began  to  grow.  The  small  bulk 
permitted  by  the  need  of  com- 
pactness, the  high  filament  resist- 
ance required  for  simplicity  of  the 
general  power  scheme  of  the  sys- 
tem, and  the  need  of  considerable 

sturdiness  in  the  completed  thing  have  made  the  task  a  hard  one. 
The  practical  result,  however,  is  a  signal  lamp  which  is  highly 
satisfactory. 

The  nature  of  carbon  and  certain  earths  being  that  their  conduc- 
tivity rises  with  the  temperature  and  that  of  metals  being  that  their 
conductivity  falls  with  the  temperature,  has  enabled  the  Nernst  lamp 
to  be  successful.  The  same  relation  of  properties  has  enabled  in- 
candescent-lamp signals  to  be  connected  direct  to  lines  without  re- 
lays, but  compensated  against  too  great  a  current  by  causing  the 
resistance  in  series  with  the  lamp  to  be  increased  inversely  as  the 

resistance  of  the  filament.  Em- 
ployment of  a  "ballast"  resistance 
in  this  way  is  referred  to  in  Chap- 
ter XL  In  Fig.  27  is  shown  its 
relation  to  a  signal  lamp  directly 
in  the  line.  1  is  the  carbon-fila- 
ment lamp;  2  is  the  ballast.  The 
latter's  conductor  is  fine  iron  wire  in  a  vacuum.  The  resistance  of 
the  lamp  falls  as  that  of  the  ballast  rises.  Within  certain  limits,  these 
changes  balance  each  other,  widening  the  range  of  allowable  change 
in  the  total  resistance  of  the  line. 


Fig.  27.     Lamp  Signal  and  Ballast 


CHAPTER  IV 
TELEPHONE  LINES 

The  line  is  a  path  over  which  the  telephone  current  passes  from 
telephone  to  telephone.  The  term  "telephone  line  circuit"  is  equivalent. 
"Line"  and  "line  circuit"  mean  slightly  different  things  to  some  per- 
sons, "line"  meaning  the  out-of-doors  portion  of  the  line  and  "line 
circuit"  meaning  the  indoor  portion,  composed  of  apparatus  and 
associated  wiring.  Such  shades  of  meaning  are  inevitable  and  serve 
useful  purposes.  The  opening  definition  hereof  is  accurate. 

A  telephone  line  consists  of  two  conductors.  One  of  these  con- 
ductors may  be  the  earth;  the  other  always  is  some  conducting  ma- 
terial other  than  the  earth — almost  universally  it  is  of  metal  and  in 
the  form  of  a  wire.  A  line  using  one  wire  and  the  earth  as  its  pair  of 
conductors  has  several  defects,  to  be  discussed  later  herein.  Both 
conductors  of  a  line  may  be  wires,  the  earth  serving  as  no  part  of  the 
circuit,  and  this  is  the  best  practice.  A  line  composed  of  one  wire 
and  the  earth  is  called  a  grounded  line;  a  line  composed  of  two  wires 
not  needing  the  earth  as  a  conductor  is  called  a  metallic  circuit. 

In  the  earliest  telephone  practice,  all  lines  were  grounded  ones. 
The  wires  were  of  iron,  supported  by  poles  and  insulated  from  them 
by  glass,  earthenware,  or  rubber  insulators.  For  certain  uses,  such 
lines  still  represent  good  practice.  For  telegraph  service,  they  rep- 
resent the  present  standard  practice. 

Copper  is  a  better  conductor  than  iron,  does  not  rust,  and  when 
drawn  into  wire  in  such  a  way  as  to  have  a  sufficient  tensile  strength 
to  support  itself  is  the  best  available  conductor  for  telephone  lines. 
Only  one  metal  surpasses  it  in  any  quality  for  the  purpose :  silver  is 
a  better  conductor  by  1  or  2  per  cent.  Copper  is  better  than  silver 
in  strength  and  price. 

In  the  open  country,  telephone  lines  consist  of  bare  wires  of 
copper,  of  ire ii,  of  steel,  or  of  copper-covered  steel  supported  on 


38  TELEPHONY 

insulators  borne  by  poles.  If  the  wires  on  the  poles  be  many,  cross- 
arms  carry  four  to  ten  wires  each  and  the  insulators  are  mounted  on 
pins  in  the  cross-arms.  If  the  wires  on  the  poles  be  few,  the  insu- 
lators are  mounted  on  brackets  nailed  to  the  poles.  Wires  so  carried 
are  called  open  wires. 

In  towns  and  cities  where  many  wires  are  to  be  carried  along  the 
same  route,  the  wires  are  reduced  in  size,  insulated  by  a  covering 
over  each,  and  assembled  into  a  group.  Such  a  bundle  of  insulated 
wires  is  called  a  cable.  It  may  be  drawn  into  a  duct  in  the  earth  and 
be  called  an  underground  cable;  it  may  be  laid  on  the  bottom  of  the 
sea  or  other  water  and  be  called  a  submarine  cable;  or  it  may  be  sus- 
pended on  poles  and  be  called  an  aerial  cable.  In  the  most  general 
practice  each  wire  is  insulated  from  all  others  by  a  wrapping  of  paper 
ribbon,  which  covering  is  only  adequate  when  very  dry.  Cables 
formed  of  paper-insulated  wires,  therefore,  are  covered  by  a  seamless, 
continuous  lead  sheath,  no  part  of  the  paper  insulation  of  the  wires 
being  exposed  to  the  atmosphere  during  the  cable's  entire  life  in  serv- 
ice. Telephone  cables  for  certain  uses  are  formed  of  wires  insulated 
with  such  materials  as  soft  rubber,  gutta-percha,  and  cotton  or  jute 
saturated  with  mineral  compounds.  When  insulated  with  rubber 
or  gutta-percha,  no  continuous  lead  sheath  is  essential  for  insulation, 
as  those  materials,  if  continuous  upon  the  wire,  insulate  even  when 
the  cable  is  immersed  in  water.  Sheaths  and  other  armors  can  assist 
in  protecting  these  insulating  materials  from  mechanical  injury,  and 
often  are  used  for  that  purpose.  The  uses  to  which  such  cables  are 
suitable  in  telephony  are  not  many,  as  will  be  shown. 

A  wire  supported  on  poles  requires  that  it  be  large  enough  to 
support  its  own  weight.  The  smaller  the  wire,  the  weaker  it  is,  and 
with  poles  a  given  distance  apart,  the  strength  of  the  wire  must  be 
above  a  certain  minimum. '  In  regions  where  freezing  occurs,  wires 
in  the  open  air  can  collect  .ice  in  winter  and  everywhere  open  wires 
stre  subject  to  wind  pressure;  for  these  reasons  additional  strength  is 
required.  Speaking  generally,  the  practical  and  economical  spacing  of 
poles  requires  that  wires,  to  be  strong  enough  to  meet  the  above  con- 
ditions, shall  have  a  diameter  not  less  than  .08  inch,  if  of  hard-drawn 
copper,  and  .064  inch,  if  of  iron  or  steel.  The  honor  of  developing 
ways  of  drawing  copper  wire  with  sufficient  tensile  strength  for  open- 
air  uses  belongs  to  Mr.  Thomas  B.  Doolittle  of  Massachusetts. 


TELEPHONE  LINES  39 

Lines  whose  lengths  are  limited  to  a  few  miles  do  not  require  a 
conductivity  as  great  as  that  of  copper  wire  of  .08-inch  diameter.  A 
wire  of  that  size  weighs  approximately  100  pounds  per  mile.  Less 
than  100  pounds  of  copper  per  mile  of  wire  will  not  give  strength 
enough  for  use  on  poles;  but  as  little  as  10  pounds  per  mile  of  wire 
gives  the  necessary  conductivity  for  the  lines  of  the  thousands  of 
telephone  stations  in  towns  and  cities. 

Open  wires,  being  exposed  to  the  elements,  suffer  damage  from 
storms;  their  insulation  is  injured  by  contact  with  trees;  they  may 
make  contact  with  electric  power  circuits,  perhaps  injuring  appara- 
tus, themselves,  and  persons;  they  endanger  life  and  property  by  the 
possibility  of  falling;  they  and  their  cross-arm  supports  are  less  sightly 
than  a  more  compact  arrangement. 

Grouping  small  wires  of  telephone  lines  into  cables  has,  there- 
fore, the  advantage  of  allowing  less  copper  to  be  used,  of  reducing 
the  space  required,  of  improving  appearance,  and  of  increasing 
safety.  On  the  other  hand,  this  same  grouping  introduces  negative 
advantages  as  well  as  the  foregoing  positive  ones.  It  is  not  possible 
to  talk  as  far  or  as  well  over  a  line  in  an  ordinary  cable  as  over 
a  line  of  two  open  wires.  Long-distance  telephone  circuits,  there- 
fore, have  not  yet  been  placed  in  cables  for  lengths  greater  than  200 
or  300  miles,  and  special  treatment  of  cable  circuits  is  required  to 
talk  through  them  for  even  100  miles.  One  may  talk  2,000  miles 
over  open  wires.  The  reasons  for  the  superiority  of  the  open  wires 
have  to  do  with  position  rather  than  material.  Obviously  it  is  possi- 
ble to  insulate  and  bury  any  wire  which  can  be  carried  in  the  air. 
The  differences  in  the  properties  of  lines  whose  wires  are  differently 
situated  with  reference  to  each  other  and  surrounding  things  are 
interesting  and  important. 

A  telephone  line  composed  of  two  conductors  always  possesses 
four  principal  properties  in  some  amount:  (1)  conductivity  of  the 
conductors;  (2)  electrostatic  capacity  between  the  conductors;  (3) 
inductance  of  the  circuit;  (4)  insulation  of  each  conductor  from 
other  things. 

Conductivity  of  Conductors.  The  conductivity  of  a  wire  de- 
pends upon  its  material,  its  cross-section,  its  length,  and  its  temper- 
ature. Conductivity  of  a  copper  wire,  for  example,  increases  in  direct 
ratio  to  its  weight,  in  inverse  ratio  to  its  length,  and  its  conductiv- 


40  TELEPHONY 

ity  falls  as  the  temperature  rises.  Resistance  is  the  reciprocal  of 
conductivity  and  the  properties,  conductivity  and  resistance,  are  more 
often  expressed  in  terms  of  resistance.  The  unit  of  the  latter  is  the 
ohm;  of  the  former  the  mho.  A  conductor  having  a  resistance  of 
100  ohms  has  a  conductivity  of  .01  mho.  The  exact  correlative 
terms  are  resistance  and  conductance,  resistivity  and  conductivity. 
The  use  of  the  terms  as  in  the  foregoing  is  in  accordance  with  col- 
loquial practice. 

Current  in  a  circuit  having  resistance  only,  varies  inversely  as  the 
resistance.  Electromotive  force  being  a  cause,  and  resistance  a  state, 
current  is  the  result.  The  formula  of  this  relation,  Ohm's  law,  is 


C  being  the  current  which  results  from  E,  the  electromotive  force, 
acting  upon  R,  the  resistance.  The  units  are:  of  current,  the  ampere; 
of  electromotive  force,  the  volt;  of  resistance,  the  ohm. 

As  the  conductivity  or  resistance  of  a  line  is  the  property  of  con- 
trolling importance  in  telegraphy,  a  similar  relation  was  expected 
in  early  telephony.  As  the  current  in  the  telephone  line  varies  rap- 
idly, certain  other  properties  of  the  line  assume  an  importance  they 
do  not  have  in  telegraphy  in  any  such  degree. 

The  importance  that  these  properties  assume  is,  that  if  they  did 
not  act  and  the  resistance  of  the  conductors  alone  limited  speech, 
transmission  would  be  possible  direct  from  Europe  to  America  over  a 
pair  of  wires  weighing  200  pounds  per  mile  of  wire,  which  is  less  than 
half  the  weight  of  the  wire  of  the  best  long-distance  land  lines  now 
in  service.  The  distance  from  Europe  to  America  is  about  twice 
as  great  as  the  present  commercial  radius  by  land  lines  of  435-pound 
wire.  In  other  words,  good  speech  is  possible  through  a  mere  re- 
sistance twenty  times  greater  than  the  resistance  of  the  longest  actual 
open-wire  line  it  is  possible  to  talk  through.  The  talking  ratio  be- 
tween a  mere  resistance  and  the  resistance  of  a  regular  telephone 
cable  is  still  greater. 

Electrostatic  Capacity.  It  is  the  possession  of  electrostatic  ca- 
pacity which  enables  the  condenser,  of  which  the  Leyden  jar  is  a  good 
example,  to  be  useful  in  a  telephone  line.  The  simplest  form  of  a 
condenser  is  illustrated  in  Fig.  28,  in  which  two  conducting  surfaces 


TELEPHONE  LINES 


41 


are  separated  by  an  insulating  material.  The  larger  the  surfaces, 
the  closer  they  are  together;  and  the  higher  the  specific  inductive 
capacity  of  the  insulator,  the  greater  the  capacity  of  the  device. 
An  insulator  used  in  this  relation  to  two  conducting  surfaces  is 
called  the  dielectric. 

Two  conventional  signs  are  used  to  illustrate  condensers,  the 
upper  one  of  Fig.  29  growing  out  of  the  original  condenser  of  two 


Fig.  28.  Simple  Condenser 


Fig.  29.  Condenser  Symbols 


metal  plates,  the  lower  one  suggesting  the  thought  of  interleaved  con- 
ductors of  tin  foil,  as  for  many  years  was  the  practice  in  condenser 
construction. 

\\ith  relation  to  this  property,  a  telephone  line  is  just  as  truly  a 
condenser  as  is  any  other  arrangement  of  conductors  and  insulators. 
Assume  such  a  line  to  be  open  at  the  distant  end  and  its  wires  to  be 
well  insulated  from  each  other  and  the  earth.  Telegraphy  through 
such  a  line  by  ordinary  means  would  be  impossible.  All  that  the 
battery  or  other  source  could  do  would  be  to  cause  current  to  flow 
into  the  line  for  an  infinitesimal  time,  raising  the  wires  to  its  poten- 
tial, after  which  no  current  would  flow.  But,  by  virtue  of  electro- 
static capacity,  the  condition  is  much  as  shown  in  Fig.  30.  The 


H 

ii  n 

1    IL 

1 

ii 

li  11 

Ji  n 

ii 

Fig.  30.     Line  with  Shunt  Capacity 

condensers  which  that  figure  shows  bridged  across  the  line  from  wire 
to  wire  are  intended  merely  to  fix  in  the  mind  that  there  is  a  path  for 
the  transfer  of  electrical  energy  from  wire  to  wire. 

A  simple  test  will  enable  two  of  the  results  of  a  short-circuiting 
capacity  to  be  appreciated.  Conceive  a  very  short  line  of  two  wires 
to  connect  two  local  battery  telephones.  Such  a  line  possesses  negli- 


42 


TELEPHONY 


gible  resistance,  inductance,  and  shunt  capacity.  Its  insulation  is 
practically  infinite.  Let  condensers  be  bridged  across  the  line,  one 
by  one,  while  conversation  goes  on.  The  listening  observer  will  no- 
tice that  the  sounds  reaching  his  ear  steadily  grow  less  loud  as  the 
capacity  across  the  line  increases.  The  speaking  observer  will  notice 
that  the  sounds  he  hears  through  the  receiver  in  series  with  the  line 
steadily  grow  louder  as  the  capacity  across  the  line  increases.  Fig. 
31  illustrates  the  test. 

The  speaker's  observation  in  this  test  shows  that  increasing  the 
capacity  across  the  line  increased  the  amount  of  current  entering  it. 
The  hearer's  observation  in  this  test  shows  that  increasing  the  capac- 
ity across  the  line  decreased  the  amount  of  energy  turned  into  sound 
at  his  receiver. 

The  unit  of  electrostatic  capacity  is  the  farad.  As  this  unit  is 
inconveniently  large,  for  practical  applications  the  unit  mwrofdrad — 
a  millionth  of  a  farad — is  employed.  If  quantities  are  known  in 


n   n  n  n  n 


Fig.  31.     Test  of  Line  with  Varying  Shunt  Capacity 

microfarads  and  are  to  be  used  in  calculations  in  which  the  values  of 
the  capacity  require  to  be  farads,  care  should  be  taken  to  introduce 
the  proper  corrective  factor. 

The  electrostatic  capacity  between  the  conductors  of  a  telephone 
line  depends  upon  their  surface  area,  their  length,  their  position,  and 
the  nature  of  the  materials  separating  them  from  each  other  and  from 
other  things.  For  instance,  in  an  open  wire  line  of  two  wires,  the 
electrostatic  capacity  depends  upon  the  diameter  of  the  wires,  upon 
the  length  of  the  line,  upon  their  distance  apart,  upon  their  distance 
above  the  earth,  and  upon  the  specific  inductive  capacity  of  the 
air.  Air  being  so  common  an  insulating  medium,  it  is  taken  as  a 
convenient  material  whose  specific  inductive  capacity  may  be  used 
as  a  basis  of  reference.  Therefore,  the  specific  inductive  capacity  of 
air  is  taken  as  unity.  All  solid  matter  has  higher  specific  inductive 
capacity  than  air. 


TELEPHONE  LINES  43 

The  electrostatic  capacity  of  two  open  wires  .165  inch  diameter, 
1  ft.  apart,  and  30  ft.  above  the  earth,  is  of  the  order  of  .009  micro- 
farads per  mile.  This  quantity  would  be  higher  if  the  wires  were 
closer  together;  or  nearer  the  earth;  or  if  they  were  surrounded  by  a 
gas  other  than  the  air  or  hydrogen;  or  if  the  wires  were  insulated  not 
by  a  gas  but  by  any  solid  covering.  As  another  example,  a  line  com- 
posed of  two  wires  of  a  diameter  of  .036  inch,  if  wrapped  with  paper 
and  twisted  into  a  pair  as  a  part  of  a  telephone-cable,  has  a  mutual 
electrostatic  capacity  of  approximately  .08  microfarads  per  mile,  this 
quantity  being  greater  if  the  cable  be  more  tightly  compressed. 

The  use  of  paper  as  an  insulator  for  wires  in  telephone  cables  is 
due  to  its  low  specific  inductive  capacity.  This  is  because  the  insu- 
lation of  the  wires  is  so  largely  dry  air.  Rubber  and  similar  insu- 
lating materials  give  capacities  as  great  as  twice  that  of  dry  paper. 

The  condenser  or  other  capacity  acts  as  an  effective  barrier  to 
the  steady  flow  of  direct  currents.  Applying  a  fixed  potential  causes 
a  mere  rush  of  current  to  charge  its  surface  to  a  definite  degree,  de- 
pendent upon  the  particular  conditions.  The  condenser  does  not 
act  as  such  a  barrier  to  alternating  currents,  for  it  is  possible  to  talk 
through  a  condenser  by  means  of  the  alternating  voice  currents  of 
telephony,  or  to  pass  through  it  alternating  currents  of  much  lower 
frequency.  A  condenser  is  used  in  series  with  a  polarized  ringer 
for  the  purpose  of  letting  through  alternating  current  for  ringing  the 
bell,  and  of  preventing  the  flow  of  direct  current. 

The  degree  to  which  the  condenser  allows  alternating  currents 
to  pass  while  stopping  direct  currents,  depends  on  the  capacity  of  the 
condenser  and  on  the  frequencies  of  alternating  current.  The  larger 
the  condenser  capacity  or  the  higher  the  frequency  of  the  alterna- 
tions, the  greater  will  be  the  current  passing  through  the  circuit. 
The  degree  to  which  the  current  is  opposed  by  the  capacity  is  the 
reactance  of  that  capacity  for  that  frequency.  The  formula  is 

Capacity  reactance  =  -~ — 

wherein  C  is  the  capacity  in  farads  and  o>  is  2rra,  or  twice  3.1416 
times  the  frequency. 

All  the  foregoing  leads  to  the  generalization  that  the  higher  the 
frequency,  the  less  the  opposition  of  a  capacity  to  an  alternating 


44  TELEPHONY 

current.  If  the  frequency  be  zero,  the  reactance  is  infinite,  i.  e.,  the 
circuit  is  open  to  direct  current.  If  the  frequency  be  infinite,  the 
reactance  is  zero,  i.  e.,  the  circuit  is  as  if  the  condenser  were  replaced 
by  a  solid  conductor  of  no  resistance.  Compare  this  statement  with 
the  correlative  generalization  which  follows  the  next  thought  upon 
inductance. 

Inductance  of  the  Circuit.  Inductance  is  the  property  of  a  cir- 
cuit by  which  change  of  current  in  it  tends  to  produce  in  itself  and 
other  conductors  an  electromotive  force  other  than  that  which  causes 

iQDQOXXXXXXXL 

Fig.  32.     Spiral  of  Wire  Fig.   33.     Spiral  of  Wire 

Around  Iron  Core 

the  current.  Its  unit  is  the  henry.  The  inductance  of  a  circuit  is 
one  henry  when  a  change  of  one  ampere  per  second  produces  an 
electromotive  force  of  one  volt.  Induction  between  circuits  occurs 
because  the  circuits  possess  inductance;  it  is  called  mutual  induction. 
Induction  within  a  circuit  occurs  because  the  circuit  possesses  in- 
ductance; it  is  called  self-induction.  Lenz'  law  says:  In  all  cases  of 
electromagnetic  induction,  the  induced  currents  have  such  a  direction 
that  their  reaction  tends  to  stop  the  motion  which  produced  them. 

All  conductors  possess  inductance,  but  straight  wires  used  in 
lines  have  negligible  inductance  in  most  actual  cases.  All  wires 
which  are  wound  into  coils,  such  as  electromagnets,  possess  induct- 
ance in  a  greatly  increased  degree.  A  wire  wound  into  a  spiral,  as 
indicated  in  Fig.  32,  possesses  much  greater  inductance  than  when 
drawn  out  straight.  If  iron  be  inserted  into  the  spiral,  as  shown  in 
Fig.  33,  the  inductance  is  still  further  increased.  It  is  for  the  purpose 
of  eliminating  inductance  that  resistance  coils  are  wound  with  double 
wires,  so  that  current  passing  through  such  coils  turns  in  one  direction 
half  the  way  and  in  the  other  direction  the  other  half. 

A  simple  test  will  enable  the  results  of  a  series  inductance  in  a 
line  to  be  appreciated.  Conceive  a  very  short  line  of  two  wires  to 
connect  two  local  battery  telephones.  Such  a  line  possesses  negligible 
resistance,  inductance,  and  shunt  capacity.  Its  insulation  is  practi- 
cally infinite.  Let  inductive  coils  such  as  electromagnets  be  inserted 
serially  in  the  wires  of  the  line  one  by  one,  while  conversation  goes  on. 


TELEPHONE  LINES  45 

The  /istening  observer  will  notice  that  the  sounds  reaching  his  ear 
steadily  grow  faint  as  the  inductance  in  the  line  increases  and  the 
speaking  observer  will  notice  the  same  thing  through  the  receiver  in 
series  with  the  line. 

Both  observations  in  this  test  show  that  the  amount  of  current 
entering  and  emerging  from  the  line  decreased  as  the  inductance  in- 
creased. Compare  this  with  the  test  with  bridged  capacity  and  the 
loading  of  lines  described  later  herein,  observing  the  curious  benefi- 
cial result  when  both  hurtful  properties  are  present  in  a  line.  The 
test  is  illustrated  in  Fig.  34. 

The  degree  in  which  any  current  is  opposed  by  inductance  is 
termed  the  reactance  of  that  inductance.  Its  formula  is 

Inductive  reactance  =  LQJ 

wherein  L  is  the  inductance  in  henrys  and  <*>  is  27m,  or  twice  3.1416 
times  the  frequency.  To  distinguish  the  two  kinds  of  reactance,  that 
due  to  the  capacity  is  called  capacity  reactance  and  that  due  to  induct- 
ance is  called  inductive  reactance. 

All  the  foregoing  leads  to  the  generalization  that  the  higher  the 
frequency,  the  greater  the  opposition  of  an  inductance  to  an  alter- 
nating current.  If  the  frequency  be  zero,  the  reactance  is  zero,  i.  e., 
the  circuit  conducts  direct  current  as  mere  resistance.  If  the  fre- 


Fig.  34.     Test  of  Line  with  Varying  Serial  Inductance 

quency  be  infinite,  the  reactance  is  infinite,  i.  e.,  the  circuit  is  "open" 
to  the  alternating  current  and  that  current  cannot  pass  through  it. 
Compare  this  with  the  correlative  generalization  following  the  pre- 
ceding thought  upon  capacity. 

Capacity  and  inductance  depend  only  on  states  of  matter.  Their 
reactances  depend  on  states  of  matter  and  actions  of  energy. 

In  circuits  having  both  resistance  and  capacity  or  resistance  and 
inductance,  both  properties  affect  the  passage  of  current.  The  joint 


46  TELEPHONY 

reaction  is  expressed  in  ohms  and  is  called  impedance.  Its  value 
is  the  square  root  of  the  sum  of  the  squares  of  the  resistance  and 
reactance,  or,  Z  being  impedance, 


Z  =\  R2  + 


and 

Z  = 

the  symbols  meaning  as  before. 

In  words,  these  formulas  mean  that,  knowing  the  frequency  of 
the  current  and  the  capacity  of  a  condenser,  or  the  frequency  of  the 
current  and  the  inductance  of  a  circuit  (a  line  or  piece  of  apparatus), 
and  in  either  case  the  resistance  of  the  circuit,  one  may  learn  the 
impedance  by  calculation. 

Insulation  of  Conductors.  The  fourth  property  of  telephone 
lines,  insulation  of  the  conductors,  usually  is  expressed  in  ohms  as  an 
insulation  resistance.  In  practice,  this  property  needs  to  be  intrin- 
sically high,  and  usually  is  measured  by  millions  of  ohms  resistance 
from  the  wire  of  a  line  to  its  mate  or  to  the  earth.  It  is  a  convenience 
to  employ  a  large  unit.  A  million  ohms,  therefore,  is  called  a  megohm. 
In  telephone  cables,  an  insulation  resistance  of  500  megohms  per 
mile  at  60°  Fahrenheit  is  the  usual  specification.  So  high  an  insula- 
tion resistance  in  a  paper-insulated  conductor  is  only  attained  by 
applying  the  lead  sheath  to  the  cable  when  its  core  is  made  prac- 
tically anhydrous  and  kept  so  during  the  splicing  and  terminating 
of  the  cable. 

Insulation  resistance  varies  inversely  as  the  length  of  the 
conductor.  If  a  piece  of  cable  528  feet  long  has  an  insulation 
resistance  of  6,750  megohms,  a  mile  (ten  times  as  much)  of  such 
cable,  will  have  an  insulation  resistance  of  675  megohms,  or  one- 
tenth  as  great. 

Inductance  vs.  Capacity.  The  mutual  capacity  of  a  telephone 
line  is  greater  as  its  wires  are  closer  together.  The  self-induction  of  a 
telephone  line  is  smaller  as  its  wires  are  closer  together.  The  elec- 
tromotive force  induced  by  the  capacity  of  a  line  leads  the  impressed 
electromotive  force  by  90  degrees.  The  inductive  electromotive  force 
tags  90  degrees  behind  the  impressed  electromotive  force.  And  so,  in 


TELEPHONE  LINES  47 

general,  the  natures  of  these  two  properties  are  opposite.  In  a  cable, 
the  wires  are  so  close  together  that  their  induction  is  negligible, 
while  their  capacity  is  so  great  as  to  limit  commercial  transmission 
through  a  cable  having  .06  microfarads  per  mile  capacity  and  94 
ohms  loop  resistance  per  mile,  to  a  distance  of  about  30  miles.  In 
the  case  of  open  wires  spaced  12  inches  apart,  the  limit  of  commercial 
transmission  is  greater,  not  only  because  the  wires  are  larger,  but 
because  the  capacity  is  lower  and  the  inductance  higher. 

Table  I  shows  the  practical  limiting  conversation  distance  over 
uniform  lines  with  present  standard  telephone  apparatus. 

TABLE  I 
Limiting  Transmission  Distances 


SIZE  AND  GAUGE  OF  WIHE 

LIMITING   DISTANCE 

No.     8  B.  W.  G.  copper 

900  miles 

10  B.  W.  G.  copper 

700  miles 

10  B.    &     S.  copper 

400  miles 

12  N.   B.    S.  copper 

400  miles 

12  B.    &     S.  copper 

240  miles 

14  N.   B.    S.  copper 

240  miles 

8  B.  W.  G.  iron 

135  miles 

10  B.  W.  G.  iron 

120  miles 

12  B.  W.  G.  iron 

90  miles 

16  B.  &  S.  cable,  copper 

40  miles 

19  B.  &  S.  cable,  copper 

30  miles 

22  B.   &  S.  cable,  copper 

20  miles 

In  1893,  Oliver  Heaviside  proposed  that  the  inductance  of  tele- 
phone lines  be  increased  above  the  amount  natural  for  the  inter- 
axial  spacing,  with  a  view  to  counteracting  the  hurtful  effects  of  the 
capacity.  His  meaning  was  that  the  increased  inductance — a  harm- 
ful quality  in  a  circuit  not  having  also  a  harmfully  great  capacity — 
would  act  oppositely  to  the  capacity,  and  if  properly  chosen  and 
applied,  should  decrease  or  eliminate  distortion  by  making  the  line's 
effect  on  fundamentals  and  harmonics  more  nearly  uniform,  and  as 
well  should  reduce  the  attenuation  by  neutralizing  the  action  of  the 
capacity  in  dissipating  energy. 

There  are  two  ways  in  which  inductance  might  be  introduced  into 
a  telephone  line.  As  the  capacity  whose  effects  are  to  be  neutralized 


48  TELEPHONY 

is  distributed  uniformly  throughout  the  line,  the  counteracting  in- 
ductance must  also  be  distributed  throughout  the  line.  Mere  increase 
of  distance  between  two  wires  of  the  line  very  happily  acts  both  to 
increase  the  inductance  and  to  lower  the  capacity;  unhappily  for 
practical  results,  the  increase  of  separation  to  bring  the  qualities  into 
useful  neutralizing  relation  is  beyond  practical  limits.  The  wires 
would  need  to  be  so  far  above  the  earth  and  so  far  apart  as  to  make 
the  arrangement  commercially  impossible. 

Practical  results  have  been  secured  in  increasing  the  distributed 
inductance  by  wrapping  fine  iron  wire  over  each  conductor  of  the 
line.  Such  a  treatment  increases  the  inductance  and  improves  trans- 
mission. 

The  most  marked  success  has  come  as  a  result  of  the  studies  of 
Professor  Michael  Idvorsky  Pupin.  He  inserts  inductances  in  series 
with  the  wires  of  the  line,  so  adapting  them  to  the  constants  of  the 
circuit  that  attenuation  and  distortion  are  diminished  in  a  gratifying 
degree.  This  method  of  counteracting  the  effects  of  a  distributed 
capacity  by  the  insertion  of  localized  inductance  requires  not  only  that 
the  requisite  total  amount  of  inductance  be  known,  but  that  the 
proper  subdivision  and  spacing  of  the  local  portions  of  that  induct- 
ance be  known.  Professor  Pupin's  method  is  described  in  a  paper 
entitled  "Wave  Transmission  Over  Non-uniform  Cables  and  Long- 
Distance  Air  Lines,"  read  by  him  at  a  meeting  of  the  American  In- 
stitute of  Electrical  Engineers  in  Philadelphia,  May  19,  1900. 

NOTE.  United  States  Letters  Patent  were  issued  to  Professor  Pupin  on 
June  19,  1900,  upon  his  practical  method  of  reducing  attenuation  of  electrical 
waves.  A  paper  upon  "Propagation  of  Long  Electric  Waves"  was  read  by 
Professor  Pupin  before  the  American  Institute  of  Electrical  Engineers  on 
March  22,  1899,  and  appears  in  Vol.  15  of  the  Transactions  of  that  society. 
The  student  will  find  these  documents  useful  in  his  studies  on  the  subject. 
He  is  referred  also  to  "Electrical  Papers"  and  "Electromagnetic  Theory"  of 
Oliver  Heaviside. 

Professor  Pupin  likens  the  transmission  of  electric  waves  over 
long-distance  circuits  to  the  transmission  of  mechanical  waves  over  a 
string.  Conceive  an  ordinary  light  string  to  be  fixed  at  one  end  and 
shaken  by  the  hand  at  the  other;  waves  will  pass  over  the  string 
from  the  shaken  to  the  fixed  end.  Certain  reflections  will  occur 
from  the  fixed  end.  The  amount  of  energy  which  can  be  sent  in 


TELEPHONE  LINES  4'J 

this  case  from  the  shaken  to  the  fixed  point  is  small,  but  if  the  string 
be  loaded  by  attaching  bullets  to  it,  uniformly  throughout  its  length, 
it  now  may  transmit  much  more  energy  to  the  fixed  end. 

The  addition  of  inductance  to  a  telephone  line  is  analogous  to 
the  addition  of  bullets  to  the  string,  so  that  a  telephone  line  is  said 
to  be  loaded  when  inductances  are  inserted  in  it,  and  the  inductances 
themselves  are  known  as  loading  coils, 

Fig.  35  shows  the  general  relation  of  Pupin  loading  coils  to  the 
capacity  of  the  line.  The  condensers  of  the  figure  are  merely  con- 
ventionals  to  represent  the  condenser  which  the  line  itself  forms. 
The  inductances  of  the  figure  are  the  actual  loading  coils. 


Fig.  35.     Loaded  Line 

The  loading  of  open  wires  is  not  as  successful  in  practice  as  is 
that  of  cables.  The  fundamental  reason  lies  in  the  fact  that  two  of 
the  properties  of  open  wires — insulation  and  capacity— vary  with 
atmospheric  change.  The  inserted  inductance  remaining  constant, 
its  benefits  may  become  detriments  when  the  other  two  "constants" 
change. 

The  loading  of  cable  circuits  is  not  subject  to  these  defects. 
Such  loading  improves  transmission;  saves  copper;  permits  the  use 
of  longer  underground  cables  than  are  usable  when  not  loaded; 
lowers  maintenance  costs  by  placing  interurban  cables  underground; 
and  permits  submarine  telephone  cables  to  join  places  not  otherwise 
able  to  speak  with  each  other. 

Underground  long-distance  lines  now  join  or  are  joining  Boston 
and  New  York,  Philadelphia  and  New  York,  Milwaukee  and  Chi- 
cago. England  and  France  are  connected  by  a  loaded  submarine 
cable.  There  is  no  theoretical  reason  why  Europe  and  America 
should  not  speak  to  each  other. 

The  student  wishing  to  determine  for  himself  what  are  the  effects 
of  the  properties  of  lines  upon  open  or  cable  circuits  will  find  most  of 
the  subject  in  the  following  equation.  It  tells  the  value  of  a  in  terms 
of  the  four  properties,  a  being  the  attenuation  constant  of  the  line. 


TELEPHONY 


That  is,  the  larger  a  is,  the  more  the  voice  current  is  reduced  in  pass- 
ing over  the  line.     The  equation  is 


The  quantities  are 

R  =  Resistance  in  ohms 

L  =  Inductance  in  henrys 

C  =  Mutual  (shunt)  capacity  in  farads 

o>  =  litn  =  6.2832  times  the  frequency 

S  =  Shunt  leakage  in  mhos 

The  quantity  S  is  a  measure  of  the  combined  direct-current  con- 
ductance (reciprocal  of  insulation  resistance)  and  the  apparent  con- 
ductance due  to  dielectric  hysteresis. 

NOTE.  An  excellent  paper,  assisting  such  study,  and  of  immediate 
practical  value  as  helping  the  understanding  of  cables  and  their  reasons,  is 
that  of  Mr.  Frank  B.  Jewett,  presented  at  the  Thousand  Islands  Convention 
of  the  American  Institute  of  Electrical  Engineers,  July  1,  1909. 

Chapter  43  treats  cables  in  further  detail.  They  form  a  most  important 
part  of  telephone  wire-plant  practice,  and  their  uses  are  becoming  wider  and 
more  valuable. 

Possible  Ways  of  Improving  Transmission.  Practical  ways  of 
improving  telephone  transmission  are  of  two  kinds:  to  improve  the 
lines  and  to  improve  the  apparatus.  The  foregoing  shows  what  are 
the  qualities  of  lines  and  the  ways  they  require  to  be  treated.  Ap- 
paratus treatment,  in  the  present  state  of  the  art,  is  addressed  largely 
to  the  reduction  of  losses.  Theoretical  considerations  seem  to 
show,  however,  that  great  advance  in  apparatus  effectiveness  still  is 
possible.  More  powerful  transmitters — and  more  faithful  ones — 
more  sensitive  and  accurate  receivers,  and  more  efficient  translating 
devices  surely  are  possible.  Discovery  may  need  to  intervene,  to 
enable  invention  to  restimulate. 

In  both  telegraphy  and  telephony,  the  longer  the  line  the  weaker 
the  current  which  is  received  at  the  distant  end.  In  both  telegraphy 
and  telephony,  there  is  a  length  of  line  with  a  given  kind  and  size 
of  wire  and  method  of  construction  over  which  it  is  just  possible  to 
send  intelligible  speech  or  intelligible  signals.  A  repeater,  in  teleg- 
raphy, is  a  device  in  the  form  of  a  relay  which  is  adapted  to  receive 
these  highly  attenuated  signal  impulses  and  to  re-transmit  them  with 
fresh  power  over  a  new  length  of  line.  An  arrangement  of  two  such 


TELEPHONE  LINES 


51 


relays  makes  it  possible  to  telegraph  both  ways  over  a  pair  of  lines 
^nited  by  such  a  repeater.  It  is  practically  possible  to  join  up  several 
such  links  of  lines  to  repeating  devices  and,  if  need  be,  even  subma- 
rine cables  can  be  joined  to  land  lines  within  practical  limits.  If  it 
vere  necessary,  it  probably  would  be  possible  to  telegraph  around 
he  world  in  this  way. 

If  it  were  possible  to  imitate  the  telegraph  repeater  in  telephony, 
attenuated  voice  currents  might  be  caused  to  actuate  it  so  as  to  send  on 
those  voice  currents  with  renewed  power  over  a  length  of  line,  section 
by  section.  Such  a  device  has  been  sought  for  many  years,  and  it  once 
was  quoted  in  the  public  press  that  a  reward  of  one  million  dollars 
had  been  offered  by  Charles  J.  Glidden  for  a  successful  device  of  that 
kind.  The  records  of  the  patent  offices  of  the  world  show  what 
effort  has  been  made  in  that  direction  and  many  more  devices  have 
been  invented  than  have  been  patented  in  all  the  countries  together. 

Like  some  other  problems  in  telephony,  this  one  seems  simpler 
at  first  sight  than  it  proves  to  be  after  more  exhaustive  study.  It  is 
possible  for  any  amateur  to  produce  at  once  a  repeating  device  which 
will  relay  telephone  circuits  in  one  direction.  It  is  required,  however, 
that  in  practice  the  voice  currents  be  relayed  in  both  directions,  and 
further,  that  the  relay  actually  augment  the  energy  which  passes 
through  it;  that  is,  that  it  will  send  on  a  more  powerful  current  than 
it  receives.  Most  of  the  devices  so  far  invented  fail  in  one  or  the 
other  of  these  particulars. 
Several  ways  have  been 
shown  of  assembling  re- 
peating devices  which  will 
talk  both  ways,  but  not 
many  assembling  repeating 
devices  have  been  shown 
that  will  talk  both  ways  and 
augment  in  both  directions. 

Practical  repeaters  have  been  produced,  however,  and  at  least 
one  type  is  in  daily  successful  use.  It  is  not  conclusively  shown  even 
of  it  that  it  augments  in  the  same  degree  all  of  the  voice  waves  which 
reach  it,  or  even  that  it  augments  some  of  them  at  all.  Its  action, 
however,  is  distinctly  an  improvement  in  commercial  practice.  It  is 
the  invention  of  Mr.  Herbert  E.  Shreeve  and  is  shown  in  Fig.  39. 


Fig.  36.     Shreeve  Repeater  and  Circuit 


TELEPHONY 


Primarily  it  consists  of  a  telephone  receiver,  of  a  particular  type  de- 
vised by  Gundlach,  associated  with  a  granular  carbon  transmitter 
button.  It  is  further  associated  with  an  arrangement  of  induction  coils 
or  repeating  coils,  the  object  of  these  being  to  accomplish  the  two- 
way  action,  that  is,  of  speaking  in  both  directions  and  of  preventing 

reactive  interference  between  the 
receiving  and  transmitting  ele- 
ments. The  battery  1  energizes 
the  field  of  the  receiving  element; 
the  received  line  current  varies 
that  field;  the  resulting  motion 
varies  the  resistance  of  the  carbon 
button  and  transforms  current 
from  battery  2  into  a  new  alter- 
nating line  current. 

By  reactive  interference  is 
meant  action  whereby  the  trans- 
mitter element,  in  emitting  a 
wave,  affects  its  own  controlling 
receiver  element,  thus  setting  up 
an  action  similar  to  that  which  oc- 
curs when  the  receiver  of  a  tele- 
phone is  held  close  to  its  transmitter  and  humming  or  singing  ensues. 
No  repeater  is  successful  unless  it  is  free  from  this  reactive  interference. 
Enough  has  been  accomplished  by  practical  tests  of  the  Shreeve 
device  and  others  like  it  to  show  that  the  search  for  a  method  of  re- 
laying telephone  voice  currents  is  not  looking  for  a  pot  of  gold  at  the 
end  of  the  rainbow.  The  most  remarkable  truth  established  by  the 
success  of  repeaters  of  the  Shreeve  type  is  that  a  device  embodying 
so  large  inertia  of  moving  parts  can  succeed  at  all.  It  this  mean 
anything,  it  is  that  a  device  in  which  inertia  is  absolutely  eliminated 
might  do  very  much  better.  Many  of  the  methods  already  proposed 
by  inventors  attack  the  problem  in  this  way  and  one  of  the  most  re- 
cent and  most  promising  ways  is  that  of  Mr.  J.  B.  Taylor,  the  circuit 
of  whose  telephone-relay  patent  is  shown  in  Fig.  37.  In  it,  1  is  an 
electromagnet  energized  by  voice  currents;  its  varying  field  varies  an 
arc  between  the  electrodes  2-2  and  3  in  a  vacuum  tube.  These  fluc- 
tuations are  transformed  into  line  currents  by  the  coil  4- 


LINE 


Fig.  37.     Mercury- Arc  Telephone  Relay 


CHAPTER  V 
TRANSMITTERS 

Variable  Resistance.  As  already  pointed  out  in  Chapter  II,  the 
variable-resistance  method  of  producing  current  waves,  correspond- 
ing to  sound  waves  for  telephonic  transmission,  is  the  one  that  lends 
itself  most  readily  to  practical  purposes.  Practically  all  telephone 
transmitters  of  today  employ  this  variable-resistance  principle.  The 
reason  for  the  adoption  of  this  method  instead  of  the  other  possible 
ones  is  that  the  devices  acting  on  this  principle  are  capable,  with  great 
simplicity  of  construction,  of  producing  much  more  powerful  results 
than  the  others.  Their  simplicity  is  such  as  to  make  them  capable 
of  being  manufactured  at  low  cost  and  of  being  used  successfully  by 
unskilled  persons. 

Materials.  Of  all  the  materials  available  for  the  variable-re- 
sistance element  in  telephone  transmitters,  carbon  is  by  far  the  most 
suitable,  and  its  use  is  well  nigh  universal.  Sometimes  one  of  the 
rarer  metals,  such  as  platinum  or  gold,  is  to  be  found  in  commercial 
transmitters  as  part  of  the  resistance-varying  device,  but,  even  when 
this  is  so,  it  is  always  used  in  combination  with  carbon  in  some  form 
or  other.  Most  of  the  transmitters  in  use,  however,  depend  solely 
upon  carbon  as  the  conductive  material  of  the  variable-resistance 
element. 

Arrangement  of  Electrodes.  Following  the  principles  pointed 
out  by  Hughes,  the  transmitters  of  today  always  employ  as  their  vari- 
able-resistance elements  one  or  more  loose  contacts  between  one  or 
more  pairs  of  electrodes,  which  electrodes,  as  just  stated,  are  usually 
of  carbon.  Always  the  arrangement  is  such  that  the  sound  waves  will 
vary  the  intimacy  of  contact  between  the  electrodes  and,  therefore, 
the  resistance  of  the  path  through  the  electrodes. 

A  multitude  of  arrangements  have  been  proposed  and  tried. 
Sometimes  a  single  pair  of  electrodes  has  been  employed  having  a 
single  point  of  loose  contact  between  them.  These  may  be  termed 


54 


TELEPHONY 


single-contact  transmitters.  Sometimes,  the  variable-resistance  ele- 
ment has  included  a  greater  number  of  electrodes  arranged  in  multi- 
ple, or  in  series,  or  in  series-multiple,,  and  these  have  been  termed 
multiple-electrode  transmitters,  signifying  a  plurality  of  electrodes. 
A  later  development,  an  outgrowth  of  the  multiple-electrode  trans- 
mitter, makes  use  of  a  pair  of  principal  electrodes,  between  which  is 
included  a  mass  of  finely  divided  carbon  in  the  form  of  granules  or 
small  spheres  or  pellets.  These,  regardless  of  the  exact  form  of  the 
carbon  particles,  are  called  granular-carbon  transmitters. 

Single  Electrode.     Blake.     The  most  notable  example  of  the 
single-contact  transmitter  is  the  once  familiar  Blake  instrument.     At 


Fig.  38.    Blake  Transmitter 


one  time  this  formed  a  part  of  the  standard  equipment  of  almost  every 
telephone  in  the  United  States,  and  it  was  also  largely  used  abroad. 
Probably  no  transmitter  has  ever  exceeded  it  in  clearness  of  articu- 
lation, but  it  was  decidedly  deficient  in  power  in  comparison  with 
the  modern  transmitters.  In  this  instrument,  which  is  shown  in  Fig. ' 
38,  the  variable-resistance  contact  was  that  between  a  carbon  and  a 
platinum  electrode.  The  diaphragm  1  was  of  sheet  iron  mounted, 
as  usual  in  later  transmitters,  in  a  soft  rubber  gasket  2.  The  whole 
diaphragm  was  mounted  in  a  cast-iron  ring  3,  supported  on  the  inside 
of  tiie  box  containing  the  entire  instrument.  The  front  electrode  4 
was  mounted  on  a  light  spring  5,  the  upper  end  of  which  was  supported 


TRANSMITTERS  55 

by  a  movable  bar  or  lever  6,  flexibly  supported  on  a  spring  7  secured 
to  the  casting  which  supported  the  diaphragm.  The  tension  of  this 
spring  5  was  such  as  to  cause  the  platinum  point  to  press  lightly  away 
from  the  center  of  the  diaphragm.  The  rear  electrode  was  of  carbon 
in  the  form  of  a  small  block  9,  secured  in  a  heavy  brass  button  10. 
The  entire  rear  electrode  structure  was  supported  on  a  heavier  spring 
11  carried  on  the  same  lever  as  the  spring  5.  The  tension  of  this 
latter  spring  was  such  as  to  press  against  the  front  electrode  and,  by 
its  greater  strength,  press  this  against  the  center  of  the  diaphragm. 
The  adjustment  of  the  instrument  was  secured  by  means  of  the  screw 
12,  carried  in  a  lug  extending  rearwardly  from  the  diaphragm  sup- 
porting casting,  this  screw,  by  its  position,  determining  the  strength 
with  which  the  rear  electrode  pressed  against  the  front  electrode  and 
that  against  the  diaphragm.  This  instrument  was  ordinarily  mount- 
ed in  a  wooden  box  together  with  the  induction  coil,  which  is  shown 
in  the  upper  portion  of  the  figure. 

The  Blake  transmitter  has  passed  almost  entirely  out  of  use  in 
this  country,  being  superseded  by  the  various  forms  of  granular  in- 
struments, which,  while  much  more  powerful,  are  not  perhaps  capable 
of  producing  quite  such  clear  and  distinct  articulation. 

The  great  trouble  with  the  single-contact  transmitters,  such  as 
the  Blake,  was  that  it  was  impossible  to  pass  enough  current  through 
the  single  point  of  contact  to  secure  the  desired  power  of  transmission 
without  overheating  the  contact.  If  too  much  current  is  sent  through 
such  transmitters,  an  undue  amount  of  heat  is  generated  at  the  point 
of  contact  and  a  vibration  is  set  up  which  causes  a  peculiar  humming 
or  squealing  sound  which  interferes  with  the  transmission  of  other 
sounds. 

Multiple  Electrode.  To  remedy  this  difficulty  the  so-called 
multiple-electrode  transmitter  was  brought  out.  This  took  a  very 
great  number  of  forms,  of  which  the  one  shown  in  Fig.  39  is  typ- 
ical. The  diaphragm  shown  at  1,  in  this  particular  form,  was  made 
of  thin  pine  wood.  On  the  rear  side  of  this,  suspended  from  a  rod 
3  carried  in  a  bracket  4,  were  a  number  of  carbon  rods  or  pendants  5, 
loosely  resting  against  a  rod  2,  carried  on  a  bracket  6  also  mounted 
on  the  rear  of  the  diaphragm.  The  pivotal  rod  3  and  the  rod  2, 
against  which  the  pendants  rested,  were  sometimes,  like  the  pendant 
rods,  made  of  carbon  and  sometimes  of  metal,  such  as  brass.  When 


56 


TELEPHONY 


the  diaphragm  vibrated,  the  intimacy  of  contact  between  the  pendant 
rod  5  and  the  rod  2  was  altered,  and  thus  the  resistance  of  the  path 
through  all  of  the  pendant  rods  in  multiple  was  changed. 

A  multitude  of  forms  of  such  transmitters  came  into  use  in  the 
early  eighties,  and  while  they  in  some  measure  remedied  the  difficulty 

encountered  with  the  Blake  trans- 
mitter, i.  c.,  of  not  being  able  to 
carry  a  sufficiently  large  current, 
they  were  all  subject  to  the  effects 
of  extreme  sensitiveness,  and  would 
rattle  or  break  when  called  upon 
to  transmit  sounds  of  more  than 
ordinary  loudness.  Furthermore, 
the  presence  of  such  large  masses 
Fig.  39.  Multiple-Electrode  Transmitter  of  material,  which  it  was  necessary 

to    throw   into    vibration    by    the 

sound  waves,  was  distinctly  against  this  form  of  transmitter.  The 
inertia  of  the  moving  parts  was  so  great  that  clearness  of  articulation 
was  interfered  with. 

Granular  Carbon.  The  idea  of  employing  a  mass  of  granular 
carbon,  supported  between  two  electrodes,  one  of  which  vibrated 
with  the  sound  waves  and  the  other  was  stationary,  was  proposed  by 
Henry  Runnings  in  the  early  eighties.  While  this  idea  forms  the 
basis  of  all  modern  telephone  transmitters,  yet  it  did  not  prevent  the 
almost  universal  adoption  of  the  single-contact  form  of  instrument 
during  the  next  decade. 

Western  Electric  Solid=Back  Transmitter.  In  the  early  nineties, 
however,  the  granular-carbon  transmitter  came  into  its  own  with  the 
advent  and  wide  adoption  of  the  transmitter  designed  by  Anthony  C. 
White,  known  as  the  White,  or  solid-back,  transmitter.  This  has 
for  many  years  been  the  standard  instrument  of-  the  Bell  companies 
operating  throughout  the  United  States,  and  has  found  large  use 
abroad.  A  horizontal  cross-section  of  this  instrument  is  shown  in 
Fig.  40,  and  a  rear  view  of  the  working  parts  in  Fig.  41.  The  work- 
ing parts  are  all  mounted  on  the  front  casting  1.  This  is  supported 
in  a  cup  2,  in  turn  supported  on  the  lug  3,  which  is  pivoted  on  the 
transmitter  arm  or  other  support.  The  front  and  rear  electrodes  of 
this  instrument  are  formed  of  thin  carbon  disks  shown  in  solid  black. 


TRANSMITTERS 


57 


The  rear  electrode,  the  larger  one  of  these  disks,  is  securely  attached 
by  solder  to  the  face  of  a  brass  disk  having  a  rearwardly  projecting 
screw-threaded  shank,  which  serves  to  hold  it  and  the  rear  electrode 
in  place  in  the  bottom  of  a  heavy  brass  cup  4-  The  front  electrode 
is  mounted  on  the  rear  face  of  a  stud.  Clamped  against  the  head 
of  this  stud,  by  a  screw-threaded  clamping  ring  7,  is  a  mica  washer, 
or  disk  6.  The  center  portion  of  this  mica  washer  is  therefore  rigid 
with  respect  to  the  front  electrode  and  partakes  of  its  movements. 
The  outer  edge  of  this  mica  washer  is  similarly  clamped  against  the 


Fig.  40.     White  Solid-Back  Transmitter 

front  edge  of  the  cup  4,  a  screw-threaded  ring  9  serving  to  hold  the 
edge  of  the  mica  rigidly  against  the  front  of  the  cup.  The  outer  edge 
of  this  washer  is,  therefore,  rigid  with  respect  to  the  rear  electrode, 
which  is  fixed.  Whatever  relative  movement  there  is  between  the 
two  electrodes  must,  therefore,  be  permitted  by  the  flexing  of  the 
mica  washer.  This  mica  washer  not  only  serves  to  maintain  the 
electrodes  in  their  normal  relative  positions,  but  also  serves  to  close 
the  chamber  which  contains  the  electrodes,  and,  therefore,  to  prevent 
the  granular  carbon,  with  which  the  space  between  the  electrodes 
is  filled,  from  falling  out. 


58 


TELEPHONY 


The  cup  4>  containing  the  electrode  chamber,  is  rigidly  fastened 
with  respect  to  the  body  of  the  transmitter  by  a  rearwardly  projecting 
shank  held  in  a  bridge  piece  5"  which  is  secured  at  its  ends  to  the  front 
block.  The  needed  rigidity  of  the  rear  electrode  is  thus  obtained  and 
this  is  probably  the  reason,  for  calling  the  instrument  the  solid-back. 
The  front  electrode,  on  the  other  hand,  is  fastened  to  the  center  of  the 
diaphragm  by  means  of  a  shank  on  the  stud,  which  passes  through 
a  hole  in  the  diaphragm  and  is  clamped  thereto  by  two  small  nuts. 
Against  the  rear  face  of  the  diaphragm  of  this  transmitter  there  rest 
two  damping  springs.  These  are  not  shown  in  Fig.  40  but  are  in 
Fig.  41.  They  are  secured  at  one  end  to  the  rear  flange  of  the  front 
casting  1,  and  bear  with  their  other  or  free  ends  against  the  rear  face  of 
the  d'.aphragm.  The  damping  springs  are  prevented  from  coming 
into  actual  contact  with  the  diaphragm  by  small  insulating  pads. 

The  purpose  of  the  damping 
springs  is  to  reduce  the  sensitive- 
ness of  the  diaphragm  to  extra- 
neous sounds.  As  a  result,  the 
White  transmitter  does  not  pick 
up  all  of  the  sounds  in  its  vicinity 
as  readily  as  do  the  more  sensi- 
tive transmitters,  and  thus  the 
transmission  is  not  interfered  with 
by  extraneous  noises.  On  the 
other  hand,  the  provision  of  these 
heavy  damping  springs  makes  it 
necessary  that  this  transmitter 
shall  be  spoken  into  directly  by 
the  user. 

The  action  of  this  transmitter 
is  as  follows:  Sound  waves  are  concentrated  against  the  center  of 
the  diaphragm  by  the  mouth-piece,  which  is  of  the  familiar  form. 
These  waves  impinge  against  the  diaphragm,  causing  it  to  vibrate, 
and  this,  in  turn,  produces  similar  vibrations  in  the  front  electrode. 
The  vibrations  of  the  front  electrode  are  permitted  by  the  elasticity 
of  the  mica  washer  6.  The  rear  electrode  is,  however,  held  stationary 
within  the  heavy  chambered  block  4  and  which  in  turn  is  held  im- 
movable by  its  rigid  mounting.  As  a  result,  the  front  electrode  ap- 


Fig.  41.     White  Solid-Back  Transmitter 


TRANSMITTERS 


59 


preaches  and  recedes  from  the  rear  electrode,  thus  compressing  and 
decompressing  the  mass  of  granular  carbon  between  them.  As  a 
result,  the  intimacy  of  contact  between  the  electrode  plates  and  the 
granules  and  also  between  the  granules  themselves  is  altered,  and 
the  resistance  of  the  path  from  one  electrode  to  the  other  through 
the  mass  of  granules  is  varied. 

New  Western  Electric  Transmitter.  The  White  transmitter 
was  the  prototype  of  a  large  number  of  others  embodying  the  same 
features  of  having  the 
rear  electrode  mounted 
in  a  stationary  cup  or 
chamber  and  the  front 
electrode  movable  with 
the  diaphragm,  a  washer 
of  mica  or  other  flexible 
insulating  material  serv- 
ing to  close  the  front  of 
the  electrode  chamber 
and  at  the  same  time 
to  permit  the  necessary 
vibration  of  the  front 
electrode  with  the  dia- 
phragm. 

One  of  these  trans- 
mitters,  embodying 
these  same  features  but 
with  modified  details,  is 
shown  in  Fig.  42,  this 

being  the  new  transmitter  manufactured  by  the  Western  Electric 
Company.  In  this  the  bridge  of  the  original  White  transmitter  is 
dispensed  with,  the  electrode  chamber  being  supported  by  a  pressed 
metal  cup  1,  which  supports  the  chamber  as  a  whole.  The  elec- 
trode cup,  instead  of  being  made  of  a  solid  block  as  in  the  White 
instrument,  is  composed  of  two  portions,  a  cylindrical  or  tubular 
portion  2  and  a  back  3.  The  cylindrical  portion  is  externally 
screw-threaded  so  as  to  engage  an  internal  screw  thread  in  a  flanged 
opening  in  the  center  of  the  cup  L  By  this  means  the  electrode 
chamber  is  held  in  place  in  the  cup  1,  and  by  the  same  means 


Fig.  42.     New  Western  Electric  Transmitter 


60  TELEPHONY 

the  mica  washer  4  'ls  clamped  between  the  flange  in  this  opening 
and  the  tubular  portion  2  of  the  electrode  chamber.  The  front 
electrode  is  carried,  as  in  the  White  transmitter,  on  the  mica 
washer  and  is  rigidly  attached  to  the  center  of  the  diaphragm  so 
as  to  partake  of  the  movement  thereof.  It  will  be  seen,  therefore, 
that  this  is  essentially  a  White  transmitter,  but  with  a  modified 
mounting  for  the  electrode  chamber. 

A  feature  in  this  transmitter  that  is  not  found  in  the  White  trans- 
mitter is  that  both  the  front  and  the  rear  electrodes,  in  fact,  the  entire 
working  portions  of  the  transmitter,  are  insulated  from  the  exposed 
metal  parts  of  the  instrument.  This  is  accomplished  by  insulating 
the  diaphragm  and  the  supporting  cup  1  from  the  transmitter  front. 
The  terminal  5  on  the  cup  1  forms  the  electrical  connection  for  the 
rear  electrode,  while  the  terminal  6,  which  is  mounted  on  but  in- 
sulated from  the  cup  1  and  is  connected  with  the  front  electrode 
by  a  thin  flexible  connecting  strip,  forms  the  electrical  connection 
for  the  front  electrode. 

Kellogg  Transmitter.  The  transmitter  of  the  Kellogg  Switch- 
board and  Supply  Company,  originally  developed  by  Mr.  W.  W. 
Dean  and  modified  by  his  successors  in  the  Kellogg  Company,  is 
shown  in  Fig.  43.  In  this,  the  electrode  chamber,  instead  of  being 
mounted  in  a  stationary  and  rigid  position,  as  in  the  case  of  the 
White  instrument,  is  mounted  on,  and,  in  fact,  forms  a  part  of  the 
diaphragm.  The  electrode  which  is  associated  with  the  mica 
washer  instead  of  moving  with  the  diaphragm,  as  in  the  White 
instrument,  is  rigidly  connected  to  a  bridge  so  as  to  be  as  free  as 
possible  from  all  vibrations. 

Referring  to  Fig.  43,  which  is  a  horizontal  cross-section  of  the 
instrument,  1  indicates  the  diaphragm.  This  is  of  aluminum  and  it 
has  in  its  center  a  forwardly  deflected  portion  forming  a  chamber  for 
the  electrodes.  The  front  electrode  2  of  carbon  is  backed  by  a  disk 
of  brass  and  rigidly  secured  in  the  front  of  this  chamber,  as  clearly 
indicated.  The  rear  electrode  3,  also  of  carbon,  is  backed  by  a  disk 
of  brass,  and  is  clamped  against  the  central  portion  of  a  mica  disk 
by  means  of  the  enlarged  head  of  stud  6.  A  nut  7,  engaging  the  end 
of  a  screw-threaded  shank  from  the  back  of  the  rear  electrode, 
serves  to  bind  these  two  parts  together  securely,  clamping  the  mica 
washer  between  them.  The  outer  edge  of  the  mica  washer  is 


TRANSMITTERS 


61 


clamped  to  the  main  diaphragm  1  by  an  aluminum  ring  and  rivets, 
as  clearly  indicated.  It  is  seen,  therefore,  that  the  diaphragm  itself 
contains  the  electrode  chamber  as  an  integral  part  thereof.  The 
entire  structure  of  the  diaphragm,  the  front  and  back  electrodes, 
and  the  granular  carbon  within  are  permanently  assembled  in  the 
factory  and  cannot  be  dissociated  without  destroying  some  of  the 
parts.  The  rear  electrode  is  held  rigidly  in  place  by  the  bridge  5 
and  the  stud  6,  this  stud  passing  through  a  block  9  mounted  on  the 


Fig.  43.     Kellogg  Transmitter 

bridge  but  insulated  from  it.  The  stud  6  is  clamped  in  the  block  9 
by  means  of  the  set  screw  8,  so  as  to  hold  the  rear  electrode  in 
proper  position  after  this  position  has  been  determined. 

In  this  transmitter,  as  in  the  transmitter  shown  in  Fig.  42,  all 
of  the  working  parts  are  insulated  from  the  exposed  metal  casing. 
The  diaphragm  is  insulated  from  the  front  of  the  instrument  by  means 
of  a  washer  4  of  impregnated  cloth,  as  indicated.  The  rear  electrode 
is  insulated  from  the  other  portions  of  the  instrument  by  means  of  the 
mica  washer  and  by  means  of  the  insulation  between  the  block  9  and 
the  bridge  5.  The  terminal  for  the  rear  electrode  is  mounted  on  the 
block  9,  while  the  terminal  for  the  front  electrode,  shown  at  10,  is 


62  TELEPHONY 

mounted  on,  but  insulated  from,  the  bridge.  This  terminal  10  is 
connected  with  the  diaphragm  and  therefore  with  the  front  electrode 
by  means  of  a  thin,  flexible  metallic  connection.  This  transmitter 
is  provided  with  damping  springs  similar  to  those  of  the  White  in- 
strument. 

It  is  claimed  by  advocates  of  this  type  of  instrument  that,  in  ad- 
dition to  the  ordinary  action  due  to  the  compression  and  decom- 
pression of  the  granular  carbon  between  the  electrodes,  there  exists 
another  action  due  to  the  agitation  of  the  granules  as  the  chamber  is 
caused  to  vibrate  by  the  sound  waves.  In  other  words,  in  addition 
to  the  ordinary  action,  which  may  be  termed  the  piston  action  between 
the  electrodes,  it  is  claimed  that  the  general  shaking-up  effect  of  the 
granules  when  the  chamber  vibrates  produces  an  added  effect.  Cer- 
tain it  is,  however,  that  transmitters  of  this  general  type  are  very 
efficient  and  have  proven  their  capability  of  giving  satisfactory 
service  through  long  periods  of  time. 

Another  interesting  feature  of  this  instrument  as  it  is  now  manu- 
factured is  the  use  of  a  transmitter  front  that  is  struck  up  from  sheet 
metal  rather  than  the  employment  of  a  casting  as  has  ordinarily 
been  the  practice.  The  formation  of  the  supporting  lug  for  the 
transmitter  from  the  sheet  metal  which  forms  the  rear  casing  or 
shell  of  the  instrument  is  also  an  interesting  feature. 

Automatic  Electric  Company  Transmitter.  The  transmitter  of 
the  Automatic  Electric  Company,  of  Chicago,  shown  in  Fig.  44,  is  of 
the  same  general  type  as  the  one  just  discussed,  in  that  the  electrode 
chamber  is  mounted  on  and  vibrates  with  the  diaphragm  instead  of 
being  rigidly  supported  on  the  bridge  as  in  the  case  of  the  White  or 
solid-back  type  of  instrument.  In  this  instrument  the  transmitter 
front  1  is  struck  up  from  sheet  metal  and  contains  a  rearwardly  pro- 
jecting flange,  carrying  an  internal  screw  thread.  A  heavy  inner 
cup  2,  together  with  the  diaphragm  3,  form  an  enclosure  containing 
the  electrode  chamber.  The  diaphragm  is,  in  this  case,  permanently 
secured  at  its  edge  to  the  periphery  of  the  inner  cup  2  by  a  band  of 
metal  4  so  formed  as  to  embrace  the  edges  of  both  the  cup  and  the 
diaphragm  and  permanently  lock  them  together.  This  inner  cham- 
ber is  held  in  place  in  the  transmitter  front  1  by  means  of  a  lock  ring 
5  externally  screw-threaded  to  engage  the  internal  screw-thread  on 
the  flange  on  the  front.  The  electrode  chamber  proper  is  made  in 


TRANSMITTERS 


63 


the  form  of  a  cup,  rigidly  secured  to  the  diaphragm  so  as  to  move 
therewith,  as  clearly  indicated.  The  rear  electrode  is  mounted  on 
a  screw-threaded  stud  carried  in  a  block  which  is  fitted  to  a  close 
central  opening  in  the  cup  2. 

This  transmitter  does  not  make  use  of  a  mica  washer  or  dia- 
phragm, but  employs  a  felt  washer  which  surrounds  the  shank  of 
the  rear  electrode  and  serves  to  close  and  seal  the  carbon  contain- 
ing cup.  By  this  means  the  granular  carbon  is  retained  in  the  cham- 


Fig.  44.     Automatic  Electric  Company  Transmitter 

ber  and  the  necessary  flexibility  or  freedom  of  motion  is  permitted 
between  the  front  and  the  rear  electrodes.  As  in  the  Kellogg  and  the 
later  Bell  instruments,  the  entire  working  parts  of  this  transmitter 
are  insulated  from  the  metal  containing  case,  the  inner  chamber, 
formed  by  the  cup  2  and  the  diaphragm  3,  being  insulated  from  the 
transmitter  front  and  its  locking  ring  by  means  of  insulating  washers, 
as  shown. 

Monarch  Transmitter.  The  transmitter  of  the  Monarch  Tele- 
phone Manufacturing  Company,  shown  in  Fig.  45,  differs  from  both 
the  stationary-cup  and  the  vibrating-cup  types,  although  it  has  the 
characteristics  of  both.  It  might  be  said  that  it  differs  from  each 


64 


TELEPHONY 


of  these  two  types  of  transmitters  in  that  it  has  the  characteristics 
of  both. 

This  transmitter,  it  will  be  seen,  has  two  flexible  mica  washers 
between  the  electrodes  and  the  walls  of  the  electrode  cup.  The 
front  and  the  back  electrodes  are  attached  to  the  diaphragm  and  the 
bridge,  respectively,  by  a  method  similar  to  that  employed  in  the 
solid-back  transmitters,  while  the  carbon  chamber  itself  is  free  to 
vibrate  with  the  diaphragm  as  is  characteristic  of  the  Kellogg 
transmitter. 

An  aluminum  diaphragm  is  employed,  the  circumferential  edge 
of  which  is  forwardly  deflected  to  form  a  seat.  The  edge  of  the 


Fig.  45.     Monarch  Transmitter 

diaphragm  rests  against  and  is  separated  from  the  brass  front  by 
means  of  a  one-piece  gasket  of  specially  treated  linen.  This  forms 
an  insulator  which  is  not  affected  by  heat  or  moisture.  As  in  the 
transmitters  previously  described,  the  electrodes  are  firmly  soldered 
to  brass  disks  which  have  solid  studs  extending  from  their  centers. 
In  the  case  of  both  the  front  and  the  rear  electrodes,  a  mica  disk  is 
placed  over  the  supporting  stud  and  held  in  place  by  a  brass  hub 


TRANSMITTERS  65 

which  has  a  base  of  the  same  size  as  the  electrode.  The  carbon- 
chamber  wall  consists  of  a  brass  ring  to  which  are  fastened  the  mica 
disks  of  the  front  and  the  back  electrodes  by  means  of  brass  collars 
clamped  over  the  edge  of  the  mica  and  around  the  rim  of  the  brass 
ring  forming  the  chamber. 

Electrodes.  The  electrode  plates  of  nearly  all  modern  trans- 
mitters are  of  specially  treated  carbon.  These  are  first  copper-plated 
and  soldered  to  their  brass  supporting  disks.  After  this'  they  are 
turned  and  ground  so  as  to  be  truly  circular  in  form  and  to  present 
absolutely  flat  faces  toward  each  other.  These  faces  are  then  highly 
polished  and  the  utmost  effort  is  made  to  keep  them  absolutely  clean. 
Great  pains  are  taken  to  remove  from  the  pores  of  the  carbon,  as  well 
as  from  the  surface,  all  of  the  acids  or  other  chemicals  that  may  have 
entered  them  during  the  process  of  electroplating  them  or  of  solder- 
ing them  to  the  brass  supporting  disk.  That  the  two  electrodes, 
when  mounted  in  a  transmitter,  should  be  parallel  with  each  other, 
is  an  item  of  great  importance  as  will  be  pointed  out  later. 

In  a  few  cases,  as  previously  stated,  gold  or  platinum  has  been 
substituted  for  the  carbon  electrodes  in  transmitters.  These  are  ca- 
pable of  giving  good  results  when  used  in  connection  with  the  proper 
form  of  granular  carbon,  but,  on  the  whole,  the  tendency  has  been  to 
abandon  all  forms  of  electrode  material  except  carbon,  and  its  use 
is  now  well  nigh  universal. 

Preparation  of  Cdrbon.  The  granular  carbon  is  prepared  from 
carefully  selected  anthracite  coal,  which  is  specially  treated  by 
roasting  or  "re-carbonizing"  and  is  then  crushed  to  approximately 
the  proper  fineness.  The  crushed  carbon  is  then  screened  with 
extreme  care  to  eliminate  all  dust  and  to  retain  only  granules  of 
uniform  size. 

Packing.  In  the  earlier  forms  of  granular-carbon  transmitters 
a  great  deal  of  trouble  was  experienced  due  to  the  so-called  packing  of 
the  instrument.  This,  as  the  term  indicates,  was  a  trouble  due  to 
the  tendency  of  the  carbon  granules  to  settle  into  a  compact  mass  and 
thus  not  respond  to  the  variable  pressure.  This  was  sometimes  due 
to  the  presence  of  moisture  in  the  electrode  chamber;  sometimes  to 
the  employment  of  granules  of  varying  sizes,  so  that  they  would  finally 
arrange  themselves  under  the  vibration  of  the  diaphragm  into  a  faHy 
compact  mass;  or  sometimes,  and  more  frequently,  to  the  granules 


66  TELEPHONY 

in  some  way  wedging  the  two  electrodes  apart  and  holding  them  at 
a  greater  distance  from  each  other  than  their  normal  distance.  The 
trouble  due  to  moisture  has  been  entirely  eliminated  by  so  sealing  the 
granule  chambers  as  to  prevent  the  entrance  of  moisture.  The 
trouble  due  to  the  lack  of  uniformity  in  size  of  the  granules  has  been 
entirely  eliminated  by  making  them  all  of  one  size  and  by  making 
them  of  sufficient  hardness  so  that  they  would  not  break  up  into  gran- 
ules of  smaller  size.  The  trouble  due  to  the  settling  of  the  granules 
and  wedging  the  electrodes  apart  has  been  practically  eliminated 
in  well-designed  instruments,  by  great  mechanical  nicety  in  manu- 
facture. 

Almost  any  transmitter  may  be  packed  by  drawing  the  diaphragm 
forward  so  as  to  widely  separate  the  electrodes.  This  allows  the  gran- 
ules to  settle  to  a  lower  level  than  they  normally  occupy  and  when  the 
diaphragm  is  released  and  attempts  to  resume  its  normal  position  it  is 
prevented  from  doing  so  by  the  mass  of  granules  between.  Trans- 
mitters of  the  early  types  could  be  packed  by  placing  the  lips  against 
the  mouthpiece  and  drawing  in  the  breath.  The  slots  now  provided 
at  the  base  of  standard  mouthpieces  effectually  prevent  this. 

In  general  it  may  be  said  that  the  packing  difficulty  has  been 
almost  entirely  eliminated,  not  by  the  employment  of  remedial  de- 
vices, such  as  those  often  proposed  for  stirring  up  the  carbon,  but  by 
preventing  the  trouble  by  the  design  and  manufacture  of  the  instru- 
ments in  such  forms  that  they  will  not  be  subject  to  the  evil. 

Carrying  Capacity.  Obviously,  the  power  of  a  transmitter  is 
dependent  on  the  amount  of  current  that  it  may  carry,  as  well  as  on 
the  amount  of  variation  that  it  may  make  in  the  resistance  of  the 
path  through  it.  Granular  carbon  transmitters  are  capable  of  carry- 
ing much  heavier  current  than  the  old  Blake  or  other  single  or  multi- 
ple electrode  types.  If  forced  to  carry  too  much  current,  however,  the 
same  frying  or  sizzling  sound  is  noticeable  as  in  the  earlier  types. 
This  is  due  to  the  heating  of  the  electrodes  and  to  small  arcs  that 
occur  between  the  electrodes  and  the  granules. 

One  way  to  increase  the  current-carrying  capacity  of  a  trans- 
mitter is  to  increase  the  area  of  its  electrodes,  but  a  limit  is  soon 
reached  in  this  direction  owing  to  the  increased  inertia  of  the  moving 
electrode,  which  necessarily  comes  with  its  larger  size. 

The  carrying  capacity  of  transmitters  may  also  be  increased  by 


TRANSMITTERS  67 

providing  special  means  for  carrying  away  the  heat  generated  in  the 
variable-resistance  medium.  Several  schemes  have  been  proposed 
for  this.  One  is  to  employ  unusually  heavy  metal  for  the  electrode 
chamber,  and  this  practice  is  best  exemplified  in  the  White  solid- 
back  instrument.  It  has  also  been  proposed  by  others  to  water- 
jacket  the  electrode  chamber,  and  also  to  keep  it  cool  by  placing  it  in 
close  proximity  to  the  relatively  cool  joints  of  a  thermopile.  Neither 
of  these  two  latter  schemes  seems  to  be  warranted  in  ordinary  com- 
mercial practice. 

Sensitiveness.  In  all  the  transmitters  so  far  discussed  damping 
springs  of  one  form  or  another  have  been  employed  to  reduce  the 
sensitiveness  of  the  instrument.  For  ordinary  commercial  use  too 
great  a  degree  of  sensitiveness  is  a  fault,  as  has  already  been  pointed 
out.  There  are,  however,  certain  adaptations  of  the  telephone  trans- 
mitter which  make  a  maximum  degree  of  sensitiveness  desirable. 
One  of  these  adaptations  is  found  in  the  telephone  equipments  for  as- 
sisting partially  deaf  people  to  hear.  In  these  the  transmitter  is  car- 
ried on  some  portion  of  the  body  of  the  deaf  person,  the  receiver  is 
strapped  or  otherwise  held  at  his  ear,  and  a  battery  for  furnishing 
the  current  is  carried  in  his  pocket.  It  is  not  feasible,  for  this  sort 
of  use,  that  the  sound  which  this  transmitter  is  to  reproduce  shall 
always  occur  immediately  in  front  of  the  transmitter.  It  more  often 
occurs  at  a  distance  of  several  feet.  For  this  reason  the  transmitter 
is  made  as  sensitive  as  possible,  and  yet  is  so  constructed  that  it  will 
not  be  caused  to  produce  too  loud  or  unduly  harsh  sounds  in  response 
to  a  loud  sound  taking  place  immediately  in  front  of  it.  Another 
adaptation  of  such  highly  sensitive  transmitters  is  found  in  the  special 
intercommunicating  telephone  systems  for  use  between  the  various 
departments  or  desks  in  business  offices.  In  these  it  is  desirable  that 
the  transmitter  shall  be  able  to  respond  adequately  to  sounds  occur- 
ring anywhere  in  a  small-sized  room,  for  instance. 

Acousticon  Transmitter.  In  Fig.  46  is  shown  a  transmitter 
adapted  for  such  use.  This  has  been  termed  by  its  makers  the 
acousticon  transmitter.  Like  all  the  transmitters  previously  dis- 
cussed, this  is  of  the  variable-resistance  type,  but  it  differs  from  them 
all  in  that  it  has  no  damping  springs ;  in  that  carbon  balls  are  substi- 
tuted for  carbon  granules;  and  in  that  the  diaphragm  itself  serves  as 
the  front  electrode. 


TELEPHONY 


This  transmitter  consists  of  a  cup  1,  into  which  is  set  a  cylin- 
drical block  2,  in  one  face  of  which  are  a  number  of  hemispherical 
recesses.  The  diaphragm  3  is  made  of  thin  carbon  and  is  so  placed 
in  the  transmitter  as  to  cover  the  openings  of  the  recesses  in  the  car- 
bon block,  and  lie  close  enough  to  the  carbon  block,  without  engag- 
ing it,  to  prevent  the  carbon  particles  from  falling  out.  The  dia- 
phragm thus  serves  as  the  front  electrode  and  the  carbon  block  as  the 
rear  electrode.  The  recesses  in  the  carbon  block  are  about  two- 
thirds  filled  with  small  carbon  balls,  which  are  about  the  size  of  fine 
sand.  The  front  piece  4  °f  the  transmitter  is  of 
,  sheet  metal  and  serves  to  hold  the  diaphragm  in 
place.  To  admit  the  sound  waves  it  is  provided 
with  a  circular  opening  opposite  to  and  about  the 
size  of  the  rear  electrode  block.  On  this  front  piece 
are  mounted  the  two  terminals  of  the  transmitter, 
connected  respectively  to  the  two  electrodes,  ter- 
minal 5  being  insulated  from  the  front  piece  and 
connected  by  a  thin  metal  strip  with  the  dia- 
phragm, while  terminal  6  is  mounted  directly  on 
the  front  piece  and  connected  through  the  cup  1 
with  the  carbon  block  2,  or  back  electrode  of  the 
transmitter. 

When  this  transmitter  is    used    in    connection 

Pig  46.    Acousticon    with  outfits  for  the  deaf,  it  is  placed  in  a  hard 
Transmitter  ,  ,  .    .    .  ...  e 

rubber  containing   case,  consisting   ot    a    hollow 

cylindrical  piece  7,  which  has  fastened  to  it  a  cover  8.  This  cover 
has  a  circular  row  of  openings  or  holes  near  its  outer  edge,  as 
shown  at  9,  through  which  the  sound  waves  may  pass  to  the  cham- 
ber within,  and  thence  find  their  way  through  the  round  hole  in  the 
center  of  the  front  plate  4  to  the  diaphragm  3.  It  is  probable  also 
that  the  front  face  of  the  cover  8  of  the  outer  case  vibrates,  and  in 
this  way  also  causes  sound  waves  to  impinge  against  the  diaphragm. 
This  arrangement  provides  a  large  receiving  surface  for  the  sound 
waves,  but,  owing  to  the  fact  that  the  openings  in  the  containing 
case  are  not  opposite  the  opening  in  the  transmitter  proper,  the 
sound  waves  do  not  impinge  directly  against  the  diaphragm.  This 
peculiar  arrangement  is  probably  the  result  of  an  endeavor  to  pre- 
vent the  transmitter  from  being  too  strongly  actuated  by  violent 


TRANSMITTERS 


69 


sounds  close  to  it.     Instruments  of  this  kind  are  very  sensitive  and 

under   proper  conditions  are  readily  responsive  to  words  spoken  in 

an  ordinary  tone  ten  feet  away. 

Switchboard  Transmitter.     Another  special   adaptation  of  the 

telephone  transmitter  is  that  for  use  of  telephone  operators  at  central- 

office  switchboards.     The  requirements  in  this  case  are  such  that  the 

operator  must  always  be  able  to  speak  into  the  transmitter  while 

seated  before  the  switchboard,  and  yet  allow  both  of  her  hands  to  be 

free    for    use.      This    was 

formerly  accomplished   by 

suspending     an     ordinary 

granular-carbon    transmit- 

ter in  front  of  the  operator, 

but  a  later  development  has 

resulted  in  the  adoption  of 

the  so-called  breast  trans- 

mitter, shown  in  Fig.  47. 

This  is  merely  an  ordinary 

granular-carbon    transmit- 

ter   mounted    on    a   plate 

which  is  strapped    on  the 

breast  of  the  operator,  the 

transmitter   being   provided  with  a  long  curved  mouthpiece  which 

projects  in  such  a  manner  as  to  lie  just  in  front  of  the  operator's 

lips.     This  device  has  the  advantage  of  automatically  following  the 

operator  in  her  movements.     The  breast  transmitter  shown  in  Fig. 

47,  is  that  of  the  Dean  Electric  Company. 

Conventional   Diagram.    There  are  several  common  ways  of 

illustrating  transmitters  in  diagrams  of  circuits  in  which  they  are 

employed.  The  three  most  com- 
mon  ways  are  shown  in  Fig.  48. 
The  one  at  the  left  is  supposed  to 
be  a  side  view  of  an  ordinarv 
instrument,  the  one  in  the  center 

a  front  view,  and  the  one  at  the  right  to  be  merely  a  suggestive 

arrangement  of  the  diaphragm  and  the    rear    electrode.    The   one 

at  the  right  is  best  and  perhaps  most  common;  the   center  one  is 

the  poorest  and  least  used. 


Pig.  47.     Switchboard  Transmitter 


™_        .01,, 
Fig.  48.     Transmitter  Symbols 


RECEIVERS 

The  telephone  receiver  is  the  device  which  translates  the  energy 
ot  the  voice  currents  into  the  energy  of  corresponding  sound  waves. 
All  telephone  receivers  today  are  of  the  electromagnetic  type,  the 
voice  currents  causing  a  varying  magnetic  pull  on  an  armature  or 
diaphragm,  which  in  turn  produces  the  sound  waves  corresponding 
to  the  undulations  of  the  voice  currents. 

Early  Receivers.  The  early  forms  of  telephone  receivers  were 
of  the  single-pole  type;  that  is,  the  type  wherein  but  one  pole  of  the 
electromagnet  was  presented  to  the  diaphragm.  The  single-pole 
receiver  that  formed  the  companion  piece  to  the  old  Blake  transmitter 
and  that  was  the  standard  of  the  Bell  companies  for  many  years,  is 
shown  in  Fig.  49.  While  this  has  almost  completely  passed  out  of 
use,  it  may  be  profitably  studied  in  order  that  a  comparison  may  be 
made  between  certain  features  of  its  construction  and  those  of  the 
later  forms  of  receivers. 

The  coil  of  this  receiver  was  wound  on  a  round  iron  core  2, 
flattened  at  one  end  to  afford  means  for  attaching  the  permanent 
magnet.  The  permanent  magnet  was  of  laminated  construction, 
consisting  of  four  hard  steel  bars  1,  extending  nearly  the  entire  length 
of  the  receiver  shell.  These  steel  bars  were  all  magnetized  separately 
and  placed  with  like  poles  together  so  as  to  form  a  single  bar  magnet. 
They  were  laid  together  in  pairs  so  as  to  include  between  the  pairs 
the  flattened  end  of  the  pole  piece  2  at  one  end  and  the  flattened 
portion  of  the  tail  piece  3  at  the  other  end.  This  whole  magnet 
structure,  including  the  core,  the  tail  piece,  and  the  permanently 
magnetized  steel  bars,  was  clamped  together  by  screws  as  shown. 
The  containing  shell  was  of  hard  rubber  consisting  of  three  pieces, 
the  barrel  4,  the  ear-piece  5,  and  the  tail  cap  6.  The  barrel  and  the 
ear  piece  engaged  each  other  by  means  of  a  screw  thread  and  served 
to  clamp  the  diaphragm  between  them.  The  compound  bar  magnet 


RECEIVERS 


71 


was  held  in  place  within  the  shell  by  means  of  a  screw  7  passing 
through  the  hard  rubber  tail  cap  6  and  into  the  tail  block  3  of  the 
magnet.  External  binding  posts  mounted  on  the  tail  cap,  as  shown, 
were  connected  by  heavy  leading-in  wires  to  the  terminals  of  the 
electromagnet. 

A  casual  consideration  of  the  magnetic  circuit  of  this  instrument 
will  show  that  it  was  inefficient,  since  the  return  path  for  the  lines  of 
force  set  up  by  the  bar  magnet  was 
necessarily  through  a  very  long  air  path. 
Notwithstanding  this,  these  receivers 
were  capable  of  giving  excellent  articula- 
tion and  were  of  marvelous  delicacy  of 
action.  A  very  grave  fault  was  that  the 
magnet  was  supported  in  the  shell  at  the 
end  farthest  removed  from  the  dia- 
phragm. As  a  result  it  was  difficult  to 
maintain  a  permanent  adjustment  be- 
tween the  pole  piece  and  the  diaphragm. 
One  reason  for  this  was  that  hard  rubber 
and  steel  contract  and  expand  under 
changes  of  temperature  at  very  different 
rates,  and  therefore  the  distance  be- 
tween the  pole  piece  and  the  diaphragm 
changed  with  changes  of  temperature. 
Another  grave  defect,  brought  about  by 
this  tying  together  of  the  permanent 
magnet  and  the  shell  which  supported 
the  diaphragm  at  the  end  farthest  from 
the  diaphragm,  was  that  any  mechanical 
shocks  were  thus  given  a  good  chance  to 
alter  the  adjustment. 

Modern  Receivers.  Receivers  of  to- 
day differ  from  this  old  single-pole  receiver  in  two  radical  respects. 
In  the  first  place,  the  modern  receiver  is  of  the  bi-polar  type,  con- 
sisting essentially  of  a  horseshoe  magnet  presenting  both  of  its  poles 
to  the  diaphragm.  In  the  second  place,  the  modern  practice  is  to 
either  support  all  of  the  working  parts  of  the  receiver,  i.  e.,  the 
magnet,  the  coils,  and  the  diaphragm,  by  an  inner  metallic  frame 


Fig.  49.     Single-Pole  Receiver 


TELEPHONY 


entirely  independent  of  the  shell;  or,  if  the  shell  is  used  as  a  part  of 
the  structure,  to  rigidly  fasten  the  several  parts  close  to  the  diaphragm 
rather  than  at  the  end  farthest  removed  from  the  diaphragm. 

Western  Electric  Receiver.  The  standard  bi-polar  receiver  of 
the  Western  Electric  Company,  in  use  by  practically  all  of  the  Bell 
operating  companies  throughout  this  country  and  in  large  use  abroad, 

is  shown  in  Fig.  50.     In  this  the  shell 
is   of   three  pieces,  consisting  of  the 
barrel  .7,  the  ear  cap  2,  and  the  tail  cap 
3.     The  tail  cap  and  the  barrel  are 
permanently  fastened  together  to  form 
substantially  a   single    piece.      Two 
permanently  magnetized  bar  magnets 
4~4  are  employed,  these  being  clamped 
together     at    their    upper    ends,    as 
shown,  so  as  to  include  the  soft  iron 
block  5  between   them.     The  north 
pole    of    one    of    these    magnets    is 
clamped  to  the  south  pole  of  the  other, 
so  that  in  reality  a  horseshoe  magnet 
is  formed.     At  their  lower  ends,  these 
two  permanent  magnets  are  clamped 
against  the  soft  iron  pole  pieces  6-6,  a 
threaded  block  7  also  being  clamped 
rigidly  between  these  pole  pieces  at 
this  point.     On  the  ends  of  the  pole 
pieces  the  bobbins  are  wound.     The 
whole   magnet    structure    is  secured 
within  the  shell  1  by  means  of  a  screw  thread  on  the  block  7  which 
engages  a  corresponding  internal  screw  thread  in  the  shell  1.     As 
a  result  of  this  construction  the  whole  magnet   structure  is  bound 
rigidly  to  the  shell  structure  at  a  point  close  to  the  diaphragm,  com- 
paratively speaking,  and  as  a  result  of  this  close  coupling,  the  rela- 
tion between  the  diaphragm  and  the  pole  piece  is  very  much  more 
rigid  and  substantial  than  in  the  case  where  the  magnet  structure 
and  the  shell  were  secured  together  at  the  end  farthest  removed  from 
the  diaphragm. 

Although  this  receiver  shown  in  Fig.  50  is  the  standard  in  use 


Fig.  50.    Western  Electric  Receiver 


RECEIVERS 


73 


by  the  Bell  companies  throughout  this  country,  its  numbers  running 
well  into  the  millions,  it  cannot  be  said  to  be  a  strictly  modern  receiver, 
because  of  at  least  one  rather  antiquated  feature.  The  binding  posts, 
by  which  the  circuit  conductors  are  led  to  the  coils  of  this  instrument, 
are  mounted  on  the  outside  of  the  receiver  shell,  as  indicated,  and  are 
thus  subject  to  danger  of  mechanical  injury  and  they  are  also  exposed 
to  the  touch  of  the  user,  so  that  he  may,  in  case  of  the  wires  being 
charged  to  an  abnormal  potential,  receive  a  shock.  Probably  a 
more  serious  feature  than  either  one  of  these  is  that  the  terminals  of  the 
flexible  cords  which  attach  to  these  binding  posts  are  attached  out- 
side of  the  receiver  shell,  and  are  therefore  exposed  to  the  wear  and 
tear  of  use,  rather  than  being  protected  as  they  should  be  within  the 
shell.  Notwithstanding  this  undesirable  feature,  this  receiver  is  a 
very  efficient  one  and  is  excellently  constructed. 

Kellogg  Receiver.  In  Fig.  51  is  shown  a  bi-polar  receiver  with 
internal  or  concealed  binding  posts.  This  particular  receiver  is 
typical  of  a  large  number  of  similar 
kinds  and  is  manufactured  by  the  Kel- 
logg Switchboard  and  Supply  Company. 
Two  straight  permanently  magnetized 
bar  magnets  1-1  are  clamped  together  at 
their  opposite  ends  so  as  to  form  a  horse- 
shoe magnet.  At  the  end  opposite  the 
diaphragm  these  bars  clamp  between 
them  a  cylindrical  piece  of  iron  2,  so  as 
to  complete  the  magnetic  circuit  at  the 
end.  At  the  end  nearest  the  diaphragm 
they  clamp  between  them  the  ends  of  the 
soft  iron  pole  pieces  3-3,  and  also  a  block 
of  composite  metal  4  having  a  large  cir- 
cular flange  4'  which  serves  as  a  means 
for  supporting  the  magnet  structure  with- 
in the  shell.  The  screws  by  means  of 
which  the  disk  4'  is  clamped  to  the  shoul- 
dered seat  in  the  shell  do  not  enter  the 
shell  directly,  but  rather  enter  screw-threaded  brass  blocks  which  are 
moulded  into  the  structure  of  the  shell.  It  is  seen  from  this  con- 
struction that  the  diaphragm  and  the  pole  pieces  and  the  magnet 


Fig.  51.     Kellogg  Receiver 


74  TELEPHONY 

structure  itself  are  all  rigidly  secured  together  through  the  medium 
of  the  shell  at  a  point  as  close  as  possible  to  the  diaphragm. 

Between  the  magnets  1-1  there  is  clamped  an  insulating  block 
5,  to  which  are  fastened  the  terminal  plates  6,  one  on  each  side  of  the 
receiver.  These  terminal  plates  are  thoroughly  insulated  from  the 
magnets  themselves  and  from  all  other  metallic  parts  by  means  of 
sheets  of  fiber,  as  indicated  by  the  heavy  black  lines.  On  these 
plates  6  are  carried  the  binding  posts  for  the  receiver  cord  terminals. 
A  long  tongue  extends  from  each  of  the  plates  6  through  a  hole  in  the 
disk  4',  into  the  coil  chamber  of  the  receiver,  at  which  point  the  ter- 
minal of  the  magnet  winding  is  secured  to  it.  This  tongue  is  insu- 
lated from  the  disk  4',  where  it  passes  through  it,  by  means  of  in- 
sulating bushing,  as  shown.  The  other  terminal  of  the  magnet 
coils  is  brought  out  to  the  other  plate  6  by  means  of  a  similar  tongue 
on  the  other  side. 

In  order  that  the  receiver  terminals  proper  may  not  be  sub- 
jected to  any  strain  in  case  the  receiver  is  dropped  and  its  weight 
caught  on  the  receiver  cord,  a  strain  loop  is  formed  as  a  continuation 
of  the  braided  covering  of  the  receiver  cord,  and  this  is  tied  to  the 
permanent  magnet  structure,  as  shown.  By  making  this  strain 
loop  short,  it  is  obvious  that  whatever  pull  the  cord  receives  will  not 
be  taken  by  the  cord  conductors  leading  to  the  binding  posts  or  by 
the  binding  posts  or  the  cord  terminals  themselves. 

A  number  of  other  manufacturers  have  gone  even  a  step  further 
than  this  in  securing  permanency  of  adjustment  between  the  re- 
ceiver diaphragm  and  pole  pieces.  They  have  done  this  by  not  de- 
pending at  all  on  the  hard  rubber  shell  as  a  part  of  the  structure,  but 
by  enclosing  the  magnet  coil  in  a  cup  of  metal  upon  which  the  dia- 
phragm is  mounted,  so  that  the  permanency  of  relation  between  the 
diaphragm  and  the  pole  pieces  is  dependent  only  upon  the  metallic 
structure  and  not  at  all  upon  the  less  durable  shell. 

Direct=Current  Receiver.  Until  about  the  middle  of  the  year 
1909,  it  was  the  universal  practice  to  employ  permanent  magnets  for 
giving  the  initial  polarization  to  the  magnet  cores  of  telephone  re- 
ceivers. This  is  still  done,  and  necessarily  so,  in  receivers  employed 
in  connection  with  magneto  telephones.  In  common-battery  systems, 
however,  where  the  direct  transmitter  current  is  fed  from  the  central 
office  to  the  local  stations,  it  has  been  found  that  this  current  which 


RECEIVERS 


75 


must  flow  at  any  rate  through  the  line  may  be  made  to  serve  the 
additional  purpose  of  energizing  the  receiver  magnets  so  as  to  give 
them  the  necessary  initial  polarity.  A  type  of  receiver  has  come 
into  wide  use  as  a  result,  which  is  com- 
monly called  the  direct-current  receiver, 
deriving  its  name  from  the  fact  that  it 
employs  the  direct  current  that  is  flowing 
in  the  common-battery  line  to  magnetize 
the  receiver  cores.  The  Automatic  Elec- 
tric Company,  of  Chicago,  was  probably 
the  first  company  to  adopt  this  form  of  re- 
ceiver as  its  standard  type.  Their  re- 
ceiver is  shown  in  cross-section  in  Fig.  52, 
and  a  photograph  of  the  same  instrument 
partially  disassembled  is  given  in  Fig.  53. 
The  most  noticeable  thing  about  the  con- 
struction of  this  receiver  is  the  absence  of 
permanent  magnets.  The  entire  working 
parts  are  contained  within  the  brass  cup 
1,  which  serves  not  only  as  a  container 
for  the  magnet,  but  also  as  a  seat  for  the 

Pig.  52.     Automatic  Electric 

diaphragm.      This    receiver    is    therefore        Company  Direct-Current 

Receiver 

illustrative  of  the  type  mentioned  above, 

wherein  the  relation  between  the  diaphragm  and  the  pole  pieces  is 

not  dependent  upon  any  connection  through  the  shell. 


Pig.  53.     Automatic  Electric  Company  Direct-Current  Receiver 

The  coil  of  this  instrument  consists  of  a  single  cylindrical  spool 
2,  mounted  on  a  cylindrical  core.  This  bobbin  lies  within  a  soft 
iron-punching  3,  the  form  of  which  is  most  clearly  shown  in  Fig.  53, 


76  TELEPHONY 

and  this  punching  affords  a  return  path  to  the  diaphragm  for  the 
lines  of  force  set  up  in  the  magnet  core.  Obviously  a  magnetizing 
current  passing  through  the  winding  of  the  coil  will  cause  the  end  of  the 
core  toward  the  diaphragm  to  be  polarized,  say  positively, while  the  end 
of  the  enclosing  shell  will  be  polarized  in  the  other  polarity,  nega- 
tively. Both  poles  of  the  magnet  are  therefore  presented  to  the 
diaphragm  and  the  only  air  gap  in  the  magnetic  circuit  is  that  between 
the  diaphragm  and  these  poles.  The  magnetic  circuit  is  therefore 
one  of  great  efficiency,  since  it  consists  almost  entirely  of  iron,  the 
only  air  gap  being  that  across  which  the  attraction  of  the  diaphragm 
is  to  take  place. 

The  action  of  this  receiver  will  be  understood  when  it  is  stated 
that  in  common-battery  practice,  as  will  be  shown  in  later  chapters, 
a  steady  current  flows  over  the  line  for  energizing  the  transmitter. 
On  this  current  is  superposed  the  incoming  voice  currents  from  a 
distant  station.  The  steady  current  flowing  in  the  line  will,  in  the 
case  of  this  receiver,  pass  through  the  magnet  winding  and  establish 
a  normal  magnetic  field  in  the  same  way  as  if  a  permanent  magnet 
were  employed.  The  superposed  incoming  voice  currents  will  then 
be  able  to  vary  this  magnetic  field  in  exactly  the  same  way  as  in  the 
ordinary  receiver. 

An  astonishing  feature  of  this  recent  development  of  the  so- 
called  direct-current  receiver  is  that  it  did  not  come  into  use  until  after 
about  twenty  years  of  common-battery  practice.  There  is  nothing 
new  in  the  principles  involved,  as  all  of  them  were  already  under- 
stood and  some  of  them  were  employed  by  Bell  in  his  original  tele- 
phone; in  fact,  the  idea  had  been  advanced  time  and  again,  and 
thrown  aside  as  not  being  worth  consideration.  This  is  an  illus- 
tration of  a  frequent  occurrence  in  the  development  of  almost  any 
rapidly  growing  art.  Ideas  that  are  discarded  as  worthless  in  the 
early  stages  of  "the  art  are  finally  picked  up  and  made  use  of.  The 
reason  for  this  is  that  in  some  cases  the  ideas  come  in  advance  of  the 
art,  or  they  are  proposed  before  the  art  is  ready  to  use  them.  In 
other  cases  the  idea  as  originally  proposed  lacked  some  small  but 
essential  detail,  or,  as  is  more  often  the  case,  the  experimenter  in 
the  early  days  did  not  have  sufficient  skill  or  knowledge  to  make  it 
fit  the  requirements  as  he  saw  them. 

Monarch   Receiver.    The   receiver  of   the  Automatic  Electric 


RECEIVERS 


77 


Company  just  discussed  employs  but  a  single  electromagnet  by  which 
the  initial  magnetization  of  the  cores  and  also  the  variable  magnet- 
ization necessary  for  speech  reproduction  is  secured.  The  problem 
of  the  direct-current  receiver  has  been  attacked  in  another  way 
by  Ernest  E.  Yaxley,  of  the  Monarch  Telephone  Manufacturing 
Company,  with  the  result  shown  in  Fig.  54.  The  construction  in 
this  case  is  not  unlike  that  of  an  ordinary  permanent-magnet  receiver, 
except  that  in  the  place  of  the  permanent 
magnets  two  soft  iron  cores  1-1  are  em- 
ployed. On  these  are  wound  two  long 
bobbins  of  insulated  wire  so  that  the  direct 
current  flowing  over  the  telephone  line  will 
pass  through  these  and  magnetize  the  cores 
to  the  same  degree  and  for  the  same  pur- 
pose as  in  the  case  of  permanent  magnets. 
These  soft  iron  magnet  cores  1-1  continue 
to  a  point  near  the  coil  chamber,  where 
they  join  the  two  soft  iron  pole  pieces  2-2, 
upon  which  the  ordinary  voice-current  coils 
are  wound.  The  two  long  coils  4~4>  which 
may  be  termed  the  direct-current  coils,  are 
of  somewhat  lower  resistance  than  the  two 
voice-current  coils  3-3.  They  are,  however, 
by  virtue  of  their  greater  number  of  turns 
and  the  greater  amount  of  iron  that  is  in- 
cluded in  their  cores,  of  much  higher  im- 
pedance than  the  voice-current  coils  3-3. 

These  two  sets  of  coils  4~4  and  3-3  are  connected  in  multiple.  As 
a  result  of  their  lower  ohmic  resistance  the  coils  4~4  will  take  a 
greater  amount  of  the  steady  current  which  comes  over  the  line, 
and  therefore  the  greater  proportion  of  the  steady  current  will  be 
employed  in  magnetizing  the  bar  magnets.  On  account  of  their 
higher  impedance  to  alternating  currents,  however,  nearly  all  of  the 
voice  currents  which  are  superposed  on  the  steady  currents,  flowing 
in  the  line  will  pass  through  the  voice-current  coils  3-3,  and,  being 
near  the  diaphragm,  these  currents  will  so  vary  the  steady  magnet- 
ism in  the  cores  2-2  as  to  produce  the  necessary  vibration  of  the 
diaphragm. 


Fig.  54.     Monarch  Direct- 
Current  Receiver 


78 


TELEPHONY 


This  receiver,  like  the  one  of  the  Automatic  Electric  Company, 
does  not  rely  on  the  shell  in  any  respect  to  maintain  the  permanency 
of  relation  between  the  pole  pieces  and  the  diaphragm.  The  cup  5, 
which  is  of  pressed  brass,  contains  the  voice-current  coils  and  also 
acts  as  a  seat  for  the  diaphragm.  The  entire  working  parts  of  this 
receiver  may  be  removed  by  merely  unscrewing  the  ear  piece  from 
the  hard  rubber  shell,  thus  permitting  the  whole  works  to  be  with- 
drawn in  an  obvious  manner. 

Dean  Receiver.  Of  such  decided  novelty  as  to  be  almost  revo- 
lutionary in  character  is  the  receiver  recently  put  on  the  market  by 

the  Dean  Electric  Company  and  shown  in 
Fig.  55.  This  receiver  is  of  the  direct- 
current  type  and  employs  but  a  single 
cylindrical  bobbin  of  wire.  The  core  of 
this  bobbin  and  the  return  path  for  the 
magnetic  lines  of  force  set  up  in  it  are 
composed  of  soft  iron  punchings  of  sub- 
stantially E  shape.  These  punchings  are 
laid  together  so  as  to  form  a  laminated 
soft-iron  field,  the  limbs  of  which  are  about 
square  in  cross-section.  The  coil  is  wound 
on  the  center  portion  of  this  E  as  a  core, 
the  core  being,  as  stated,  approximately 
square  in  cross-section.  The  general  form 
of  magnetic  circuit  in  this  instrument  is 
therefore  similar  to  that  of  the  Automatic 
Electric  Company's  receiver,  shown  in 
Figs.  52  and  53,  but  the  core  is  laminated 
instead  of  being  solid  as  in  that  instru- 
ment. 

The  most  unusual  feature  of  this  Dean  receiver  is  that  the  use 
of  hard  rubber  or  composition  does  not  enter  into  the  formation  of 
the  shell,  but  instead  a  shell  composed  entirely  of  steel  stampings 
has  been  substituted  therefor.  The  main  portion  of  this  shell  is  the 
barrel  1,  Great  skill  has  evidently  been  exercised  in  the  forming 
of  this  by  the  cold-drawn  process,  it  presenting  neither  seams  nor 
welds.  The  ear  piece  2  is  also  formed  of  steel  of  about  the  same 
gauge  as  the  barrel  1.  Instead  of  screw-threading  the  steel  parts, 


Fig.  55.     Dean  Steel  Shell 
Receiver 


RECEIVERS  79 

so  that  they  would  directly  engage  each  other,  the  ingenious  device 
has  been  employed  of  swaging  a  brass  ring  3  in  the  barrel  portion  and 
a  similar  brass  ring  4  in  the  ear  cap  portion,  these  two  being  slotted 
and  keyed,  as  shown  at  8,  so  as  to  prevent  their  turning  in  their  re- 
spective seats.  The  ring  3  is  provided  with  an  external  screw  thread 
and  the  ring  4  with  an  internal  screw  thread,  so  that  the  receiver  cap 
is  screwed  on  to  the  barrel  in  the  same  way  as  in  the  ordinary  rubber 
shell.  By  the  employment  of  these  brass  screw-threaded  rings,  the 
rusting  together  of  the  parts  so  that  they  could  not  be  separated 
when  required — a  difficulty  heretofore  encountered  in  steel  construc- 
tion of  similar  parts — has  been  remedied. 

The  entire  working  parts  of  this  receiver  are  contained  within 
the  cup  5,  the  edge  of  which  is  flanged  outwardly  to  afford  a  seat  for 


Pig.   56.     Working  Parts  of  Dean  Receiver 

the  diaphragm.  The  diaphragm  is  locked  in  place  on  the  shell  by  a 
screw-inreaded  ring  6,  as  is  clearly  indicated.  A  ring  7  of  insulating 
material  is  seated  within  the  enlarged  portion  of  the  barrel  1,  and 
against  this  the  flange  of  the  cup  5  rests  and  is  held  in  place  by  the  cap 
2  when  it  is  screwed  home.  The  working  parts  of  this  receiver  par- 
tially disassembled  are  shown  in  Fig.  56,  which  gives  a  clear  idea 
of  some  of  the  features  not  clearly  illustrated  in  Fig.  55. 

It  cannot  be  denied  that  one  of  the  principal  items  of  mainte- 
nance of  subscribers'  station  equipment  has  been  due  to  the  breakage 
of  receiver  shells.  The  users  frequently  allow  their  receiver  to  fall  and 
strike  heavily  against  the  wall  or  floor,  thus  not  only  subjecting  the 
cords  to  great  strain,  but  sometimes  cracking  or  entirely  breaking 
the  receiver  shell.  The  innovation  thus  proposed  by  the  Dean  Com- 


80 


TELEPHONY 


pany  of  making  the  entire  receiver  shell  of  steel  is  of  great  interest. 
The  shell,  as  will  be  seen,  is  entirely  insulated  from  the  circuit  of  the 
receiver  so  that  no  contact  exists  by  which  a  user  could  receive  a 
shock.  The  shell  is  enameled  inside  and  out  with  a  heavy  black 
insulating  enamel  baked  on,  and  said  to  be  of  great  durability.  How 
this  enamel  will  wear  remains  to  be  seen.  The  insulation  of  the  in- 
terior portions  of  the  receiver  is  further  guarded  by  providing  a 
lining  of  fiber  within  the  shell  at  all  points  where  it  seems 
possible  that  a  cross  could  occur  between  some  of  the  work- 
ing parts  and  the  metal  of  the  shell.  This  type  of  receiver  has 
not  been  on  the  market  long  enough  to  draw  definite  conclusions, 
based  on  experience  in  use,  as  to  what  its  permanent  performance 
will  be. 

Thus  far  in  this  chapter  only  those  receivers  which  are  commonly 
called  hand  receivers  have  been  discussed.  These  are  the  receiv- 
ers that  are  ordinarily  employed  by  the  general  public. 

Operator's  Receiver.  At  th^  central  office  in  telephone  ex- 
changes the  operators  are  provided  with  receivers  in  order  that  they 


Fig.  57.     Operator's  Receiver 

may  communicate  with  the  subscribers  or  with  other  operators.  In 
order  that  they  may  have  both  of  their  hands  free  to  set  up  and  take 
down  the  connections  and  to  perform  all  of  the  switching  operations 
required,  a  special  form  of  receiver  is  employed  for  this  purpose, 
which  is  worn  as  a  part  of  a  head-gear  and  is  commonly  termed  a 
head  receiver,  These  are  necessarily  of  very  light  construction,  in 


RECEIVERS 


order  not  to  be  burdensome  to  the  operators,  and  obviously  they  must 
be  efficient.  They  are  ordinarily  held  in  place  at  the  ear  by  a  metallic 
head  band  fitting  over  the  head  of  the  operator. 

Such  a  receiver  is  shown  in  cross-section  in  Fig.  57,  and  com- 
pletely assembled  with  its  head  band  in  Fig.  58.  Referring  to  Fig. 
57  the  shell  1  of  the  receiver  is  of  aluminum  and  the  magnets  are 
formed  of  steel  rings  2,  cross- 
magnetized  so  as  to  present  a 
north  pole  on  one  side  of  the  ring 
and  a  south  pole  on  the  other. 
The  two  L-shaped  pole  pieces  3 
are  secured  by  screws  to  the 
poles  of  these  ring  magnets,  and 
these  pole  pieces  carry  the  mag- 
net coils,  as  is  clearly  indicated. 

These  poles  are  presented  to  a  soft  iron  diaphragm  in  exactly 
the  same  way  as  in  the  larger  hand  receivers,  the  diaphragm 
being  clamped  in  place  by  a  hard  rubber  ear  piece,  as  shown.  The 
head  bands  are  frequently  of  steel  covered  with  leather.  They  have 
assumed  numerous  forms,  but  the  general  form  shown  in  Fig.  58  is 
the  one  commonly  adopted. 

Conventional  Symbols.  The  usual  diagrammatic  symbols  for 
hand  and  head  receivers  are  shown  in  Fig.  59.  They  are  self-ex- 
planatory. The  symbol  at  the  left  in  this  figure,  showing  the  general 


Fig.  58.     Operator's  Receiver  and  Cord 


Fig.  59.     Receiver  Symbols 

outline  of  the  receiver,  is  the  one  most  commonly  used  where  any 
sort  of  a  receiver  is  to  be  indicated  in  a  circuit  diagram,  but  where 
it  becomes  desirable  to  indicate  in  the  diagram  the  actual  connec- 
tions with  the  coil  or  coils  of  the  receiver,  the  symbol  shown  at  the 
right  is  to  be  preferred,  and  obviously  it  may  be  modified  as  to 
number  of  windings  and  form  of  core  as  desired. 


CHAPTER  VII 
PRIMARY  CELLS 

Galvani,  an  Italian  physician,  discovered,  in  1786,  that  a  cur- 
rent of  electricity  could  be  produced  by  chemical  action.  In  1800, 
Volta,  a  physicist,  also  an  Italian,  threw  further  light  on  Galvani's 
discovery  and  produced  what  we  know  as  the  voltaic,  or  galvanic, 
cell.  In  honor  of  these  two  discoverers  we  have  the  words  volt,  gal- 
vanic, and  the  various  words  and  terms  derived  therefrom. 

Simple  Voltaic  Cell.  A  very  simple  voltaic  cell  may  be  made  by 
placing  two  plates,  one  of  copper  and  one  of  zinc,  in  a  glass  vessel 
partly  filled  with  dilute  sulphuric  acid,  as  shown  in  Fig.  60.  When 
the  two  plates  are  not  connected  by  a  wire  or  other  conductor,  ex- 
periment shows  that  the  copper  plate  bears  a  positive  charge  with 
respect  to  the  zinc  plate,  and  the  zinc  plate  bears  a  negative  charge 
with  respect  to  the  copper.  When  the  two  plates  are  connected  by 
a  wire,  a  current  flows  from  the  copper  to  the  zinc  plate  through  the 
metallic  path  of  the  wire,  just  as  is  to  be  expected  when  any  conductor 
of  relatively  high  electrical  potential  is  joined  to  one  of  relatively 
low  electrical  potential.  Ordinarily,  when  one  charged  body  is  con- 
nected to  another  of  different  potential,  the  resulting  current  is  of  but 
momentary  duration,  due  to  the  redistribution  of  the  charges  and 
consequent  equalization  of  potential.  In  the  case  of  the  simple  cell, 
however,  the  current  is  continuous,  showing  that  some  action  is 
maintaining  the  charges  on  the  two  plates  and  therefore  maintaining 
the  difference  of  potential  between  them.  The  energy  of  this  cur- 
rent is  derived  from  the  chemical  action  of  the  acid  on  the  zinc.  The 
cell  is  in  reality  a  sort  of  a  zinc-burning  furnace. 

In  the  action  of  the  cell,  when  the  two  plates  are  joined  by  a 
wire,  it  may  be  noticed  that  the  zinc  plate  is  consumed  and  that 
bubbles  of  hydrogen  gas  are  formed  on  the  surface  of  the  copper 
plate. 


PRIMARY  CELLS 


83 


Theory.  Just  why  or  how  chemical  action  in  a  voltaic  cell 
results  in  the  production  of  a  negative  charge  on  the  consumed  plate 
is  not  known.  Modern  theory  has  it  that  when  an  acid  is  diluted  in 
water  the  molecules  of  the  acid  are  split  up  or  dissociated  into  two 
oppositely  charged  atoms,  or  groups  of  atoms,  one  bearing  a  positive 
charge  and  the  other  a  negative  charge  of  electricity.  Such  charged 
atoms  or  groups  of  atoms  are  called 
ions.  This  separation  of  the  mole- 
cules of  a  chemical  compound  into 
positively  and  negatively  charged  ions 
is  called  dissociation. 

Thus,  in  the  simple  cell  under 
consideration  the  sulphuric  acid,  by 
dissociation,  splits  up  into  hydrogen 
ions  bearing  positive  charges,  and 
SO4  ions  bearing  negative  charges. 
The  solution  as  a  whole  is  neutral 
in  potential,  having  an  equal  num- 
ber of  equal  and  opposite  charges. 

It  is  known  that  when  a  metal  is 
being  dissolved  by  an  acid,  each  atom 
of  the  metal  which  is  torn  off  by  the 
solution  leaves  the  metal  as  a  posi- 
tively charged  ion.  The  carrying 
away  of  positive  charges  from  a 
hitherto  neutral  body  leaves  that  body  with  a  negative  charge. 
Hence  the  zinc,  or  consumed  plate,  becomes  negatively  charged. 

In  the  chemical  attack  of  the  sulphuric  acid  on  the  zinc,  the 
positive  hydrogen  ions  are  liberated,  due  to  the  affinity  of  the  nega- 
tive SO4  ions  for  the  positive  zinc  ions,  this  resulting  in  the  formation 
of  zinc  sulphate  in  the  solution.  Now  the  solution  itself  becomes 
positively  charged,  due  to  the  positive  charges  leaving  the  zinc  plate 
with  the  zinc  ions,  and  the  free  positively  charged  hydrogen  ions 
liberated  in  the  solution  as  just  described  are  repelled  to  the  copper 
plate,  carrying  their  positive  charges  thereto.  Hence  the  copper 
plate,  or  the  unconsumed  plate,  becomes  positively  charged  and  also 
coated  with  hydrogen  bubbles. 

The  plates  or  electrodes  of  a  voltaic  cell  need  not  consist  of  zinc 


Fig.  60.     Simple  Voltaic  Cell 


84  TELEPHONY 

and  copper,  nor  need  the  fluid,  called  the  electrolyte,  be  of  sulphuric 
acid;  any  two  dissimilar  elements  immersed  in  an  electrolyte  that 
attacks  one  of  them  more  readily  than  the  other  will  form  a  voltaic 
cell.  In  every  such  cell  it  will  be  found  that  one  of  the  plates  is  con- 
sumed, and  that  on  the  other  plate  some  element  is  deposited,  this 
element  being  sometimes  a  gas  and  sometimes  a  solid.  The  plate 
which  is  consumed  is  always  the  negative  plate,  and  the  one  on  which 
the  element  is  deposited  is  always  the  positive,  the  current  through 
the  connecting  wire  always  being,  therefore,  from  the  unconsumed  to 
the  consumed  plate.  Thus,  in  the  simple  copper-zinc  cell  just  con- 
sidered, the  zinc  is  consumed,  the  element  hydrogen  is  deposited  on 
the  copper,  and  the  current  flow  through  the  external  circuit  is  from 
the  copper  to  the  zinc. 

The  positive  charges,  leaving  the  zinc,  or  consumed,  plate,  and 
passing  through  the  electrolyte  to  the  copper,  or  unconsumed, 
plate,  constitute  in  effect  a  current  of  electricity  flowing  within  the 
electrolyte.  The  current  within  the  cell  passes,  therefore,  from  the 
zinc  plate  to  the  copper  plate.  The  zinc  is,  therefore,  said  to  be 
positive  with  respect  to  the  copper. 

Difference  of  Potential.  The  amount  of  electromotive  force 
that  is  generated  between  two  dissimilar  elements  immersed  in  an 
electrolyte  is  different  for  different  pairs  of  elements  and  for  differ- 
ent electrolytes.  For  a  given  electrolyte  each  element  bears  a  certain 
relation  to  another;  i.  e.,  they  are  either  electro-positive  or  electro- 
negative relative  to  each  other.  In  the  following  list  a  group  of 
elements  are  arranged  with  respect  to  the  potentials  which  they 
assume  with  respect  to  each  other  with  dilute  sulphuric  acid  as  the 
electrolyte.  The  most  electro-positive  elements  are  at  the  top  and 
the  most  electro-negative  at  the  bottom. 

+  Sodium  Lead  Copper 

Magnesium  Iron  Silver 

Zinc  Nickel  Gold 

Cadmium  Bismuth  Platinum 

Tin  Antimony  —Graphite  (Carbon) 

Any  two  elements  selected  from  this  list  and  immersed  in  dilute 
sulphuric  acid  will  form  a  voltaic  cell,  the  amount  of  difference  of 
potential,  or  electromotive  force,  depending  on  the  distance  apart  in 
this  series  of  the  two  elements  chosen.  The  current  within  the  cell 


PRIMARY  CELLS 


85 


will  always  flow  from  the  one  nearest  the  top  of  the  list  to  the  one 
nearest  the  bottom,  i.  c.,  from  the  most  electro-positive  to  the  most 
electro-negative;  and,  therefore,  the  current  in  the  wire  joining  the 
two  plates  will  flow  from  the  one  lowest  down  in  the  list  to  the  one 
highest  up. 

From  this  series  it  is  easy  to  see  why  zinc  and  copper,  and  also 
zinc  and  carbon,  are  often  chosen  as  elements  of  voltaic  cells.  They 
are  widely  separated  in  the  series  and  comparatively  cheap. 

This  series  may  not  be  taken  as  correct  for  all  electrolytes,  for 
different  electrolytes  alter  somewhat  the  order  of  the  elements  in  the 
series.  Thus,  if  two  plates,  one  of  iron  and  the  other  of  copper,  are 
immersed  in  dilute  sulphuric  acid,  a  current  is  set  up  which  proceeds 
through  the  liquid  from  the  iron  to  the  copper;  but,  if  the  plates 
after  being  carefully  washed  are  placed  in  a  solution  of  potassium 
sulphide,  a  current  is  produced  in  the  opposite  direction.  The  cop- 
per is  now  the  positive  element. 

Table  II  shows  the  electrical  deportment  of  the  principal  metals 
in  three  different  liquids.  It  is  arranged  like  the  preceding  one, 
each  metal  being  electro-positive  to  any  one  lower  in  the  list. 

TABLE  II 
Behavior  of  Metals  in  Different  Electrolytes 


CAUSTIC  POTASH 

HYDROCHLORIC  ACID 

POTASSIUM  SULPHIDE 

+  Zinc 

+  Zinc 

+  Zinc 

Tin 
Cadmium 

Cadmium 
Tin 

Copper 
Cadmium 

Antimony 
Lead 

Lead 
Iron 

Tin 

Silver 

Bismuth 
Iron 

Copper 

Bismuth 

Antimony 
Lead 

Copper 
Nickel 

Nickel 

Silver 

Bismuth 
Nickel 

—  Silver 

—  Antimony 

—  Iron 

It  is  important  to  remember  that  in  all  cells,  no  matter  what 
elements  or  what  electrolyte  are  used,  the  electrode  which  is  con- 
sumed is  the  one  that  becomes  negatively  charged  and  its  terminal, 
therefore,  becomes  the  negative  terminal  or  pole,  while  the  electrode 
which  is  not  consumed  is  the  one  that  becomes  positively  charged,  and 
its  terminal  is,  therefore,  the  positive  terminal  or  pole  of  the  cell.  How- 


86  TELEPHONY 

ever,  because  the  current  in  the  electrolyte  flows  from  the  consumed 
plate  to  the  unconsumed  plate,  the  consumed  plate  is  called  the  posi- 
tive plate  and  the  unconsumed,  the  negative.  This  is  likely  to  be- 
come confusing,  but  if  one  remembers  that  the  active  plate  is  the 
positive  plate,  because  it  sends  forth  positive  ions  in  the  electrolyte, 
and,  therefore,  itself  becomes  negatively  charged,  one  will  have  the 
proper  basis  always  to  determine  the  direction  of  the  current  flow, 
which  is  the  important  thing. 

Polarization.  If  the  simple  cell  already  described  have  its  ter- 
minals connected  by  a  wire  for  some  time,  it  will  be  found  that  the 
current  rapidly  weakens  until  it  ceases  to  be  manifest.  This  weak- 
ening results  from  two  causes:  first,  the  hydrogen  gas  which  is  lib- 
erated in  the  action  of  the  cell  is  deposited  in  a  layer  on  the  copper 
plate,  thereby  covering  the  plate  and  reducing  the  area  of  contact 
with  the  liquid.  This  increases  the  internal  resistance  of  the  cell, 
since  hydrogen  is  a  non-conductor.  Second,  the  plate  so  covered 
becomes  in  effect  a  hydrogen  electrode,  and  hydrogen  stands  high  as 
an  electro-positive  element.  There  is,  therefore,  actual  reduction  in 
the  electromotive  force  of  the  cell,  as  well  as  an  increase  in  internal 
resistance.  This  phenomenon  is  known  as  polarization,  and  in  com- 
mercial cells  means  must  be  taken  to  prevent  such  action  as  far  as 
possible. 

The  means  by  which  polarization  of  cells  is  prevented  or  reduced 
in  practice  may  be  divided  into  three  general  classes : 

First — mechanical  means.  If  the  hydrogen  bubbles  be  simply  brushed 
away  from  the  surface  of  the  electrode  the  resistance  and  the  counter  polarity 
which  they  cause  will  be  diminished.  The  same  result  may  be  secured  if  air 
be  blown  into  the  solution  through  a  tube,  or  if  the  liquid  be  kept  agitated. 
If  the  surface  of  the  electrode  be  roughened  or  covered  with  points,  the  bub- 
bles collect  more  freely  at  the  points  and  are  more  quickly  carried  away  to 
the  surface  of  the  liquid.  These  means  are,  however,  hardly  practical  except 
in  cells  for  laboratory  use. 

Second — chemical  means.  If  a  highly  oxidizing  substance  be  added  to 
the  electrolyte,  it  will  destroy  the  hydrogen  bubbles  by  combining  with  them 
while  they  are  in  a  nascent  state,  and  this  will  prevent  the  increase  in  internal 
resistance  and  the  opposing  electromotive  force.  Such  substances  are  bi- 
chromate of  potash,  nitric  acid,  and  chlorine,  and  are  largely  used. 

Third — electro-chemical  means.  Double  cells,  arranged  to  separate  the 
elements  and  liquids  by  means  of  porous  partitions  or  by  gravity,  may  be  so 
arranged  that  solid  copper  is  liberated  instead  of  hydrogen  at  a  point  where  the 
current  leaves  the  liquid,  thereby  entirely  obviating  polarization.  This 
method  also  is  largely  used. 


PRIMARY  CELLS  87 

Local  Action.  When  a  simple  cell  stands  idle,  *'.  e.,  with  its 
circuit  open,  small  hydrogen  bubbles  may  be  noticed  rising  from 
the  zinc  electrode  instead  of  from  copper,  as  is  the  case  where  the 
circuit  is  closed.  This  is  due  to  impurities  in  the  zinc  plate,  such  as 
particles  of  iron,  tin,  arsenic,  carbon,  etc.  Each  of  these  particles 
acts  with  the  surrounding  zinc  just  as  might  be  expected  of  any 
pair  of  dissimilar  elements  opposed  to  each  other  in  an  electrolyte; 
in  other  words,  they  constitute  small  voltaic  cells.  Local  currents, 
therefore,  are  generated,  circulating  between  the  two  adjacent  metals, 
and,  as  a  result,  the  zinc  plate  and  the  electrolyte  are  needlessly 
wasted  and  the  general  condition  of  the  cell  is  impaired.  This  is 
called  local  action. 

Amalgamated  Zincs,  Local  action  might  be  prevented  by  the 
use  of  chemically  pure  zinc,  but  this,  on  account  of  its  expense, 
cannot  be  employed  commercially.  Local  action,  however,  may  be 
overcome  to  a  great  extent  by  amalgamating  the  zinc,  i.  e.,  coating 
it  with  mercury.  The  iron  particles  or  other  impurities  do  not  dis- 
solve in  the  mercury,  as  does  the  zinc,  but  they  float  to  the  sur- 
face, whence  the  hydrogen  bubbles  which  may  form  speedily  carry 
them  off,  and,  in  other  cases,  the  impurities  fall  to  the  bottom 
of  the  cell.  As  the  zinc  in  the  pasty  amalgam  dissolves  in  the 
acid,  the  film  of  mercury  unites  with  fresh  zinc,  and  so  always  pre- 
sents a  clear,  bright,  homogeneous  surface  to  the  action  of  the 
electrolyte. 

The  process  of  amalgamating  the  zinc  may  be  performed  by 
dipping  it  in  a  solution  composed  of 

Nitric  Acid 1  Ib. 

Muriatic  Acid 2  Ibs. 

Mercury 8  oz. 

The  acids  should  be  first  mixed  and  then  the  mercury  slowly  added 
until  dissolved.  Clean  the  zinc  with  lye  and  then  dip  it  in  the 
solution  for  a  second  or  two.  Rinse  in  clean  water  and  rub  with 
a  brush. 

Another  method  of  amalgamating  zincs  is  to  clean  them  by  dip- 
ping them  in  dilute  sulphuric  acid  and  then  in  mercury,  allowing 
the  surplus  to  drain  off. 

Commercial  zincs,  for  use  in  voltaic  cells  as  now  manufactured, 


88  TELEPHONY 

usually  have  about  4  per  cent  of  mercury  added  to  the  molten  zinc 
before  casting  into  the  form  of  plates  or  rods. 

Series  and  Multiple  Connections.  When  a  number  of  voltaic 
cells  are  joined  in  series,  the  positive  pole  of  one  being  connected  to 
the  negative  pole  of  the  next  one,  and  so  on  throughout  the  series, 
the  electromotive  forces  of  all  the  cells  are  added,  and  the  electromo- 
tive force  of  the  group,  therefore,  becomes  the  sum  of  the  electromo- 
tive forces  of  the  component  cells.  The  currents  through  all  the  cells 
in  this  case  will  be  equal  to  that  of  one  cell. 

If  the  cells  be  joined  in  multiple,  the  positive  poles  all  being 
connected  by  one  wire  and  the  negative  poles  by  another,  then  the 
currents  of  all  the  cells  will  be  added  while  the  electromotive  force 
of  the  combination  remains  the  same  as  that  of  a  single  cell,  as- 
suming all  the  cells  to  be  alike  in  electromotive  force. 

Obviously  combinations  of  these  two  arrangements  may  be  made, 
as  by  forming  strings  of  cells  connected  in  series,  and  connecting  the 
strings  in  multiple  or  parallel. 

The  term  battery  is  frequently  applied  to  a  single  voltaic  cell, 
but  this  term  is  more  properly  used  to  designate  a  plurality  of  cells 
joined  together  in  series,  or  in  multiple,  or  in  series  multiple  so  as  to 
combine  their  actions  in  causing  current  to  flow  through  an  external 
circuit.  We  may  therefore  refer  to  a  battery  of  so  many  cells.  It  has, 
however,  become  common,  though  technically  improper,  to  refer  to  a 
single  cell  as  a  battery,  so  that  the  term  battery,  as  indicating  nec- 
essarily more  than  one  cell,  has  largely  lost  its  significance. 

Cells  may  be  of  two  types,  primary  and  secondary. 

Primary  cells  are  those  consisting  of  electrodes  of  dissimilar 
elements  which,  when  placed  in  an  electrolyte,  become  immediately 
ready  for  action. 

Secondary  cells,  commonly  called  storage  cells  and  accumulators, 
consist  always  of  two  inert  plates  of  metal,  or  metallic  oxide,  im- 
mersed in  an  electrolyte  which  is  incapable  of  acting  on  either  of  them 
until  a  current  has  first  been  passed  through  the  electrolyte  from  one 
plate  to  the  other.  On  the  passage  of  a  current  in  this  way,  the  de- 
composition of  the  electrolyte  is  effected  and  the  composition  of  the 
plates  is  so  changed  that  one  of  them  becomes  electro-positive  and  the 
other  electro-negative.  The  cell  is  then,  when  the  charging  current 
ceases,  capable  of  acting  as  a  voltaic  cell. 


PRIMARY  CELLS  89 

This  chapter  is  devoted  to  the  primary  cell  or  battery  alone. 

Types  of  Primary  Cells.  Primary  cells  may  be  divided  into 
two  general  classes:  first,  those  adapted  to  furnish  constant  current; 
and  second,  those  adapted  to  furnish  only  intermittent  currents. 
The  difference  between  cells  in  this  respect  rests  largely  in  the 
means  employed  for  preventing  or  lessening  polarization.  Obviously 
in  a  cell  in  which  polarization  is  entirely  prevented  the  current  may 
be  allowed  to  flow  constantly  until  the  cell  is  completely  exhausted ; 
that  is,  until  the  zinc  is  all  eaten  up  or  until  the  hydrogen  is  exhausted 
from  the  electrolyte  or  both.  On  the  other  hand  some  cells  are 
so  constituted  that  polarization  takes  place  faster  than  the  means 
intended  to  prevent  it  can  act.  In  other  words,  the  polarization 
gradually  gains  on  the  preventive  means  and  so  gradually  reduces  the 
current  by  increasing  the  resistance  of  the  cell  and  lowering  its 
electromotive  force.  In  cells  of  this  kind,  however,  the  arrange- 
ment is  such  that  if  the  cell  is  allowed  to  rest,  that  is,  if  the  exter- 
nal circuit  is  opened,  the  depolarizing  agency  will  gradually  act  to 
remove  the  hydrogen  from  the  unattacked  electrode  and  thus  place 
the  cell  in  good  condition  for  use  again. 

Of  these  two  types  of  primary  cells  the  intermittent-current 
cell  is  of  far  greater  use  in  telephony  than  the  constant-current  cell. 
This  is  because  the  use  of  primary  batteries  in  telephony  is,  in  the 
great  majority  of  cases,  intermittent,  and  for  that  reason  a  cell  which 
will  give  a  strong  current  for  a  few  minutes  and  which  after  such  use 
will  regain  practically  all  of  its  initial  strength  and  be  ready  for  use 
again,  is  more  desirable  than  one  which  will  give  a  weaker  current 
continuously  throughout  a  long  period  of  time. 

Since  the  cells  which  are  adapted  to  give  constant  current  are 
commonly  used  in  connection  with  circuits  that  are  continuously 
closed,  they  are  called  closed-circuit  cells.  The  other  cells,  which  are 
better  adapted  for  intermittent  current,  are  commonly  used  on 
circuits  which  stand  open  most  of  the  time  and  are  closed  only  occa- 
sionally when  their  current  is  desired.  For  this  reason  these  are 
termed  open-circuit  cells. 

Open-Circuit  Cells.  LeClanche  Cell:— By  far  the  most  impor- 
tant primary  cell  for  telephone  work  is  the  so-called  LeClanche"  cell. 
This  assumes  a  large  variety  of  forms,  but  always  employs  zinc  as 
the  negatively  charged  element,  carbon  as  the  positively  charged  ele- 


90 


TELEPHONY 


ment,  and  a  solution  of  sal  ammoniac  as  the  electrolyte.  This  cell 
employs  a  chemical  method  of  taking  care  of  polarization,  the  depo- 
larizing agent  being  peroxide  of  manganese,  which  is  closely  asso- 
ciated with  the  carbon  element. 

The  original  form  of  the  LeClanche*  cell,  a  form  in  which  it 
was  very  largely  used  up  to  within  a  short  time  ago,  is  shown  in  Fig. 
61.  In  this  the  carbon  element  is  placed  within  a  cylindrical  jar 
of  porous  clay,  the  walls  of  this  jar  being  of 
such  consistency  as  to  allow  moisture  slowly 
to  permeate  through  it.  Within  this  porous 
cup,  as  it  is  called,  a  plate  or  disk  of  carbon 
is  placed,  and  around  this  the  depolarizing 
agent,  consisting  of  black  oxide  of  manganese. 
This  is  usually  mixed  with  broken  carbon, 
so  as  to  increase  the  effective  area  of  the 
carbon  element  in  contact  with  the  depolar- 
izing agent,  and  also  to  reduce  the  total  in- 
ternal resistance  of  the  cell.  The  zinc  elec- 
trode usually  consisted  merely  in  a  rod  of 
zinc,  as  shown,  with  a  suitable  terminal  at 
its  upper  end. 

The  chemical  action   taking   place   writhin 
the   LeClanche"   cell    is,    briefly,   as   follows: 

Sal  ammoniac  is  chemically  known  as  chloride  of  ammonium  and 
is  a  combination  of  chlorine  and  ammonia.  In  the  action  which 
is  assumed  to  accompany  the  passage  of  current  in  this  cell,  the 
sal  ammoniac  is  decomposed,  the  chlorine  leaving  the  ammonia 
to  unite  with  an  atom  of  the  zinc  plate,  forming  chloride  of  zinc 
and  setting  free  ammonia  and  hydrogen.  The  ammonia  is  im- 
mediately dissolved  in  the  water  of  the  cell,  and  the  hydrogen 
enters  the  porous  cup  and  would  speedily  polarize  the  cell  by  ad- 
hering to  the  carbon  plate  but  for  the  fact  that  it  encounters  the  per- 
oxide of  manganese.  This  material  is  exceedingly  rich  in  oxygen 
and  it  therefore  readily  gives  up  a  part  of  its  oxygen,  which  forms 
water  by  combination  with  the  already  liberated  hydrogen  and 
leaves  what  is  termed  a  sesquioxide  of  manganese.  This  absorp- 
tion or  combination  of  the  hydrogen  prevents  immediate  polarization, 
but  hydrogen  is  evolved  during  the  operation  of  the  cell  more  rapidly 


Fig.  61.    LeClanchfi  Cell 


PRIMARY  CELLS 


91 


GLA-SS  JAR 


than  it  can  combine  wth  the  oxygen  of  the  manganese,  thereby  lead- 
ing to  polarization  more  rapidly  than  the  depolarizer  can  prevent 
it  when  the  cell  is  heavily  worked.  When,  however,  the  cell  is  left 
with  its  external  circuit  open  for  a  time,  depolarization  ensues  by  the 
gradual  combination  of  the  hydrogen  with  the  oxygen  of  the  per- 
oxide of  manganese,  and  as  a  result  the  cell  recuperates  and  in  a 
short  time  attains  its  normal  electromotive  force. 

The  electromotive  force  of  this  cell  when  new  is  about  1.47  volts. 
The  internal  resistance  of  the  cell  of  the  type  shown  in  Fig.  61  is 
approximately  1  ohm,  ordinarily  less  rather  than  more. 

A  more  recent  form  of  LeClanche*  cell  is  shown  in  cross-section 
in  Fig.  62.  This  uses  practically  the  same  materials  and  has  the 
same  chemical  action  as  the  old 

1-    1         T       n\  \.e  11          1  • 

disk  LeClanche  cell  shown  in 
Fig.  61.  It  dispenses,  however, 
with  the  porous  cup  and  instead 
employs  a  carbon  electrode,which 
in  itself  forms  a  *cup  for  the 
depolarizing  agent. 

The  carbon  electrode  is  in 
the  form  of  a  corrugated  hollow 
cylinder  which  engages  by  means 
of  an  internal  screw  thread  a 
corresponding  screw  thread  on 
the  outer  side  of  the  carbon  cover. 
Within  this  cylinder  is  contained 
a  mixture  of  broken  carbon  and 
peroxide  of  manganese.  The 
zinc  electrode  is  in  the  form  of  a 
hollow  cylinder  almost  surrounding  the  carbon  electrode  and  sepa- 
rated therefrom  by  means  of  heavy  rubber  bands  stretched  around 
the  carbon.  The  rod,  forming  the  terminal  of  the  zinc,  passes 
through  a  porcelain  bushing  on  the  cover  plate  to  obviate  short 
circuits.  This  type  of  cell  has  an  electromotive  force  of  about 
1.55  volts  and  recuperates  very  quickly  after  severe  use.  It  also  has 
considerably  lower  internal  resistance  than  the  type  of  LeClanche* 
cell  employing  a  porous  cup,  and,  therefore,  is  capable  of  generating 
a  considerably  larger  current. 


BROKEN 
CARBON  AND 

OXIDE  OF 
MANGANESE. 


SAL-AMMOMiAC 
SQLUT/ON 


Fig.  62.    Carbon  Cylinder  LeClanchg  Cell 


92  TELEPHONY 

Cells  of  this  general  type  have  assumed  a  variety  of  forms.  In 
some  the  carbon  electrode,  together  with  the  broken  carbon  and 
peroxide  of  manganese,  were  packed  into  a  canvas  bag  which  was 
suspended  in  the  electrolyte  and  usually  surrounded  by  the  zinc 
electrode.  In  other  forms  the  carbon  electrode  has  moulded  with  it 
the  manganese  depolarizer. 

In  order  to  prevent  the  salts  within  the  cell  from  creeping  over 
the  edge  of  the  containing  glass  jar  and  also  over  the  upper  portion 
of  the  carbon  electrode,  it  is  common  practice  to  immerse  the  upper 
end  of  the  carbon  element  and  also  the  upper  edge  of  the  glass  jar 
in  hot  paraffin. 

In  setting  up  the  LeClanche'  cell,  place  not  more  than  four  ounces 
of  white  sal  ammoniac  in  the  jar,  fill  the  jar  one-third  full  of  water, 
and  stir  until  the  sal  ammoniac  is  all  dissolved.  Then  put  the  car- 
bon and  zinc  elements  in  place.  A  little  water  poured  in  the  vent 
hole  of  the  porous  jar  or  carbon  cylinder  will  tend  to  hasten  the  action. 

An  excess  of  sal  ammoniac  should  not  be  used,  as  a  saturated 
solution  tends  to  deposit  crystals  on  the  zinc; 'on  the  other  hand, 
the  solution  should  not  be  allowed  to  become  too  weak,  as  in  that 
case  the  chloride  of  zinc  will  form  on  the  zinc.  Both  of  these  causes 
materially  increase  the  resistance  of  the  cell. 

A  great  advantage  of  the  LeClanche  cell  is  that  when  not  in  use 
there  is  but  little  material  waste.  It  contains  no  highly  corrosive 
chemicals.  Such  cells  require  little  attention,  and  the  addition  of 
water  now  and  then  to  replace  the  loss  due  to  evaporation  is  about 
all  that  is  required  until  the  elements  become  exhausted.  They 
give  a  relatively  high  electromotive  force  and  have  a  moderately  low 
internal  resistance,  so  that  they  are  capable  of  giving  rather  large 
currents  for  short  intervals  of  time.  If  properly  made,  they  recuper- 
ate quickly  after  polarization  due  to  heavy  use. 

Dry  Cell,  All  the  forms  of  cells  so  far  considered  may  be  quite 
properly  termed  wet  cells  because  of  the  fact  that  a  free  liquid  electro- 
lyte is  used.  This  term  is  employed  in  contradistinction  to  the  later 
developed  cell,  commonly  termed  the  dry  cell.  This  term  "dry  cell" 
is  in  some  respects  a  misnomer,  since  it  is  not  dry  and  if  it  were  dry 
it  would  not  work.  It  is  essential  to  the  operation  of  these  cells  that 
they  shall  be  moist  within,  and  when  such  moisture  is  dissipated  the 
cell  is  no  longer  usable,  as  there  is  no  further  useful  chemical  action. 


PRIMARY  CELLS 


93 


ME6AT/VE 
TEP.M/HAL 


CAKBOff 
ELECTQOOE 


PAPER 

Soaked  *nt-f> 
solution  of  ch/o  - 
nc/e  of  ammonium 


The  dry  cells  are  all  of  the  LeClanche'  type,  the  liquid  electro- 
lyte of  that  type  being  replaced  by  a  semi-solid  substance  that  is 
capable  of  retaining  moisture  for  a  considerable  period. 

As  in  the  ordinary  wet  LeClanche  cell,  the  electrodes  are  of  car- 
bon and  zinc,  the  zinc  element  being  in  the  form  of  a  cylindrical  cup 
and  forming  the  retaining  vessel  of  the  cell,  while  the  carbon  element 
is  in  the  form  of  a  rod  or  plate  and  occupies  a  central  position  with 
regard  to  the  zinc,  being  held  out  of  contact  with  the  zinc,  however, 
at  all  points. 

A  cross-section  of  an  excellent  form  of  dry  cell  is  shown  in  Fig. 
63.  The  outer  casing  is  of  zinc, 
formed  in  the  shape  of  a  cylin- 
drical cup,  and  serves  not  only 
as  the  retaining  vessel,  but  as  the 
negatively  charged  electrode.  The 
outer  surface  of  the  zinc  is  com- 
pletely covered  on  its  sides  and 
bottom  with  heavy  pasteboard  so 
as  to  insulate  it  from  bodies  with 
which  it  may  come  in  contact, 
and  particularly  from  the  zinc 
cups  of  other  cells  used  in  the 
same  battery.  The  positively 
charged  electrode  is  a  carbon 
rod  corrugated  longitudinally,  as 
shown,  in  order  to  obtain  greater 
surface.  This  rod  is  held  in  the 
center  of  the  zinc  cup  out  of 
contact  therewith,  and  the  inter- 
vening space  is  filled  with  a  mix- 
ture of  peroxide  of  manganese,  powdered  carbon,  and  sal  am- 
moniac. Several  thicknesses  of  blotting  paper  constitute  a  lining 
for  the  inner  portion  of  the  zinc  electrode  and  serve  to  prevent  the 
manganese  mixture  from  coming  directly  into  contact  therewith. 
The  cell  is  sealed  with  pitch,  which  is  placed  on  a  layer  of  sand  and 
sawdust  mixed  in  about  equal  parts. 

The  electrolyte  in  such  cells  varies  largely  as  to  quantities  and 
proportions  of  the  materials  employed  in  various  types  of  cells,  and 


Pig.   63.     Dry  Cell 


94  TELEPHONY 

also  varies  in  the  method  in  which  the  elements  are  introduced  into 
the  container. 

The  following  list  and  approximate  proportions  of  material  will 
serve  as  a  fair  example  of  the  filling  mixture  in  well-known  types 

of  cells. 

Manganese  dioxide 45  per  cent 

Carbon  or  graphite,  or  both 45  per  cent 

Sal  ammoniac 7  per  cent 

Zinc  chloride 3  per  cent 

Water  is  added  to  the  above  and  a  sufficient  amount  of  mixture 
is  taken  for  each  cell  to  fill  the  zinc  cup  about  seven-eighths  full 
when  the  carbon  is  in  place.  The  most  suitable  quantity  of  water 
depends  upon  the  original  dryness  and  fineness  of  material  and  upon 
the  quality  of  the  paper  lining. 

In  some  forms  of  dry  batteries,  starch  or  other  paste  is  added  to 
improve  the  contact  of  the  electrolyte  with  the  zinc  and  promote  a 
more  even  distribution  of  action  throughout  the  electrolyte.  Mer- 
cury, too,  is  often  added  to  effect  amalgamation  of  the  zinc. 

As  in  the  ordinary  wet  type  of  LeClanche  cell,  the  purpose  of  the 
manganese  is  to  act  as  a  depolarizer;  the  carbon  or  graphite  being 
added  to  give  conductivity  to  the  manganese  and  to  form  a  large 
electrode  surface.  It  is  important  that  the  sal  ammoniac,  which  is 
the  active  agent  of  the  cell,  should  be  free  from  lumps  in  order  to 
mix  properly  with  the  manganese  and  carbon. 

A  small  local  action  takes  place  in  the  dry  cell,  caused  by  the 
dissimilar  metals  necessarily  employed  in  soldering  up  the  zinc  cup 
and  in  soldering  the  terminal  rod  of  zinc  to  the  zinc  cup  proper.  This 
action,  however,  is  slight  in  the  better  grades  of  cells.  As  a  result  of 
this,  and  also  of  the  gradual  drying  out  of  the  moisture  within  the  cell, 
these  cells  gradually  deteriorate  even  when  not  in  use — this  is  com- 
monly called  shelf-wear.  Shelf-wear  is  much  more  serious  in  the 
very  small  sizes  of  dry  cells  than  in  the  larger  ones. 

Dry  cells  are  made  in  a  large  number  of  shapes  and  sizes.  The 
most  useful  form,  however,  is  the  ordinary  cylindrical  type.  These 
are  made  in  sizes  varying  from  one  and  one-half  inches  high  and 
three-quarters  inch  in  diameter  to  eight  inches  high  and  three  and 
three-quarters  inches  in  diameter.  The  most  used  and  standard 
size  of  dry  cell  is  of  cylindrical  form  six  inches  high  and  two  and  three- 
quarters  inches  in  diameter.  The  dry  cell  when  new  and  in  good 


PRIMARY  CELLS  95 

condition  has  an  open-circuit  voltage  of  from  1.5  to  1.6  volts.  Per- 
haps 1.55  represents  the  usual  average. 

A  cell  of  the  two  and  three-quarters  by  six-inch  size  will  give 
throughout  its  useful  life  probably  thirty  ampere  hours  as  a  maximum, 
but  this  varies  greatly  with  the  condition  of  use  and  the  make  of  cell. 
Its  effective  voltage  during  its  useful  life  averages  about  one  volt, 
and  if  during  this  life  it  gives  a  total  discharge  of  thirty  ampere  hours, 
the  fair  energy  rating  of  the  cell  will  be  thirty  watt-hours.  This  may 
not  be  taken  as  an  accurate  figure,  however,  as  the  watt-hour  capac- 
ity of  a  cell  depends  very  largely,  not  only  on  the  make  of  the  cell,  but 
on  the  rate  of  its  discharge. 

An  examination  of  Fig.  63  shows  that  the  dry  cell  has  all  of  the 
essential  elements  of  the  LeClanche*  cell.  The  materials  of  which 
the  electrodes  are  made  are  the  same  and  the  porous  cup  of  the  disk 
LeClanche"  cell  is  represented  in  the  dry  cell  by  the  blotting-paper 
cylinder,  which  separates  the  zinc  from  the  carbon  electrode.  The 
positively  charged  electrode  must  not  be  considered  as  merely  the  car- 
bon plate  or  rod  alone,  but  rather  the  carbon  rod  with  its  surround- 
ing mixture  of  peroxide  of  manganese  and  broken  carbon.  Such 
being  the  case,  it  is  obvious  that  the  separation  between  the  elec- 
trodes is  very  small,  while  the  surface  presented  by  both  electrodes  is 
very  large.  As  a  result,  the  internal  resistance  of  the  cell  is  small 
and  the  current  which  it  will  give  on  a  short  circuit  is  correspond- 
ingly large.  A  good  cell  of  the  two  and  three-quarters  by  six-inch 
size  will  give  eighteen  or  twenty  amperes  on  short-circuit,  when  new. 

As  the  action  of  the  cell  proceeds,  zinc  chloride  and  am- 
monia are  formed,  and  there  being  insufficient  water  to  dissolve 
the  ammonia,  there  results  the  formation  of  double  chlorides  of  zinc 
and  ammonium.  These  double  chlorides  are  less  soluble  than  the 
chlorides  and  finally  occupy  the  pores  of  the  paper  lining  between 
the  electrolyte  and  the  zinc  and  greatly  increase  the  internal  resist- 
ance of  the  cell.  This  increase  of  resistance  is  further  contributed 
to  by  the  gradual  drying  out  of  the  cell  as  its  age  increases. 

Within  the  last  few  years  dry  batteries  have  been  so  per- 
fected mechanically,  chemically,  and  electrically  that  they  have  far 
greater  outputs  and  better  recuperative  power  than  any  of  the 
other  types  of  LeClanche  batteries,  while  in  point  of  convenience 
and  economy,  resulting  from  their  small  size  and  non-break- 


96 


TELEPHONY 


able,   non-spillable   features    and    low  cost,   they    leave   no    room 
for  comparison. 

Closed-Circuit  Cells.  Gravity-Cell: — Coming  now  to  the  con- 
sideration of  closed-circuit  or  constant-current  cells,  the  most  im- 
portant is  the  well-known  gravity,  or  blue-stone,  cell,  devised  by 
Daniell.  It  is  largely  used  in  telegraphy,  and  often  in  telephony  in 
such  cases  as  require  a  constantly  flowing  current  of  small  quantity. 
Such  a  cell  is  shown  in  Fig.  64. 

The  elements  of  the  gravity  cell  are  electrodes  of  copper  and  zinc. 
The  solution  in  which  the  copper  plate  is  immersed  is  primarily  a 
solution  of  copper  sulphate,  commonly  known  as  blue-stone,  in  water. 
The  zinc  plate  after  the  cell  is  in  action  is  immersed  in  a  solution  of 
sulphate  of  zinc  which  is  formed  around  it. 

The  glass  jar  is  usually  cylindrical,  the  standard  sizes  being  5 
inches  diameter  and  7  inches  deep;  and  also  6  inches  diameter  and  8 

inches  deep.  The  copper  elec- 
trode is  of  sheet  copper  of  the 
form  shown,  and  it  is  partly  cov- 
ered with  crystals  of  blue-stone  or 
copper  sulphate.  Frequently,  in 
later  forms  of  cells,  the  copper 
electrode  consists  merely  of  a 
straight,  thick,  rectangular  bar  of 
copper  laid  horizontally,  directly 
on  top  of  the  blue-stone  crystals. 
In  all  cases  a  rubber-insulated 
wire  is  attached  by  riveting  to  the 
copper  electrode,  and  passes  up 
through  the  electrolyte  to  form 
the  positive  terminal. 

The  zinc  is,  as  a  rule,  of 
crowfoot  form,  as  shown,  whence  this  cell  derives  the  commonly 
applied  name  of  crowfoot  cell.  This  is  essentially  a  two-fluid  cell, 
for  in  its  action  zinc  sulphate  is  formed,  and  this  being  lighter  than 
copper  sulphate  rises  to  the  top  of  the  jar  and  surrounds  the  zinc. 
Gravity,  therefore,  serves  to  keep  the  two  fluids  separate. 

In  the  action  of  the  cell,  when  the  external  circuit  is  closed,  sul- 
phuric acid  is  formed  which  attacks  the  zinc  to  form  sulphate  of  zinc 


Fig.  64.     Gravity  Cell 


PRIMARY  CELLS  97 

and  to  liberate  hydrogen,  which  follows  its  tendency  to  attach  itself  to 
the  copper  plate.  But  in  so  doing  the  hydrogen  necessarily  passes 
through  the  solution  of  sulphate  of  copper  surrounding  the  copper 
plate.  The  hydrogen  immediately  combines  with  the  SO4  radical, 
forming  therewith  sulphuric  acid,  and  liberating  metallic  copper. 
This  sulphuric  acid,  being  lighter  than  the  copper  sulphate,  rises  to 
the  surface  of  the  zinc  and  attacks  the  zinc,  thus  forming  more  sul- 
phate of  zinc.  The  metallic  copper  so  formed  is  deposited  on  the 
copper  plate,  thereby  keeping  the  surface  bright  and  clean.  Since 
hydrogen  is  thus  diverted  from  the  copper  plate,  polarization  does 
not  ensue. 

The  zinc  sulphate  being  colorless,  while  the  copper  sulphate  is 
of  a  dark  blue  color,  the  separating  line  of  the  two  liquids  is  easily 
distinguishable.  This  line  is  called  the  blue  line  and  care  should  be 
taken  that  it  does  not  reach  the  zinc  and  cause  a  deposit  of  copper 
io  be  placed  thereon. 

As  has  been  stated,  these  two  liquids  do  not  mix  readily,  but  they 
will  eventually  mingle  unless  the  action  of  the  cell  is  sufficient  to  use 
up  the  copper  sulphate  as  speedily  as  it  is  dissolved.  Thus  it  will  be 
seen  that  while  the  cell  is  free  from  polarization  and  local  action, 
there  is,  nevertheless,  a  deteriorating  effect  if  the  cell  is  allowed  to 
remain  long  on  open  circuit.  Therefore,  it  should  be  used  when  a 
constant  current  is  required. 

Prevention  of  Creeping: — Much  trouble  has  been  experienced 
in  gravity  cells  due  to  the  creeping  of  the  salts  over  the  edge  of  the 
jar.  Frequently  the  upper  edges  of  the  jars  are  coated  by  dipping 
in  hot  paraffin  wax  in  the  hope  of  preventing  this.  Sometimes  oil 
is  poured  on  top  of  the  fluid  in  the  jar  to  prevent  the  creeping  of  the 
salts  and  the  evaporation  of  the  electrolyte.  The  following  account 
of  experiments  performed  by  Mr.  William  Reid,  of  Chicago,  throws 
light  on  the  relative  advantages  of  these  and  other  methods  of 
preventing  creeping. 

The  experiment  was  made  with  gravity  cells  having  5-inch  by  7-inch 
glass  jars.  Four  cells  were  made  up  and  operated  in  a  rather  dry,  warm  place, 
although  perhaps  under  no  more  severe  local  conditions  than  would  be  found 
in  most  telephone  exchanges.  Cell  No.  I  was  a  plain  cell  as  ordinarily  used. 
Cell  No.  2  had  the  top  of  the  rim  of  the  jar  treated  with  paraffin  wax  by  dip- 
ping the  rim  to  about  one  inch  in  depth  in  melte;l  paraffin  wax.  Cell  No.  3 
had  melted  paraffin  wax  poured  over  the  surface  of  the  liquid  forming  a  seal 


98  TELEPHONY 

about  -fs  inch  in  thickness.  After  cooling,  a  few  small  holes  were  bored  through 
the  seal  to  let  gases  escape.  Cell  No.  4  had  a  layer  of  heavy  paraffin  oil  nearly 
^  inch  in  thickness  (about  G  oz.  being  used)  on  top  of  the  solutions. 

These  cells  were  all  run  on  a  load  of  .22  to  .29  amperes  for  15^  hours 
per  day  for  thirty  days,  after  which  the  following  results  were  noted: 

(a)  The  plain  cell,  or  cell  No.  1,  had  to  have  26  ounces  of  water  added 
to  it  to  replace  that  which  had  evaporated.  The  creeping  of  zinc  sulphate 
salts  was  very  bad. 

(6)  The  waxed  rim  cell,  or  cell  No.  2,  evaporated  26  ounces  of  water  and 
the  creeping  of  zinc  sulphate  salts  was  not  prevented  by  the  waxed  rim.  The 
wax  proved  of  no  value. 

(c)  The  wax  sealed  cell,  or  cell  No.  3,  showed  practically  no  evaporation 
and  only  very  slight  creeping  of  zinc  sulphate  salts.  The  creeping  of  salts  that 
took  place  was  only  around  spots  where  the  edges  of  the  seal  were  loose  from 
the  jar. 

(rf)  The  paraffin  oil  sealed  cell,  or  cell  No.  4,  showed  no  evaporation  and 
no  creeping  of  salts. 

It  was  concluded  by  Mr.  Reid  from  the  above  experiments  that 
the  wax  applied  to  the  rim  of  the  jar  is  totally  ineffective  and  has  no 
merits.  The  wax  seal  loosens  around  the  edges  and  does  not 
totally  prevent  creeping  of  the  zinc  sulphate  salts,  although  nearly  so. 
The  wax-sealed  jar  must  have  holes  drilled  in  it  to  allow  the  gases  to 
escape.  The  method  is  hardly  commercial,  as  it  is  difficult  to  make 
a  neat  appearing  cell,  besides  making  it  almost  impossible  to  ma- 
nipulate its  contents.  A  coat  of  paraffin  oil  approximately  |  inch  in 
thickness  (about  6  ounces)  gives  perfect  protection  against  evap- 
oration and  creeping  of  the  zinc  sulphate  salts.  The  cell,  having 
the  paraffin-oil  seal,  had  a  very  neat,  clean  appearance  as  compared 
with  cells  No.  1  and  No.  2.  It  was  found  that  the  zinc  could  be 
drawn  out  through  the  oil,  cleaned,  and  replaced  with  no  appreciable 
effect  on  voltage  or  current. 

Setting  Up: — In  setting  up  the  battery  the  copper  electrode  is 
first  unfolded  to  form  a  cross  and  placed  in  the  bottom  of  the  jar. 
Enough  copper  sulphate,  or  blue-stone  crystals,  is  then  dropped  into 
the  jar  to  almost  cover  the  copper.  The  zinc  crowfoot  is  then  hung 
in  place,  occupying  a  position  about  4  inches  above  the  top  of  the 
copper.  Clear  water  is  then  poured  in  sufficient  to  fill  the  jar  within 
about  an  inch  of  the  top. 

If  it  is  not  required  to  use  the  cell  at  once,  it  may  be  placed  on 
short  circuit  for  a  time  and  allowed  to  form  its  own  zinc  sulphate. 
The  cell  may,  however,  be  made  immediately  available  for  use  by 


PRIMARY  CELLS  99 

drawing  about  one-half  pint  of  a  solution  of  zinc  sulphate  from  a 
cell  already  in  use  and  pouring  it  into  the  jar,  or,  when  this  is 
not  convenient,  by  putting  into  the  liquid  four  or  five  ounces  of 
pulverized  sulphate  of  zinc,  or  by  adding  about  ten  drops  of  sulphuric 
acid.  When  the  cell  is  in  proper  working  condition,  one-half  inch 
in  thickness  of  heavy  paraffin  oil  of  good  quality  may  be  added. 

If  the  blue  line  gets  too  low,  and  if  there  is  in  the  bottom  of  the 
cell  a  sufficient  quantity  of  sulphate  of  copper,  it  may  be  raised  by 
drawing  off  a  portion  of  the  zinc  sulphate  with  a  battery  syringe  and 
replacing  this  with  water.  If  the  blue  line  gets  too  high,  it  may  be 
lowered  by  short-circuiting  the  cell  for  a  time,  or  by  the  addition  of 
more  sulphate  of  zinc  solution  from  another  battery.  If  the  copper 
sulphate  becomes  exhausted,  it  should  be  replenished  by  dropping 
in  more  crystals. 

Care  should  be  taken  in  cold  weather  to  maintain  the  temperature 
of  the  battery  above  65°  or  70°  Fahrenheit.  If  below  this  tem- 
perature, the  internal  resistance  of  a  cell  increases  very  rapidly,  so 
much  so  that  even  at  50°  Fahrenheit  the  action  becomes  very  much 
impaired.  This  follows  from  the  facts  that  the  resistance  of  a  liquid 
decreases  as  its  temperature  rises,  and  that  chemical  action  is  much 
slower  at  lower  temperatures. 

The  gravity  cell  has  a  practically  constant  voltage  of  1.08  volts. 
Its  internal  resistance  is  comparatively  high,  seldom  falling  below 
1  ohm  and  often  rising  to  6  ohms.  At  best,  therefore,  it  is  only  capa- 
ble of  producing  about  1  ampere.  The  gravity  cell  is  perhaps  the 
most  common  type  of  cell  wherein  depolarization  is  affected  by 
electro-chemical  means. 

Fuller  Cell: — A  form  of  cell  that  is  adapted  to  very  heavy 
open-circuit  work  and  also  closed-circuit  work  where  heavier  currents 
are  required  than  can  be  supplied  by  the  gravity  battery  is  the  Fuller. 
In  this  the  electrodes  are  of  zinc  and  carbon,  respectively,  the  zinc 
usually  being  in  the  form  of  a  heavy  cone  and  placed  within  a  porous 
cup.  The  electrolyte  of  the  Fuller  cell  is  known  as  electropoion  fluid, 
and  consists  of  a  mixture  of  sodium  or  potassium  bichromate,  sul- 
phuric acid,  and  water. 

The  various  parts  of  the  standard  Fuller  cell,  as  once  largely 
employed  by  the  various  Bell  operating  companies,  are  shown  in 
Fig.  65.  In  this  the  jar  was  made  of  flint  glass,  cylindrical  in  form, 


100  TELEPHONY 

six  inches  in  diameter  and  eight  inches  deep.  It  is  important  that 
a  good  grade  of  glass  be  used  for  the  jar  in  this  cell,  because,  on 
account  of  the  nature  of  the  electrolyte,  breakage  is  disastrous  in 
the  effects  it  may  produce  on  adjacent  property.  The  carbon  plate 
is  rectangular  in  form,  about  four  inches  wide,  eight  and  three- 
quarters  inches  long,  and  one-quarter  inch  thick.  The  metal  ter- 
minal at  the  top  of  the  carbon  block  is  of  bronze,  both  it  and  the 
lock  nuts  and  bolts  being  nickel-plated  to  minimize  corrosion.  The 
upper  end  of  the  carbon  block  is  soaked  in  paraffin  so  hot  as  to  drive 
all  of  the  moisture  out  of  the  paraffin  and  out  of  the  pores  of  the 
block  itself. 

The  zinc,  as  is  noted  from  the  cut,  is  in  the  form  of  a  truncated 
cone.  It  is  about  two  and  one-eighth  inches  in  diameter  at  the  base 
and  two  and  one-half  inches  high.  Cast  into  the  zinc  is  a  soft  copper 
wire  about  No.  12  B.  &  S.  gauge.  This  wire  extends  above  the  top  of 
the  jar  so  as  to  form  a  convenient  terminal  for  the  cell. 

The  porous  cup  is  cylindrical  in  form,  about  three  inches  in 
diameter  and  seven  inches  deep.  The  wooden  cover  is  of  kiln-dried 
white  wood  thoroughly  coated  with  two  coats  of  asphalt  paint.  It 
is  provided  with  a  slot  for  the  carbon  and  a  hole  for  the  copper 
wire  extending  to  the  zinc. 

The  electrolyte  for  this  cell  is  made  as  follows: 

Sodium  bichromate 6  oz. 

Sulphuric  acid 17  oz. 

Soft  water 56  oz. 

This  solution  is  mixed  by  dissolving  the  bichromate  of  sodium  in  the 
water  and  then  adding  slowly  the  sulphuric  acid.  Potassium  bi- 
chromate may  be  substituted  for  the  sodium  bichromate. 

In  setting  up  this  cell,  the  amalgamated  zinc  is  placed  within 
the  porous  cup,  in  the  bottom  of  which  are  about  two  teaspoonfuls 
of  mercury,  the  latter  serving  to  keep  the  zinc  well  amalgamated. 
The  porous  cup  is  then  placed  in  the  glass  jar  and  a  sufficient 
quantity  of  the  electrolyte  is  placed  in  the  outer  jar  to  come  within 
about  one  and  one-half  inches  of  the  top  of  the  porous  cup.  About 
two  teaspoonfuls  of  salt  are  then  placed  in  the  porous  cup  and 
sufficient  soft  water  added  to  bring  the  level  of  the  liquid  within  the 
porous  cup  even  with  the  level  of  the  electrolyte  in  the  jar  surrounding 
the  cup.  The  carbon  is  then  placed  through  the  slot  in  the  cover,  and 


PRIMARY  CELLS 


101 


the  wire  from  the  zinc  is  passed  through  the  hole  in  the  cover  pro- 
vided for  it,  and  the  cover  is  allowed  to  fall  in  place.  The  cell  is 
now  ready  for  immediate  use. 

The  action  of  this  cell  is  as  follows:  The  sulphuric  acid  attacks 
the  zinc  and  forms  zinc  sulphate,  liberating  hydrogen.  The  hydro- 
gen attempts  to  pass  to  the  carbon  plate  as  usual,  but  in  so  doing  it 
meets  with  the  oxygen  of  the  chromic  acid  and  forms  water  therewith. 
The  remainder  of  the  chromic  acid  combines  with  the  sulphuric  acid 
to  form  chromium  sulphate. 

The  mercury  placed  in  the  bottom  of  the  porous  cup  with  the 
zinc  keeps  the  zinc  in  a  state  of  perpetual  amalgamation.  This  it 


GLASS 
JAR 


POROUS  Ci/P 


CARBON 

PI-ATE 


Fig    65.     Fuller  Cell 

does  by  capillary  action,  as  the  mercury  spreads  over  the  entire  sur- 
face of  the  zinc.  The  initial  amalgamation,  while  not  absolutely 
essential,  helps  in  a  measure  this  capillary  action. 

In  another  well-known  type  of  the  Fuller  battery  the  carbon 
is  a  hollow  cylinder,  surrounding  the  porous  cup.  In  this  type 
the  zinc  usually  took  the  form  of  a  long  bar  having  a  cross-shaped 
section,  the  length  of  this  bar  being  sufficient  to  extend  the  entire 
depth  of  the  porous  cup.  This  type  of  cell  has  the  advantage  of  a 
somewhat  lower  internal  resistance  than  the  standard  form  just 
described. 

Should  the  electrolyte  become  supersaturated  by  virtue  of  the 
battery  being  neglected  or  too  heavily  overworked,  a  set  of  secon- 
dary reactions  will  occur  in  the  cell,  resulting  in  the  formation  of  the 


102  TELEPHONY 

yellow  crystals  upon  the  carbon.  This  seriously  affects  the  e.  m.  f. 
of  the  cell  and  also  its  internal  resistance.  Should  this  occur,  some 
of  the  solution  should  be  withdrawn  and  dilute  sulphuric  acid  in- 
serted in  its  place  and  the  crystals  which  have  formed  on  the  carbon 
should  be  carefully  washed  off.  Should  the  solution  lose  its  orange 
tint  and  turn  blue,  it  indicates  that  more  bichromate  of  potash  or 
bichromate  of  sodium  is  needed.  This  cell  gives  an  electromotive 
force  of  2.1  volts  and  a  very  large  current  when  it  is  in  good  condition, 
since  its  internal  resistance  is  low. 

The  Fuller  cell  was  once  largely  used  for  supplying  current 
to  telephone  transmitters  at  subscribers'  stations,  where  very  heavy 
service  was  demanded,  but  the  advent  of  the  so-called  common-bat- 
tery systems,  in  some  cases,  and  of  the  high-resistance  transmitter, 
in  other  cases,  has  caused  a  great  lessening  in  its  use.  This  is  fortu- 
nate as  the  cell  is  a  "dirty"  one  to  handle  and  is  expensive  to  main- 
tain. 

The  Fuller  cell  still  warrants  attention,  however,  as  an  available 
source  of  current,  which  may  be  found  useful  in  certain  cases  of 
emergency  work,  and  in  supplying  special  but  temporary  needs  for 
heavier  current  than  the  LeClanche  or  gravity  cell  can  furnish. 

Lalande  Cell: — A  type  of  cell,  specially  adapted  to  constant- 
current  work,  and  sometimes  used  as  a  central  source  of  current  in 
very  small  common-battery  exchanges  is  the  so-called  copper  oxide, 
or  Lalande  cell,  of  which  the  Edison  and  the  Gordon  are  types.  In 
all  of  these  the  negatively  charged  element  is  of  zinc,  the  positively 
charged  element  a  mass  of  copper  oxide,  and  the  electrolyte  a  solu- 
tion of  caustic  potash  in  water.  In  the  Edison  cell  the  copper  oxide 
is  in  the  form  of  a  compressed  slab  which  with  its  connecting  copper 
support  forms  the  electrode.  In  the  Gordon  and  other  cells  of  this 
type  the  copper  oxide  is  contained  loosely  in  a  perforated  cylinder 
of  sheet  copper.  The  copper  oxide  serves  not  only  as  an  electrode,  but 
also  as  a  depolarizing  agent,  the  liberated  hydrogen  in  the  electrolyte 
uniting  with  the  oxygen  of  the  copper  oxide  to  form  water,  and  leav- 
ing free  metallic  copper. 

On  open  circuit  the  elements  are  not  attacked,  therefore  there  is 
no  waste  of  material  while  the  cell  is  not  in  use.  This  important 
feature,  and  the  fact  that  the  internal  resistance  is  low,  make  this 
cell  well  adapted  for  all  forms  of  heavy  open-circuit  work.  The 


PRIMARY  CELLS 


103 


fact  that  there  is  no  polarizing  action  within  the  cell  makes  it  further 
adaptable  to  heavy  closed-circuit  service. 

These  cells  are  intended  to  be  so  proportioned  that  all  of  their 
parts  become  exhausted  at  once  so  that  when  the  cell  fails,  complete 
renewals  are  necessary.  Therefore,  there  is  never  a  question  as  to 
which  of  the  elements  should  be  renewed. 

After  the  elements  and  solution  are  in  place  about  one-fourth 
of  an  inch  of  heavy  paraffin  oil  is  poured  upon  the  surface  of  the  so- 
lution in  order  to  prevent  evaporation.  This  cell  requires  little  at- 
tention and  will  maintain  a  constant  e.  m.  f.  of  about  two-thirds  of 
a  volt  until  completely  exhausted.  It  is  non-freezable  at  all  ordinary 
temperatures.  Its  low  voltage  is  its  principal  disadvantage. 

Standard  Cell.  Chloride  of 
Silver  Cell:— The  chloride  of 
silver  cell  is  largely  used  as  a 
standard  for  testing  purposes.  Its 
compactness  and  portability  and 
its  freedom  from  local  action 
make  it  particularly  adaptable 
to  use  in  portable  testing  outfits 
where  constant  electromotive 
force  and  very  small  currents 
are  required. 

A  cross-section  of  one  form 
of  the  cell  is  shown  in  Fig.  66. 
Its  elements  are  a  rod  of  chem- 
ically-pure zinc  and  a  rod  of 
chloride  of  silver  immersed  in  a  water  solution  of  sal  ammoniac. 
As  ordinarily  constructed,  the  glass  jar  or  tube  is  usually  about  2| 
inches  long  by  1  inch  in  diameter.  After  the  solution  is  poured  in 
and  the  elements  are  in  place  the  glass  tube  is  hermetically  sealed 
with  a  plug  of  paraffin  wax. 

The  e.  m.  f.  of  a  cell  of  this  type  is  1.03  volts  and  the  external 
resistance  varies  with  the  age  of  the  cell,  being  about  4  ohms  at  first. 
Care  should  be  taken  not  to  short-circuit  these  cells,  or  use  them  in 
any  but  high-resistance  circuits,  as  they  have  but  little  energy  and 
become  quickly  exhausted  if  compelled  to  work  in  low-resistance 
circuits. 


Fig.  66.     Chlori.le  of  Silver  Cell 


104  TELEPHONY 

Conventional  Symbol.  The  conventional  symbol  for  a  cell,  either 
of  the  primary  or  the  secondary  type,  consists  of  a  long  thin  line  and 
a  short  heavy  line  side  by  side  and  parallel.  A  battery  is  represented 
by  a  number  of  pairs  of  such  lines,  as  in  Fig.  67.  The  two  lines  of 
each  pair  are  supposed  to  represent  the  two  electrodes  of  a  cell. 
Where  any  significance  is  to  be  placed  on  the  polarity  of  the  cell  or 
battery  the  long  thin  line  is  supposed  to  represent  the  positively 


Fig.  67.     Battery  Symbols 

charged  plate  and  the  short  thick  line  the  negatively  charged  plate. 
The  number  of  pairs  may  indicate  the  number  of  cells  in  the  battery. 
Frequently,  however,  a  few  pairs  of  such  lines  are  employed  merely 
for  the  purpose  of  indicating  a  battery  without  regard  to  its  polarity 
or  its  number  of  cells. 

In  Fig.  67  the  representation  at  A  is  that  of  a  battery  of 
a  number  of  cells  connected  in  parallel;  that  at  B  of  a  battery 
with  the  cells  connected  in  series;  and  that  at  C  of  a  battery  with 
one  of  its  poles  grounded. 


CHAPTER  VIII 
MAGNETO  SIGNALING  APPARATUS 

Method  of  Signaling.  The  ordinary  apparatus,  by  which  speech 
is  received  telephonically,  is  not  capable  of  making  sufficiently  loud 
sounds  to  attract  the  attention  of  people  at  a  distance  from  the  in- 
strument. For  this  reason  it  is  necessary  to  employ  auxiliary  appara- 
tus for  the  purpose  of  signaling  between  stations.  In  central  offices 
where  an  attendant  is  always  on  hand,  the  sense  of  sight  is  usually 
appealed  to  by  the  use  of  signals  which  give  a  visual  indication,  but 
in  the  case  of  telephone  instruments  for  use  by  the  public,  the  sense 
of  hearing  is  appealed  to  by  employing  an  audible  rather  than  a 
visual  signal. 

Battery  Bell.  The  ordinary  vibrating  or  battery  bell,  such  as  is 
employed  for  door  bells,  is  sometimes,  though  not  often,  employed 
in  telephony.  It  derives  its  current  from  primary  batteries  or  from 
any  direct-current  source.  The  reason  why  they  are  not  em- 
ployed to  a  greater  extent  in  telephony  is  that  telephone  signals  usu- 
ally have  to  be  sent  over  lines  of  considerable  length  and  the  voltage 
that  would  be  required  to  furnish  current  to  operate  such  bells  over 
such  lengths  of  line  is  higher  than  would  ordinarily  be  found  in  the 
batteries  commonly  employed  in  telephone  work.  Besides  this  the 
make-and-break  contacts  on  which  the  ordinary  battery  bell  depends 
for  its  operation  are  an  objectionable  feature  from  the  standpoint 
of  maintenance. 

Magneto  Bell.  Fortunately,  however,  there  has  been  developed 
a  simpler  type  of  electric  bell,  which  operates  on  smaller  currents, 
and  which  requires  no  make-and-break  contacts  whatever.  This 
simpler  form  of  bell  is  commonly  known  as  the  polarized,  or  magneto, 
bell  or  ringer.  It  requires  for  its  operation,  in  its  ordinary  form,  an 
alternating  current,  though  in  its  modified  forms  it  may  be  used  with 
pulsating  currents,  that  is,  with  periodically  recurring  impulses  of 
current  always  in  the  same  direction. 


106 


TELEPHONY 


Magneto  Generator.  In  the  early  days  of  telephony  there  was 
nearly  always  associated  with  each  polarized  bell  a  magneto  gener- 
ator for  furnishing  the  proper  kind  of  current  to  ring  such  bells. 
Each  telephone  was  therefore  equipped,  in  addition  to  the  transmitter 
and  receiver,  with  a  signal-receiving  device  in  the  form  of  a  polarized 
bell,  and  with  a  current  generator  by  which  the  user  was  enabled  to 
develop  his  own  currents  of  suitable  kind  and  voltage  for  ringing  the 
bells  of  other  stations. 

Considering  the  signaling  apparatus  of  the  telephones  alone, 
therefore,  each  telephone  was  equipped  with  a  power  plant  for  gen- 
erating currents  used  by  that  station  in  signaling  other  stations,  the 

prime  mover  being  the  muscles 
of  the  user  applied  to  the  turning 
of  a  crank  on  the  side  of  the  in- 
strument; and  also  with  a  cur- 
rent-consuming device  in  the  form 
of  a  polarized  electromagnetic 
bell  adapted  to  receive  the  cur- 
rents generated  at  other  stations 
and  to  convert  a  portion  of  their 
energy  into  audible  signals. 

The  magneto  generator  is 
about  the  simplest  type  of  dy- 
namo-electric machine,  and  it 
depends  upon  the  same  principles 
of  operation  as  the  much  larger 
generators,  employed  in  electric- 
Fig.  68.  Principles  of  Magneto  Generator  lighting  and  street-railway  power 

plants,  for  instance.     Instead  of 

developing  the  necessary  magnetic  field  by  means  of  electromagnets, 
as  in  the  case  of  the  ordinary  dynamo,  the  field  'of  the  magneto 
generator  is  developed  by  permanent  magnets,  usually  of  the  horse- 
shoe form.  Hence  the  name  magneto.  . 

In  order  to  concentrate  the  magnetic  field  within  the  space  in 
which  the  armature  revolves,  pole  pieces  of  iron  are  so  arranged  in 
connection  with  the  poles  of  the  permanent  magnet  as  to  afford  a 
substantially  cylindrical  space  in  which  the  armature  conductors  may 
revolve  and  through  which  practically  all  the  magnetic  lines  of  force 


MAGNETO  SIGNALING  APPARATUS  107 

set  up  by  the  permanent  magnets  will  pass.  In  Fig.  68  there  is  shown, 
diagrammatically,  a  horseshoe  magnet  with  such  a  pair  of  pole  pieces, 
between  which  a  loop  of  wire  is  adapted  to  rotate.  The  magnet  1  is 
of  hardened  steel  and  permanently  magnetized.  The  pole  pieces  are 
shown  at  2  and  3,  each  being  of  soft  iron  adapted  to  make  good  mag- 
netic contact  on  its  flat  side  with  the  inner  flat  surface  of  the  bar 
magnet,  and  being  bored  out  so  as  to  form  a  cylindrical  recess  between 
them  as  indicated.  The  direction  of  the  magnetic  lines  of  force  set 
up  by  the  bar  magnet  through  the  interpolar  space  is  indicated  by 
the  long  horizontal  arrows,  this  flow  being  from  the  north  pole  (N) 
to  the  south  pole  (S)  of  the  magnet.  At  4  there  is  shown  a  loop  of 
wire  supposed  to  revolve  in  the  magnetic  field  of  force  on  the  axis  5-5. 

Theory.  In  order  to  understand  how  currents  will  be  generated 
in  this  loop  of  wire  4>  it  is  only  necessary  to  remember  that  if  a  con- 
ductor is  so  moved  as  to  cut  across  magnetic  lines  of  force,  an  electro- 
motive force  will  be  set  up  in  the  conductor  which  will  tend  to  make 
the  current  flow  through  it.  The  magnitude  of  the  electromotive 
force  will  depend  on  the  rate  at  which  the  conductor  cuts  through  the 
lines  of  force,  or,  in  other  words,  on  the  number  of  lines  of  force  that 
are  cut  through  by  the  conductor  in  a  given  unit  of  time.  Again,  the 
direction  of  the  electromotive  force  depends  on  the  direction  of  the 
cutting,  so  that  if  the  conductor  be  moved  in  one  direction  across  the 
lines  of  force,  the  electromotive  force  and  the  current  will  be  in  one 
direction;  while  if  it  moves  in  the  opposite  direction  across  the  lines 
of  force,  the  electromotive  force  and  the  current  will  be  in  the  reverse 
direction. 

It  is,  evident  that  as  the  loop  of  wire  4  revolves  in  the  field  of 
force  about  the  axis  5-5,  the  portions  of  the  conductor  parallel  to  the 
axis  will  cut  through  the  lines  of  force,  first  in  one  direction  and  then 
in  the  other,  thus  producing  electromotive  forces  therein,  first  in  one 
direction  and  then  in  the  other. 

Referring  now  to  Fig.  68,  and  supposing  that  the  loop  4  ls  re~ 
volving  in  the  direction  of  the  curved  arrow  shown  between  the  upper 
edges  of  the  pole  pieces,  it  will  be  evident  that  just  as  the  loop  stands 
in  the  vertical  position,  its  horizontal  members  will  be  moving  in  a 
horizontal  direction,  parallel  with  the  lines  of  force  and,  therefore, 
not  cutting  them  at  all.  The  electromotive  force  and  the  current  will, 
therefore,  be  zero  at  this  time. 


108  TELEPHONY 

As  the  loop  advances  toward  the  position  shown  in  dotted  lines, 
the  upper  portion  of  the  loop  that  is  parallel  with  the  axis  will  begin 
to  cut  downwardly  through  the  lines  of  force,  and  likewise  the  lower 
portion  of  the  loop  that  is  parallel  with  the  axis  will  begin  to  cut  up- 
wardly through  the  lines  of  force.  This  will  cause  electromotive 
forces  in  opposite  directions  to  be  generated  in  these  portions  of  the 
loop,  and  these  will  tend  to  aid  each  other  in  causing  a  current  to 
circulate  in  the  loop  in  the  direction  shown  by  the  arrows  associated 
with  the  dotted  representation  of  the  loop.  It  is  evident  that  as  the 
motion  of  the  loop  progresses,  the  rate  of  cutting  the  lines  of  force 
will  increase  and  will  be  a  maximum  when  the  loop  reaches  a  hori- 
zontal position,  or  at  that  time  the  two  portions  of  the  loop  that  are 
parallel  with  the  axis  will  be  traveling  at  right  angles  to  the  lines  of 
force.  At  this  point,  therefore,  the  electromotive  force  and  the 
current  will  be  a  maximum. 

From  this  point  until  the  loop  again  assumes  a  vertical  position, 
the  cutting  of  the  lines  of  force  will  still  be  in  the  same  direction,  but 
at  a  constantly  decreasing  rate,  until,  finally,  when  the  loop  is 
vertical  the  movement  of  the  parts  of  the  loop  that  are  parallel  with 
the  axis  will  be  in  the  direction  of  the  lines  of  force  and,  therefore,  no 
cutting  will  take  place.  At  this  point,  therefore,  the  electromotive 
force  and  the  current  in  the  loop  again  will  be  zero.  We  have  seen, 
therefore,  that  in  this  half  revolution  of  the  loop  from  the  time  when 
it  was  in  a  vertical  position  to  a  time  when  it  was  again  in  a  vertical 
position  but  upside  down,  the  electromotive  force  varied  from  zero 
to  a  maximum  and  back  to  zero,  and  the  current  did  the  same. 

It  is  easy  to  see  that,  as  the  loop  moves  through  the  next  half 
revolution,  an  exactly  similar  rise  and  fall  of  electromotive  force  and 
current  will  take  place;  but  this  will  be  in  the  opposite  direction,  since 
that  portion  of  the  loop  which  was  going  down  through  the  lines 
of  force  is  now  going  up,  and  the  portion  which  was  previously  going 
up  is  now  going  down. 

The  law  concerning  the  generation  of  electromotive  force  and 
current  in  a  conductor  that  is  cutting  through  lines  of  magnetic  force, 
may  be  stated  in  another  way,  when  the  conductor  is  bent  into  the 
form  of  a  loop,  as  in  the  case  under  consideration:  Thus,  if  the 
number  of  lines  of  force  which  pass  through  a  conducting  loop  be  varied, 
electromotive  forces  will  be  generated  in  the  loop.  This  will  be  true 


MAGNETO  SIGNALING  APPARATUS  109 

whether  the  number  of  lines  passing  through  the  loop  be  varied  by 
moving  the  loop  within  the  field  of  force  or  by  varying  the  field  of 
force  itself.  In  any  case,  if  the  number  of  lines  of  force  be  increased, 
the  current  will  flow  in  one  way,  and  if  it  be  diminished  the  current 
will  flow  in  the  other  way.  The  amount  of  the  current  will  depend, 
other  things  being  equal,  on  the  rate  at  which  the  lines  of  force 
through  the  loop  are  being  varied,  regardless  of  the  method  by  which 
the  variation  is  made  to  take  place.  One  revolution  of  the  loop, 
therefore,  results  in  a  complete  cycle  of  alternating  current  consisting 
of  one  positive  followed  by  one  negative  impulse. 

The  diagram  of  Fig.  68  is  merely  intended  to  illustrate  the  prin- 
ciple involved.  In  the  practical  construction  of  magneto  generators 
more  than  one  bar  magnet  is  used,  and,  in  addition,  the  conductors 
in  the  armature  are  so  arranged  as  to  include  a  great  many  loops  of 
wire.  Furthermore,  the  conductors  in  the  armature  are  wound  around 
an  iron  core  so  that  the  path  through  the  armature  loops  or  turns, 
may  present  such  low  reluctance  to  the  passage  of  lines  of  force  as 
to  greatly  increase  the  number  of  such  lines  and  also  to  cause  prac- 
tically all  of  them  to  go  through  the  loops  in  the  armature  conductor. 

Armature.  The  iron  upon  which  the  armature  conductors  are 
wound  is  called  the  core.  The  core  of  an  ordinary  armature  is  shown 
in  Fig.  69.  This  is  usually  made 

of  soft  gray  cast  iron,  turned  so  f 

as  to  form  bearing  surfaces  at  1 
and  2,  upon  which  the  entire 
armature  may  rotate,  and  also 
turned  so  that  the  surfaces  3  will 

be   truly  Cylindrical   with  respect  Pig.  69.     Generator  Armature 

to  the  axis  through  the  center  of 

the  shaft.  The  armature  conductors  are  put  on  by  winding  the 
space  between  the  two  parallel  faces  4  as  full  of  insulated  wire  as 
space  will  admit.  One  end  of  the  armature  winding  is  soldered  to 
the  pin  5  and,  therefore,  makes  contact  with  the  frame  of  the  gen- 
erator, while  the  other  end  of  the  winding  is  soldered  to  the  pin  6, 
which  engages  the  stud  7,  carried  in  an  insulating  bushing  in  a  longi- 
tudinal hole  in  the  end  of  the  armature  shaft.  It  is  thus  seen  that 
the  frame  of  the  machine  will  form  one  terminal  of  the  armature 
winding,  while  the  insulated  stud  7  will  form  the  other  terminal. 


no 


TELEPHONY 


Another  form  of  armature  largely  employed  in  recent  magneto 
generators  is  illustrated  in  Fig.  70.  In  this  the  shaft  on  which  the 
armature  revolves  does  not  form  an  integral  part  of  the  armature  core 


•SECTION  A-A 


Fig.  70.     Generator  Armature 


but  consists  of  two  cylindrical  studs  2  and  3  projecting  from  the 
centers  of  disks  4  and  5,  which  are  screwed  to  the  ends  of  the  core  1. 
This  H  type  of  armature  core,  as  it  is  called,  while  containing  some- 
what more  parts  than  the  simpler  type  shown  in  Fig.  69,  possesses 
distinct  advantages  in  the  matter  of  winding.  By  virtue  of  its  sim- 
pler form  of  winding  space,  it  is  easier  to  insulate  and  easier  to  wind, 
and  furthermore,  since  the  shaft  does  not  run  through  the  winding 
space,  it  is  capable  of  holding  a  considerably  greater  number  of  turns 
of  wire.  The  ends  of  the  armature  winding  are  connected,  one 
directly  to  the  frame  and  the  other  to  an  insulated  pin,  as  is  shown 
in  the  illustration. 

The  method  commonly  employed  of  associating  the  pole  pieces 
with  each  other  and  with  the  permanent  magnets  is  shown  in  Fig. 


Fig.  71.     Generator  Field  and  Armature 


71.  It  is  very  important  that  the  space  in  which  the  armature 
revolves  shall  be  truly  cylindrical,  and  that  the  bearings  for  the 
armature  shall  be  so  aligned  as  to  make  the  axis  of  rotation  of  the 
armature  coincide  with  the  axis  of  the  cylindrical  surface  of  the 


MAGNETO  SIGNALING  APPARATUS  111 

pole  pieces.  A  rigid  structure  is,  therefore,  required  and  this  is 
frequently  secured,  as  shown  in  Fig.  71,  by  joining  the  two  pole 
pieces  1  and  2  together  by  means  of  heavy  brass  rods  3  and  4,  the 
rods  being  shouldered  and  their  reduced  ends  passed  through  holes  in 
flanges  extending  from  the  pole  pieces,  and  riveted.  The  bearing 
plates  in  which  the  armature  is  journaled  are  then  secured  to  the 
ends  of  these  pole  pieces,  as  will  be  shown  in  subsequent  illustra- 
tions. This  assures  proper  rigidity  between  the  pole  pieces  and  also 
between  the  pole  pieces  and  the  armature  bearings. 

The  reason  why  this  degree  of  rigidity  is  required  is  that  it  is 
necessary  to  work  with  very  small  air  gaps  between  the  armature 
core  and  its  pole  pieces  and  unless  these  generators  are  mechanically 
well  made  they  are  likely  to  alter  their  adjustment  and  thus  allow  the 
armature  faces  to  scrape  or  rub  against  the  pole  pieces.  In  Fig.  71 
one  of  the  permanent  horseshoe  magnets  is  shown,  its  ends  resting 
in  grooves  on  the  outer  faces  of  the  pole  pieces  and  usually  clamped 
thereto  by  means  of  heavy  iron  machine  screws. 

With  this  structure  in  mind,  the  theory  of  the  magneto  generator 
developed  in  connection  with  Fig.  68  may  be  carried  a  little  further. 
When  the  armature  lies  in  the  position  shown  at  the  left  of  Fig.  71,  so 
that  the  center  position  of  the  core  is  horizontal,  a  good  path  is  afford- 
ed for  the  lines  of  force  passing  from  one  pole  to  the  other.  Prac- 
tically all  of  these  lines  will  pass  through  the  iron  of  the  core  rather 
than  through  the  air,  and,  therefore,  practically  all  of  them  will  pass 
through  the  convolutions  of  the  armature  winding. 

When  the  armature  has  advanced,  say  45  degrees,  in  its  rotation 
in  the  direction  of  the  curved  arrow,  the  lower  right-hand  portion  of 
the  armature  flange  will  still  lie  opposite  the  lower  face  of  the  right- 
hand  pole  piece  and  the  upper  left-hand  portion  of  the  armature 
flange  will  still  lie  opposite  the  upper  face  of  the  left-hand  pole  piece. 
As  a  result  there  will  still  be  a  good  path  for  the  lines  of  force  through 
the  iron  of  the  core  and  comparatively  little  change  in  the  number 
of  lines  passing  through  the  armature  winding.  As  the  corners  of  the 
armature  flange  pass  away  from  the  corners  of  the  pole  pieces,  how- 
ever, there  is  a  sudden  change  in  condition  which  may  be  best  under- 
stood by  reference  to  the  right-hand  portion  of  Fig.  71.  The  lines 
of  force  now  no  longer  find  path  through  the  center  portion  of  the 
armature  core — that  lying  at  right  angles  to  their  direction  of  flow. 


112  TELEPHONY 

Two  other  paths  are  at  this  time  provided  through  the  now  horizontal 
armature  flanges  which  serve  almost  to  connect  the  two  pole  pieces. 
The  lines  of  force  are  thus  shunted  out  of  the  path  through  the  arma- 
ture coils  and  there  is  a  sudden  decrease  from  a  large  number  of 
lines  through  the  turns  of  the  winding  to  almost  none.  As  the  arma- 
ture continues  in  its  rotation  the  two  paths  through  the  flanges  are 
broken,  and  the  path  through  the  center  of  the  armature  core  and, 
therefore,  through  the  coils  themselves,  is  reestablished. 

As  a  result  of  this  consideration  it  will  be  seen  that  in  actual  prac- 
tice the  change  in  the  number  of  lines  passing  through  the  armature 
winding  is  not  of  the  gradual  nature  that  would  be  indicated  by  a 
consideration  of  Fig.  68  alone,  but  rather,  is  abrupt,  as  the  corners 


Fig.  72.     Generator  with  Magnets  Removed 

of  the  armature  flanges  leave  the  corners  of  the  pole  pieces.  This 
abrupt  change  produces  a  sudden  rise  in  electromotive  force  just  at 
these  points  in  the  rotation,  and,  therefore,  the  electromotive  force  and 
the  current  curves  of  these  magneto  generators  is  not  usually  of  the 
smooth  sine-wave  type  but  rather  of  a  form  resembling  the  sine  wave 
with  distinct  humps  added  to  each  half  cycle. 

As  is  to  be  expected  from  any  two-pole  alternating  generator, 
there  is  one  cycle  of  current  for  each  revolution  of  the  armature. 
Under  ordinary  conditions  a  person  is  able  to  turn  the  generator 
handle  at  the  rate  of  about  two  hundred  revolutions  a  minute,  and  as 
the  ratio  of  gearing  is  about  five  to  one,  this  results  in  about  one 
thousand  revolutions  per  minute  of  the  generator,  and,  therefore,  in  a 


MAGNETO  SIGNALING  APPARATUS  113 

current  of  about  one  thousand  cycles  per  minute,  this  varying  widely 
according  to  the  person  who  is  doing  the  turning. 

The  end  plates  which  support  the  bearings  for  the  armature  are 
usually  extended  upwardly,  as  shown  in  Fig.  72,  so  as  to  afford  bear- 
ings for  the  crank  shaft.  The  crank  shaft  carries  a  large  spur  gear 
which  meshes  with  a  pinion  in  the  end  of  the  armature  shaft,  so  that 
the  user  may  cause  the  armature  to  revolve  rapidly.  The  construc- 
tion shown  in  Fig.  72  is  typical  of  that  of  a  modern  magneto  generator, 
it  being  understood  that  the  permanent  magnets  are  removed  for 
clearness  of  illustration. 

Fig.  73  is  a  view  of  a  completely  assembled  generator  such  as 
is  used  for  service  requiring  a  comparatively  heavy  output.  Other 


Fig.  73.    Five-Bar  Generator 

types  of  generators  having  two,  three,  or  four  permanent  magnets 
instead  of  five,  as  shown  in  this  figure,  are  also  standard. 

Referring  again  to  Fig.  69,  it  will  be  remembered  that  one  end 
of  the  armature  winding  shown  diagrammatically  in  that  figure,  is 
terminated  in  the  pin  6,  while  the  other  terminates  in  the  pin  7.  When 
the  armature  is  assembled  in  the  frame  of  the  generator  it  is  evident 
that  the  frame  itself  is  in  metallic  connection  with  one  end  of  the  arm- 
ature winding,  since  the  pin  5  is  in  metallic  contact  with  the  armature 
casting  and  this  is  in  contact  with  the  frame  of  the  generator  through 
the  bearings.  The  frame  of  the  machine  is,  therefore,  one  terminal 
of  the  generator.  When  the  generator  is  assembled  a  spring  of  one 
form  or  another  always  rests  against  the  terminal  pin  7  of  the  arm- 
ature so  as  to  form  a  terminal  for  the  armature  winding  of  such  a 


114 


TELEPHONY 


nature  as  to  permit  the  armature  to  .rotate  freely.     Such  spring, 
therefore,  forms  the  other  terminal  of  the  generator. 

Automatic  Shunt.  Under  nearly  all  conditions  of  practice  it  is 
desirable  to  have  the  generator  automatically  perform  some  switch- 
ing function  when  it  is  operated.  As  an  example,  when  the  genera- 
tor is  connected  so  that  its  armature  is  in  series  in  a  telephone  line, 

it  is  quite  obvious  that 
the  presence  of  the  re- 
sistance and  the  impe- 
dance of  the  armature 
winding  would  be  objec- 
tionable if  left  in  the 
circuit  through  which  the 
voice  currents  had  to 
pass.  For  this  reason, 
what  is  termed  an  auto- 
matic shunt  is  employed 
Pig  74.  Generator  Shunt  Switch  on  generators  designed 

for  series  work;  this  shunt 

is  so  arranged  that  it  will  automatically  shunt  or  short-circuit  the 
armature  winding  when  it  is  at  rest  and  also  break  this  shunt  when 
the  generator  is  operated,  so  as  to  allow  the  current  to  pass  to  line. 
A  simple  and  much-used  arrangement  for  this  purpose  is  shown 
in  Fig.  74,  where  1  is  the  armature;  2  is  a  wire  leading  from  the  frame 
of  the  generator  and  forming  one  terminal  of  the  generator  circuit; 
and  3  is  a  wire  forming  the  other  terminal  of  the  generator  circuit, 
this  wire  being  attached  to  the  spring  4)  which  rests  against  the  cen- 
ter pin  of  the  armature  so  as  to  make  contact  with  the  opposite  end 
of  the  armature  winding  to  that  which  is  connected  with  the  frame. 
The  circuit  through  the  armature  may  be  traced  from  the  terminal 
wire  2  through  the  frame;  thence  through  the  bearings  to  the  arma- 
ture 1  and  through  the  pin  to  the  right-hand  side  of  the  armature 
winding.  Continuing  the  circuit  through  the  winding  itself,  it  passes 
to  the  center  pin  projecting  from  the  left-hand  end  of  the  armature 
shaft;  thence  to  the  spring  4  which  rests  against  this  pin;  and  thence 
to  the  terminal  wire  3. 

Normally,  this  path  is  shunted  by  what  is  practically  a  short 
circuit,  which  may  be  traced  from  the  terminal  2  through  the  frame 


MAGNETO  SIGNALING  APPARATUS 


115 


of  the  generator  to  the  crank  shaft  5;  thence  to  the  upper  end  of  the 
spring  4  and  out  by  the  terminal  wire  3.  This  is  the  condition  which 
ordinarily  exists  and  which  results  in  the  removal  of  the  resistance 
and  the  impedance  on  the  armature  winding  from  any  circuit  in  which 
the  generator  is  placed,  as  long  as  the  generator  is  not  operated. 

An  arrangement  is  provided,  however,  whereby  the  crank  shaft 
5  will  be  withdrawn  automatically  from  engaging  with  the  upper  end 
of  the  spring  4,  thus  breaking  the  shunt  around  the  armature  circuit, 
whenever  the  generator  crank  is  turned.  In  order  to  accomplish 
this  the  crank  shaft  5  is  capable  of  partial  rotation  and  of  slight  longi- 
tudinal movement  within  the  hub  of  the  large  gear  wheel.  A  spring 
7  usually  presses  the  crank  shaft  toward  the  left  and  into  engagement 
with  the  spring  4-  A  Pm  #  carried  by  the  crank  shaft,  rests  in  a  V- 
shaped  notch  in  the  end  of  the  hub  6  and  as  a  result,  when  the  crank 
is  turned  the  pin  rides  on  the  surface  of  this  notch  before  the  large 
gear  wheel  starts  to  turn,  and  thus  moves  the  crank  shaft  5  to  the  right 
and  breaks  the  contact  between  it  and  the  spring  4-  Thus,  as  long  as 
the  generator  is  being  operated,  its  armature  is  connected  in  the  cir- 
cuit of  the  line,  but  as  soon  as  it  becomes  idle  the  armature  is  automat- 
ically short-circuited.  Such  devices  as  this  are  termed  automatic  shunts. 

In  still  other  cases  it  is  desirable  to  have  the  generator  circuit 
normally  open  so  that  it  will  not  affect  in  any  way  the  electrical  char- 
acteristics of  the  line  while  the  line  is  being  used  for  talking.     In  this 
case  the  arrangement  is 
made  so  that  the  gener- 
ator will   automatically 
be  placed  in  proper  cir- 
cuit  relation    with    the 
line  when  it  is  operated. 

A  common  arrange- 
ment for  doing  this  is 
shown  in  Fig.  75,  where- 
in the  spring  1  normally 
rests  against  the  contact 

pin  of  the  armature  and  forms  one  terminal  of  the  armature  circuit. 
The  spring  2  is  adapted  to  form  the  other  terminal  of  the  armature 
circuit  but  it  is  normally  insulated  from  everything.  The  circuit  of 
the  generator  is,  therefore,  open  between  the  spring  2  and  the  shaft  S. 


Fig.  75.     Generator  Cut-In  Switch 


116 


TELEPHONY 


but  as  soon  as  the  generator  is  operated  the  crank  shaft  is  bodily  moved 
to  the  left  by  means  of  the  V-shaped  notch  in  the  driving  collar  4 
and  is  thus  made  to  engage  the  spring  2.  The  circuit  of  the  generator 
is  then  completed  from  the  spring  1  through  the  armature  pin  to  the 
armature  winding;  thence  to  the  frame  of  the  machine  and  through 
shaft  3  to  the  spring  2.  Such  devices  as  this  are  largely  used  in 
connection  with  so-called  "bridging"  telephones  in  which  the  gener- 
ators and  bells  are  adapted  to  be  connected  in  multiple  across  the  line. 
A  better  arrangement  for  accomplishing  the  automatic  switching 
on  the  part  of  the  generator  is  to  make  no  use  of  the  crank  shaft  as  a 
part  of  the  conducting  path  as  is  the  case  in  both  Figs.  74  and  75,  but 
to  make  the  crank  shaft,  by  its  longitudinal  movement,  impart  the 
necessary  motion  to  a  switch  spring  which,  in  turn,  is  made  to  engage 
or  disengage  a  corresponding  contact  spring.  An  arrangement  of 
this  kind  that  is  in  common  use  is  shown  in  Fig.  76.  This  needs  no 

further  explanation  than 
to  say  that  the  crank 
shaft  is  provided  on  its 
end  with  an  insulating 
stud  1,  against  which  a 
switching  spring  2  bears. 
This  spring  normally 
rests  against  another 
switch  spring  3,  but  when 
the  generator  crank  shaft 
moves  to  the  right  upon 
the  turning  of  the  crank, 
the  spring  2  disengages  spring  3  and  engages  spring  4,  thus  com- 
pleting the  circuit  of  the  generator  armature.  It  is  seen  that  this 
operation  accomplishes  the  breaking  of  one  circuit  and  the  making 
of  another,  a  function  that  will  be  referred  to  later  on  in  this  work. 

Pulsating  Current.  Sometimes  it  is  desirable  to  have  a  gener- 
ator capable  of  developing  a  pulsating  current  instead  of  an  alterna- 
ting current;  that  is,  a  current  which  will  consist  of  impulses  all  in 
one  direction  rather  than  of  impulses  alternating  in  direction.  It  is 
obvious  that  this  may  be  accomplished  if  the  circuit  of  the  generator 
be  broken  during  each  half  revolution  so  that  its  circuit  is  completed 
only  when  current  is  being  generated  in  one  direction. 


Fig.  76.     Generator  Cut-In  Switch 


MAGNETO  SIGNALING  APPARATUS 


117 


Pulsating-Current  Commutator 


Such  an  arrangement  is  indicated  diagrammatically  in  Fig.  77. 
Instead  of  having  one  terminal  of  the  armature  winding  brought 
out  through  the  frame  of  the  generator  as  is  ordinarily  done,  both 
terminals   are  brought  out   to  a 
commuting  device  carried  on  the 
end  of  the  armature  shaft.    Thus, 
one  end  of  the  loop  representing 
the   armature    winding  is  shown 
connected  directly  to  the  arma- 
ture pin  1,  against  which  bears  a 
spring  2,   in  the  usual  manner. 
The  other  end  of  the  armature 
winding  is  carried  directly  to  a 

disk  3,  mounted  on  but  insulated  from  the  shaft  and  revolving 
therewith.  One-half  of  the  circumferential  surface  of  this  disk  is  of 
insulating  material  4  and  a  spring  5  rests  against  this  disk  and  bears 
alternately  upon  the  conducting  portion  3  or  the  insulating  portion 
4,  according  to  the  position  of  the  armature  in  its  revolution.  It  is 
obvious  that  when  the  generator  armature  is  in  the  position  shown 
the  circuit  through  it  is  from  the  spring  2  to  the  pin  1;  thence  to  one 
terminal  of  the  armature  loop;  thence  through  the  loop  and  back  to 
the  disk  3  and  out  by  the  spring  5.  If,  however,  the  armature  were 
turned  slightly,  the  spring  5  would  rest  on  the  insulating  portion  4 
and  the  circuit  would  be  broken. 

It  is  obvious  that  if  the  brush  5  is  so  disposed  as  to  make  contact 
with  the  disk  3  only  during  that  portion  of  the  revolution  while  pos- 
itive current  is  being  generated,  the  generator  will  produce  positive 
pulsations  of  current,  all  the  neg- 
ative ones  being  cut  out.  If,  on 
the  other  hand,  the  spring  5  may 
be  made  to  bear  on  the  opposite 
side  of  the  disk,  then  it  is  evident 
that  the  positive  impulses  would 

all  be  cut  out  and  the  generator  would  develop  only  negative  im- 
pulses. Such  a  generator  is  termed  a  "direct-current"  generator  or 
a  "pulsating-current"  generator. 

The  symbols  for  magneto  or  hand  generators  usually  embody 
a  simplified  side  view,  showing  the  crank  and  the  gears  on  one  side 


Fig.  78.     Generator  Symbols 


118  TELEPHONY 

and  the  shunting  or  other  switching  device  on  the  other.  Thus  in 
Fig.  78  are  shown  three  such  symbols,  differing  from  each  other 
only  in  the  details  of  the  switching  device.  The  one  at  the  left  shows 
the  simple  shunt,  adapted  to  short-circuit  the  generator  at  all  times 
save  when  it  is  in  operation.  The  one  in  the  center  shows  the  cut-in, 
of  which  another  form  is  described  in  connection  with  Fig.  75;  while 
the  symbol  at  the  right  of  Fig.  78  is  of  the  make-and-break  device, 
discussed  in  connection  with  Fig.  70.  In  such  diagrammatic  repre- 
sentations of  generators  it  is  usual  to  somewhat  exaggerate  the  size  of 
the  switching  springs,  in  order  to  make  clear  their  action  in  respect  to 
the  circuit  connections  in  which  the  generator  is  used. 

Polarized  Ringer.  The  polarized  bell  or  ringer  is,  as  has  been 
stated,  the  device  which  is  adapted  to  respond  to  the  currents  sent 
out  by  the  magneto  generator.  In  order  that  the  alternately  opposite 
currents  may  cause  the  armature  to  move  alternately  in  opposite 
directions,  these  bells  are  polarized,  i.  e.,  given  a  definite  magnetic 
set,  so  to  speak;  so  the  effect  of  the  currents  in  the  coils  is  not  to  create 
magnetism  in  normally  neutral  iron,  but  rather  to  alter  the  magnetism 
in  iron  already  magnetized. 

Western  Electric  Ringer.  A  typical  form  of  polarized  bell  is 
shown  in  Fig.  79,  this  being  the  standard  bell  or  ringer  of  the  Western 
Electric  Company.  The  two  electromagnets  are  mounted  side  by 
side,  as  shown,  by  attaching  their  cores  to  a  yoke  piece  1  of  soft  iron. 
This  yoke  piece  also  carries  the  standards  2  upon  which  the  gongs 
are  mounted.  The  method  of  mounting  is  such  that  the  standards 
may  be  adjusted  slightly  so  as  to  bring  the  gongs  closer  to  or  farther 
from  the  tapper. 

The  soft  iron  yoke  piece  1  also  carries  two  brass  posts  3  which, 
in  turn,  carry  another  yoke  4  of  brass.  In  this  yoke  4  'ls  pivoted,  by 
means  of  trunnion  screws,  the  armature  5,  this  extending  on  each  side 
of  the  pivot  so  that  its  ends  lie  opposite  the  free  poles  of  the  electro- 
magnets. From  the  center  of  the  armature  projects  the  tapper  rod 
carrying  the  ball  or  striker  which  plays  between  the  two  gongs. 

In  order  that  the  armature  and  cores  may  be  normally  polarized, 
a  permanent  magnet  6  is  secured  to  the  center  of  the  yoke  piece  1. 
This  bends  around  back  of  the  electromagnets  and  comes  into  close 
proximity  to  the  armature  5.  By  this  means  one  end  of  each  of  the 
electromagnet  cores  is  given  one  polarity — say  north — while  the  arma- 


MAGNETO  SIGNALING  APPARATUS 


119 


ture  is  given  the  other  polarity— say  south.  The  two  coils  of  the  elec- 
tromagnet are  connected  together  in  series  in  such  a  way  that  current 
in  a  given  direction  will  act  to  produce  a  north  pole  in  one  of  the  free 
poles  and  a  south  pole  in  the  other.  If  it  be  assumed  that  the  perma- 
nent magnet  maintains  the  armature  normally  of  south  polarity  and 
that  the  current  through  the  coils  is  of  such  direction  as  to  make  the 
left-hand  core  north  and  the  right-hand  core  south,  then  it  is  evident 
that  the  left-hand  end  of  the  armature  will  be  attracted  and  the  right- 
hand  end  repelled.  This  will  throw  the  tapper  rod  to  the  right  and 
sound  the  right-hand  bell.  A  reversal  in  current  will  obviously  produce 
the  opposite  effect  and  cause  the  tapper  to  strike  the  left-hand  bell. 
An  important  feature  in  polarized  bells  is  the  adjustment  be- 
tween the  armature  and  the  pole  pieces.  This  is  secured  in  the 


Pig.  79.     Polarized  Bell 

Western  Electric  bell  by  means  of  the  nuts  7,  by  which  the  yoke  4  'ls 
secured  to  the  standards  3.  By  moving  these  nuts  up  or  down  on 
the  standards  the  armature  may  be  brought  closer  to  or  farther  from 
the  poles,  and  the  device  affords  ready  means  for  clamping  the  parts 
into  any  position  to  which  they  may  have  been  adjusted. 

Kellogg  Ringer.  Another  typical  ringer  is  that  of  the  Kellogg 
Switchboard  and  Supply  Company,  shown  in  Fig.  80.  This  differs 
from  that  of  the  Western  Electric  Company  mainly  in  the  details 
by  which  the  armature  adjustment  is  obtained.  The  armature 
supporting  yoke  1  is  attached  directly  to  the  cores  of  the  magnets,  no 
supporting  side  rods  being  employed.  Instead  of  providing  means 
whereby  the  armature  may  be  adjusted  toward  or  from  the  poles,  the 


120 


TELEPHONY 


reverse  practice  is  employed,  that  is,  of  making  the  poles  themselves 
extensible.  This  is  done  by  means  of  the  iron  screws  2  which  form 
extensions  of  the  cores  and  which  may  be  made  to  approach  or  recede 
from  the  armature  by  turning  them  in  such  direction  as  to  screw  them 
in  or  out  of  the  core  ends. 

Biased  Bell.  The  pulsating-current  generator  has  already 
been  discussed  and  its  principle  of  operation  pointed  out  in  con- 
nection with  Fig.  77.  The  companion  piece  to  this  generator  is 
the  so-called  biased  ringer.  This  is  really  nothing  but  a  common 
alternating-current  polarized  ringer  with  a  light  spring  so  arranged  as 
to  hold  the  armature  normally  in  one  of  its  extreme  positions  so  that 
the  tapper  will  rest  against  one  of  the  gongs.  Such  a  ringer  is  shown 
in  Fig.  81  and  needs  no  further  explanation.  It  is  obvious  that  if  a 
current  flows  in  the  coils  of  such  a  ringer  in  a  direction  tending  to 
move  the  tapper  toward  the  left,  then  no  sound  will  result  because  the 


Fig.  80.     Polarized  Bell 


Pig.  81.     Biased  Bell 


tapper  is  already  moved  as  far  as  it  can  be  in  that  direction.  If, 
however,  currents  in  the  opposite  direction  are  caused  to  flow  through 
the  windings,  then  the  electromagnetic  attraction  on  the  armature 
will  overcome  the  pull  of  the  spring  and  the  tapper  will  move  over 
and  strike  the  right-hand  gong.  A  cessation  of  the  current  will  allow 
the  spring  to  exert  itself  and  throw  the  tapper  back  into  engagement 
with  the  left-hand  gong.  A  series  of  such  pulsations  in  the  proper 
direction  will,  therefore,  cause  the  tapper  to  play  between  the  two 
gongs  and  ring  the  bell  as  usual.  A  series  of  currents  in  a  wrong 
direction  will,  however,  produce  no  effect. 


MAGNETO  SIGNALING  APPARATUS 


121 


Fig.  82.     Ringer  Symbols 


Conventional  Symbols.  In  Fig.  82  are  shown  six  conventional 
symbols  of  polarized  bells.  The  three  at  the  top,  consisting  merely 
of  two  circles  representing  the  magnets  in  plan  view,  are  perhaps  to 
be  preferred  as  they  are  well  standardized,  easy  to  draw,  and  rather 
suggestive.  The  three  at  the  bot- 
tom, showing  the  ringer  as  a  whole 
in  side  elevation,  are  somewhat 
more  specific,  but  are  objection- 
able in  that  they  take  more  space 
and  are  not  so  easily  drawn. 

Symbols  A  or  B  may  be  used 
for  designating  any  ordinary  polarized  ringer.  Symbols  C  and  D  are 
interchangeably  used  to  indicate  a  biased  ringer.  If  the  bell  is  de- 
signed to  operate  only  on  positive  impulses,  then  the  plus  sign  is 
placed  opposite  the  symbol,  while  a  minus  sign  so  placed  indicates 
that  the  bell  is  to  be  operated  only  by  negative  impulses. 

Some  specific  types  of  ringers  are  designed  to  operate  only  on  a 
given  frequency  of  current.  That  is,  they  are  so  designed  as  to  be 
responsive  to  currents  having  a  frequency  of  sixty  cycles  per  second, 
for  instance,  and  to  be  unresponsive  to  currents  of  any  other  frequency. 
Either  symbols  E  or  F  may  be  used  to  designate  such  ringers,  and  if  it 
is  desired  to  indicate  the  particular  frequency  of  the  ringer  this  is 
done  by  adding  the  proper  numeral  followed  by  a  short  reversed 
curve  sign  indicating  frequency.  Thus  50  -~  would  indicate  a  fre- 
quency of  fifty  cycles  per  second. 


CHAPTER  IX 
THE  HOOK  SWITCH 

Purpose.  In  complete  telephone  instruments,  comprising  both 
talking  and  signaling  apparatus,  it  is  obviously  desirable  that  the 
two  sets  of  apparatus,  for  talking  and  signaling  respectively,  shall 
not  be  connected  with  the  line  at  the  same  time.  A  certain  switching 
device  is,  therefore,  necessary  in  order  that  the  signaling  apparatus 
alone  may  be  left  operatively  connected  with  the  line  while  the  instru- 
ment is  not  being  used  in  the  transmission  of  speech,  and  in  order  that 
the  signaling  apparatus  may  be  cut  out  when  the  talking  apparatus 
is  brought  into  play. 

In  instruments  employing  batteries  for  the  supply  of  transmitter 
current,  another  switching  function  is  the  closing  of  the  battery  cir- 
cuit through  the  transmitter  and  the  induction  coil  when  the  instru- 
ment is  in  use  for  talking,  since  to  leave  the  battery  circuit  closed  all 
the  time  would  be  an  obvious  waste  of  battery  energy. 

In  the  early  forms  of  telephones  these  switching  operations 
were  performed  by  a  manually  operated  switch,  the  position  of  which 
the  user  was  obliged  to  change  before  and  after  each  use  of  the  tele- 
phone. The  objection  to  this  was  not  so  much  in  the  manual  labor 
imposed  on  the  user  as  in  the  tax  on  his  memory.  It  was  found  to  be 
practically  a  necessity  to  make  this  switching  function  automatic, 
principally  because  of  the  liability  of  the  user  to  forget  to  move  the 
switch  to  the  proper  position  after  using  the  telephone,  resulting  not 
only  in  the  rapid  waste  of  the  battery  elements  but  also  in  the  inoper- 
ative condition  of  the  signal-receiving  bell.  The  solution  of  this 
problem,  a  vexing  one  at  first,  was  found  in  the  so-called  automatic 
hook  switch  or  switch  hook,  by  which  the  circuits  of  the  instrument 
were  made  automatically  to  assume  their  proper  conditions  by  the 
mere  act,  on  the  part  of  the  user,  of  removing  the  receiver  from,  or 
placing  it  upon,  a  conveniently  arranged  hook  or  fork  projecting  from 
the  side  of  the  telephone  casing. 


THE  HOOK  SWITCH  123 

Automatic  Operation.  It  may  be  taken  as  a  fundamental  princi- 
ple in  the  design  of  any  piece  of  telephone  apparatus  that  is  to  be 
generally  used  by  the  public,  that  the  necessary  acts  which  a  person 
must  perform  in  order  to  use  the  device  must,  as  far  as  possible, 
follow  as  a  natural  result  from  some  other  act  which  it  is  perfectly 
obvious  to  the  user  that  he  must  perform.  So  in  the  case  of  the 
switch  hook,  the  user  of  a  telephone  knows  that  he  must  take  the 
receiver  from  its  normal  support  and  hold  it  to  his  ear;  and  likewise, 
when  he  is  through  with  it,  that  he  must  dispose  of  it  by  hanging 
it  upon  a  support  obviously  provided  for  that  purpose. 

In  its  usual  form  a  forked  hook  is  provided  for  supporting  the 
receiver  in  a  convenient  place.  This  hook  is  at  the  free  end  of  a 
pivoted  lever,  which  is  normally  pressed  upward  by  a  spring  when 
the  receiver  is  not  supported  on  it.  When,  however,  the  receiver 
is  supported  on  it,  the  lever  is  depressed  by  its  weight.  The  motion 
of  the  lever  is  mechanically  imparted  to  the  members  of  the  switch 
proper,  the  contacts  of  which  are  usually  enclosed  so  as  to  be  out  of 
reach  of  the  user.  This  switch  is  so  arranged  that  when  the  hook  is 
depressed  the  circuits  are  held  in  such  condition  that  the  talking 
apparatus  will  be  cut  out,  the  battery  circuit  opened,  and  the  signaling 
apparatus  connected  with  the  line.  On  the  other  hand,  when  the 
hook  is  in  its  raised  position,  the  signaling  apparatus  is  cut  out,  the 
talking  apparatus  switched  into  proper  working  relation  with  the 
line,  and  the  battery  circuit  closed  through  the  transmitter. 

In  the  so-called  common-battery  telephones,  where  no  magneto 
generator  or  local  battery  is  included  in  the  equipment  at  the  sub- 
scriber's station,  the  mere  raising  of  the  hook  serves  another  impor- 
tant function.  It  acts,  not  only  to  complete  the  circuit  through  the 
substation  talking  apparatus,  but,  by  virtue  of  the  closure  of  the  line 
circuit,  permits  a  current  to  flow  over  the  line  from  the  central-office 
battery  which  energizes  a  signal  associated  with  the  line  at  the  central 
office.  This  use  of  the  hook  switch  in  the  case  of  the  common-bat- 
tery telephone  is  a  good  illustration  of  the  principle  just  laid  down 
as  to  making  all  the  functions  which  the  subscriber  has  to  perform 
depend,  as  far  as  possible,  on  acts  which  his  common  sense  alone  tells 
him  he  must  do.  Thus,  in  the  common-battery  telephone  the  sub- 
scriber has  only  to  place  the  receiver  at  his  ear  and  ask  for  what  he 
wants.  This  operation  automatically  displays  a  signal  at  the  central 


124  TELEPHONY 

office  and  he  does  nothing  further  until  the  operator  inquires  for  the 
number  that  he  wants.  He  has  then  nothing  to  do  but  wait  until 
the  called-for  party  responds,  and  after  the  conversation  his  own 
personal  convenience  demands  that  he  shall  dispose  of  the  receiver 
in  some  way,  so  he  hangs  it  up  on  the  most  convenient  object,  the 
hook  switch,  and  thereby  not  only  places  the  apparatus  at  his  tele- 
phone in  proper  condition  to  receive  another  call,  but  also  conveys 
to  the  central  office  the  signal  for  disconnection. 

Likewise  in  the  case  of  telephones  operating  in  connection  with 
automatic  exchanges,  the  hook  switch  performs  a  number  of  func- 
tions automatically,  of  which  the  subscriber  has  no  conception;  and 
while,  in  automatic  telephones,  there  are  more  acts  required  of  the 
user  than  in  the  manual,  yet  a  study  of  these  acts  will  show  that  they 
all  follow  in  a  way  naturally  suggested  to  the  user,  so  that  he  need 
have  but  the  barest  fundamental  knowledge  in  order  to  properly  make 
use  of  the  instrument.  In  all  cases,  in  properly  designed  apparatus, 
the  arrangement  is  such  that  the  failure  of  the  subscriber  to  do  a 
certain  required  act  will  do  no  damage  to  the  apparatus  or  to  the 
system,  and,  therefore,  will  inconvenience  only  himself. 

Design.  The  hook  switch  is  in  reality  a  two-position  switch, 
and  while  at  present  it  is  a  simple  affair,  yet  its  development  to  its 
high  state  of  perfection  has  been  slow,  and  its  imperfections  in  the 
past  have  been  the  cause  of  much  annoyance. 

Several  important  points  must  be  borne  in  mind  in  the  design  of 
the  hook  switch.  The  spring  provided  to  lift  the  hook  must  be 
sufficiently  strong  to  accomplish  this  purpose  and  yet  must  not  be 
strong  enough  to  prevent  the  weight  of  the  receiver  from  moving  the 
switch  to  its  other  position.  The  movement  of  this  spring  must  be 
somewhat  limited  in  order  that  it  will  not  break  when  used  a  great 
many  times,  and  also  it  must  be  of  such  material  and  shape  that  it 
will  not  lose  its  elasticity  with  use.  The  shape  and  material  of  the 
restoring  spring  are,  of  course,  determined  to  a  considerable  extent 
by  the  length  of  the  lever  arm  which  acts  on  the  spring,  and  on  the 
space  which  is  available  for  the  spring. 

The  various  contacts  by  which  the  circuit  changes  are  brought 
about  upon  the  movement  of  the  hook-switch  lever  usually  take  the 
form  of  springs  of  German  silver  or  phosphor-bronze,  hard  rolled  so 
as  to  have  the  necessary  resiliency,  and  these  are  usually  tipped  with 


THE  HOOK  SWITCH  125 

platinum  at  the  points  of  contact  so  as  to  assure  the  necessary  char- 
acter of  surface  at  the  points  where  the  electric  circuits  are  made  or 
broken.  A  slight  sliding  movement  between  each  pair  of  contacts 
as  they  are  brought  together  is  considered  desirable,  in  that  it  tends 
to  rub  off  any  dirt  that  may  have  accumulated,  yet  this  sliding  move- 
ment should  not  be  great,  as  the  surfaces  will  then  cut  each  other 
and,  therefore,  reduce  the  life  of  the  switch. 

Contact  Material.  On  account  of  the  high  cost  of  platinum,  much 
experimental  work  has  been  done  to  find  a  substitute  metal  suitable 
for  the  contact  points  in  hook  switches  and  similar  uses  in  the  manu- 
facture of  telephone  apparatus.  Platinum  is  unquestionably  the  best 
known  material,  on  account  of  its  non-corrosive  and  heat-resisting 
qualities.  Hard  silver  is  the  next  best  and  is  found  in  some  first-class 
apparatus.  The  various  cheap  alloys  intended  as  substitutes  for 
platinum  or  silver  in  contact  points  may  be  dismissed  as  worthless,  so 
far  as  the  writers'  somewhat  extensive  investigations  have  shown. 

In  the  more  recent  forms  of  hook  switches,  the  switch  lever  itself 
does  not  form  a  part  of  the  electrical  circuit,  but  serves  merely  as  the 
means  by  which  the  springs  that  are  concerned  in  the  switching  func- 
tions are  moved  into  their  alternate  cooperative  relations.  One  ad- 
vantage in  thus  insulating  the  switch  lever  from  the  current-carrying 
portions  of  the  apparatus  and  circuits  is  that,  since  it  necessarily  pro- 
jects from  the  box  or  cabinet,  it  is  thus  liable  to  come  in  contact  with 
the  person  of  the  user.  By  insulating  it,  all  liability  of  the  user  re- 
ceiving shocks  by  contact  with  it  is  eliminated. 

Wall  Telephone  Hooks.  Kellogg.  A  typical  form  of  hook 
switch,  as  employed  in  the  ordinary  wall  telephone  sets,  is  shown  in 
Fig.  83,  this  being  the  standard  hook  of  the  Kellogg  Switchboard  and 
Supply  Company.  In  this  the  lever  1  is  pivoted  at  the  point  3  in  a 
bracket  5  that  forms  the  base  of  all  the  working  parts  and  the  means 
of  securing  the  entire  hook  switch  to  the  box  or  framework  of  the 
telephone.  This  switch  lever  is  normally  pressed  upward  by  a  spring 
2,  mounted  on  the  bracket  5,  and  engaging  the  under  side  of  the  hook 
lever  at  the  point  4-  Attached  to  the  lever  arm  1  is  an  insulated  pin 
6.  The  contact  springs  by  which  the  various  electrical  circuits  are 
made  and  broken  are  shown  at  7,  8,  9,  10,  and  11,  these  being  mount- 
ed in  one  group  with  insulated  bushings  between  them;  the  entire 
group  is  secured  by  machine  screws  to  a  lug  projecting  horizontally 


126  TELEPHONY 

from  the  bracket  5.  The  center  spring  9  is  provided  with  a  forked 
extension  which  embraces  the  pin  6  on  the  hook  lever.  It  is  obvious 
that  an  up-and-down  motion  of  the  hook  lever  will  move  the  long 
spring  9  in  such  manner  as  to  cause  electrical  contact  either  between 
it  and  the  two  upper  springs  7  and  8,  or  between  it  and  the  two  lower 
springs  10  and  11.  The  hook  is  shown  in  its  raised  position,  which 


Fig.  83.     Long  Lever  Hook  Switch 

is  the  position  required  for  talking.  When  lowered  the  two  springs 
7  and  8  are  disengaged  from  the  long  spring  9  and  from  each  other, 
and  the  three  springs  9,  10,  and  11  are  brought  into  electrical  engage- 
ment, thus  establishing  the  necessary  signaling  conditions. 

The  right-hand  ends  of  the  contact  springs  are  shown  projecting 
beyond  the  insulating  supports.  This  is  for  the  purpose  of  facilita- 
ting making  electrical  joints  between  these  springs  and  the  various 
wires  which  lead  from  them.  These  projecting  ends  are  commonly 
referred  to  as  ears,  and  are  usually  provided  with  holes  or  notches 
into  which  the  connecting  wire  is  fastened  by  soldering. 

Western  Electric.  Fig.  84  shows  the  type  of  hook  switch  quite 
extensively  employed  by  the  Western  Electric  Company  in  wall 
telephone  sets  where  the  space  is  somewhat  limited  and  a  compact 
arrangement  is  desired.  It  will  readily  be  seen  that  the  principle  on 
which  this  hook  switch  operates  is  similar  to  that  employed  in  Fig. 
83,  although  the  mechanical  arrangement  of  the  parts  differs  rad- 
ically. The  hook  lever  1  is  pivoted  at  3  on  a  bracket  2,  which 
serves  to  support  all  the  other  parts  of  the  switch.  The  contact 
springs  are  shown  at  4,  $>  and  6,  and  this  latter  spring  6  is  so  de- 
signed as  to  make  it  serve  as  an  actuating  spring  for  the  hook. 
This  is  accomplished  by  having  the  curved  end  of  this  spring  press 
against  the  lug  7  of  the  hook  and  thus  tend  to  raise  the  hook  when  it 
is  relieved  of  the  weight  of  the  receiver.  The  two  shorter  springs  8 
and  9  have  no  electrical  function  but  merely  serve  as  supports  against 
which  the  springs  4  and  5  may  rest  when  the  receiver  is  on  the 


THE  HOOK  SWITCH 


127 


hook,  these  springs  4  and  5  being  given  a  light  normal  tension  toward 
the  stop  springs  8  and  9.  It  is  obvious  that  in  the  particular 
arrangement  of  the  springs  in  this  switch  no  contacts  are  closed  when 
the  receiver  is  on  the  hook. 

Concerning  this  latter  feat- 
ure, it  will  be  noted  that  the 
particular  form  of  Kellogg  hook 
switch,  shown  in  Fig.  83,  makes 
two  contacts  and  breaks  two 
when  it  is  raised.  Similarly  the 
Western  Electric  Company's 
makes  two  contacts  but  does  not 
break  any  when  raised.  From 
such  considerations  it  is  custom- 
ary to  speak  of  a  hook  such  as  that  shown  in  Fig.  83  as  having  two 
make  and  two  break  contacts,  and  such  a  hook  as  that  shown  in 
Fig.  84  as  having  two  make  contacts. 

It  will  be  seen  from  either  of  these  switches  that  the  modification 
of  the  spring  arrangement,  so  as  to  make  them  include  a  varying 
number  of  make-and-break  contacts,  is  a  simple  matter,  and  switches 
of  almost  any  type  are  readily  modified  in  this  respect. 


Fig.  84.     Short  Lever  Hook  Switch 


Fig.  85.     Removable  Lever  Hook  Switch 


Dean.  In  Fig.  85  is  shown  a  decidedly  unique  hook  switch  for 
wall  telephone  sets  which  forms  the  standard  equipment  of  the  Dean 
Electric  Company.  The  hook  lever  1  is  pivoted  at  2t  an  auxiliary 


128  TELEPHONY 

lever  3  also  being  pivoted  at  the  same  point.  The  auxiliary  lever  3 
carries  at  its  rear  end  a  slotted  lug  4,  which  engages  the  long  contact 
spring  5,  and  serves  to  move  it  up  and  down  so  as  to  engage  and  dis- 
engage the  spring  G,  these  two  springs  being  mounted  on  a  base  lug 
extending  from  the  base  plate  7,  upon  which  the  entire  hook-switch 
mechanism  is  mounted.  The  curved  spring  8,  also  mounted  on  this 
same  base,  engages  the  auxiliary  lever  3  at  the  point  9  and  normally 
serves  to  press  this  up  so  as  to  maintain  the  contact  springs  5  in  en- 
gagement with  contact  spring  6.  The  switch  springs  are  moved 
entirely  by  the  auxiliary  lever  3,  but  in  order  that  this  lever  3  may 
be  moved  as  required  by  the  hook  lever  1,  this  lever  is  provided  with 
a  notched  lug  10  on  its  lower  side,  which  notch  is  engaged  by  a  for- 
wardly  projecting  lug  11  that  is  integral  with  the  auxiliary  lever  3. 
The  switch  lever  may  be  bodily  removed  from  the  remaining  parts 
of  the  hook  switch  by  depressing  the  lug  11  with  the  finger,  so  that  it 
disengages  the  notch  in  lug  10,  and  then  drawing  the  hook  lever  out  of 
engagement  with  the  pivot  stud  2,  as  shown  in  the  lower  portion  of 
the  figure.  It  will  be  noted  that  the  pivotal  end  of  the  hook  lever  is 
made  with  a  slot  instead  of  a  hole  as  is  the  customary  practice. 

The  advantage  of  being  able  to  remove  the  hook  switch  bodily 
from  the  other  portions  arises  mainly  in  connection  with  the  ship- 
ment or  transportation  of  instruments.  The  projecting  hooks  cause 
the  instruments  to  take  up  more  room  and  thus  make  larger  pack- 
ing boxes  necessary  than  would  otherwise  be  used.  Moreover,  in 
handling  the  telephones  in  store  houses  or  transporting  them  to  the 
places  where  they  are  to  be  used,  the  projecting  hook  switch  is  partic- 
ularly liable  to  become  damaged.  It  is  for  convenience  under  such 
conditions  that  the  Dean  hook  switch  is  made  so  that  the  switch 
lever  may  be  removed  bodily  and  placed,  for  instance,  inside  the 
telephone  box  for  transportation. 

Desk=Stand  Hooks.  The  problem  of  hook-swTitch  design  for 
portable  desk  telephones,  while  presenting  the  same  general  character- 
istics, differs  in  the  details  of  construction  on  account  of  the  neces- 
sarily restricted  space  available  for  the  switch  contacts  in  the  desk 
telephone. 

Western  Electric.  In  Fig.  86  is  shown  an  excellent  example  of 
hook-switch  design  as  applied  to  the  requirements  of  the  ordinary 
portable  desk  set.  This  figure  is  a  cross-sectional  view  of  the 


THE  HOOK  SWITCH 


129 


base  and  standard  of  a  familiar  type  of  desk  telephone.     The  base 
itself  is  of  stamped  metal  construction,  as  indicated,  and  the  stand- 
ard which  supports  the  transmitter  and  the  switch  hook  for  the  .re- 
ceiver is  composed  of  a  black  enameled  or  nickel-plated  brass  tube 
/,  attached  to  the  base  by  a  screw-threaded  joint,  as  shown.     The 
switch  lever  2  is  pivoted  at  3  in 
a  brass  plug  4,  closing  the  upper 
end    of    the    tube    forming    the 
standard.     This  brass  plug  sup- 
ports also  the  transmitter,  which 
is    not    shown    in    this    figure. 
Attached   to   the   plug  4  by  the 
screw  5  is  a  heavy  strip  6,  which 
reaches  down  through  the  tube  to 
the  base  plate  of  the    standard 
and  is  held  therein  by  a  screw  7. 
The  plug  4)  carrying  with  it  the 
switch-hook  lever  2  and  the  brass 
strip  6,  may  be  lifted  bodily  out 
of  the  standard  1  by  taking  out 
the  screw  7  which  holds  the  strip 
6  in  place,  as  is  clearly  indicated. 
On  the  strip  6  there  is  mounted         Fig   86     Desk.stand  Hook  Switch 
the  group  of  switch  springs  by 

which  the  circuit  changes  of  the  instrument  are  brought  about  when 
the  hook  is  raised  or  lowered.  The  spring  8  is  longer  than  the  others, 
and  projects  upwardly  far  enough  to  engage  the  lug  on  the  switch- 
hook  lever  2.  This  spring,  which  is  so  bent  as  to  close  the  contacts 
at  the  right  when  not  prevented  by  the  switch  lever,  also  serves  as 
an  actuating  spring  to  raise  the  lever  2  when  the  receiver  is  removed 
from  it.  This  spring,  when  the  receiver  is  removed  from  the  hook, 
engages  the  two  springs  at  the  right,  as  shown,  or  when  the  receiver  is 
placed  on  the  hook,  breaks  contact  with  the  two  right-hand  springs 
and  makes  contact  respectively  with  the  left-hand  spring  and  also  with 
the  contact  9  which  forms  the  transmitter  terminal. 

It  is  seen  from  an  inspection  of  this  switch  hook  that  it  has  two 
make  and  two  break  contacts.  The  various  contact  springs  are 
connected  with  the  several  binding  posts  shown,  these  forming  the 


130 


TELEPHONY 


connectors  for  the  flexible  cord  conductors  leading  into  the  base  and 
up  through  the  standard  of  the  desk  stand.  By  means  of  the  con- 
ductors in  this  cord  the  circuits  are  led  to  the  other  parts  of  the  in- 
strument, such  as  the  induction  coil,  call  bell,  and  generator,  if  there 
is  one,  which,  in  the  case  of  the  Western  Electric  Company's  desk 
set,  are  all  mounted  separately  from  the  portable  desk  stand  proper. 

This  hook  switch  is  accessible  in  an  easy  manner  and  yet  not 
subject  to  the  tampering  of  idle  or  mischievous  persons.  By  taking 
out  the  screw  7  the  entire  hook  switch  may  be  lifted  out  of  the  tube 

forming  the  standard,  the  cords 
leading  to  the  various  binding 
posts  being  slid  along  through  the 
tube.  By  this  means  the  con- 
nections to  the  hook  switch,  as 
well  as  the  contact  of  the  switch 
itself,  are  readily  inspected  or 
repaired  by  those  whose  duty  it 
is  to  perform  such  operations. 

Kellogg.  In  Fig.  87  is  shown 
a  sectional  view  of  the  desk- 
stand  hook  switch  of  the  Kellogg 
Switchboard  and  Supply  Com- 
pany. In  this  it  will  be  seen  that 
instead  of  placing  the  switch- 
hook  springs  within  the  standard 
or  tube,  as  in  the  case  of  the 
Pig.  87.  Desk-stand  Hook  Switch  Western  Electric  Company,  they 

are  mounted  tn  the  base   where 

they  are  readily  accessible  by  merely  taking  off  the  base  plate 
from  the  bottom  of  the  stand.  The  hook  lever  operates  on  the 
long  spring  of  the  group  of  switch  springs  by  means  of  a  toggle 
joint  in  an  obvious  manner.  This  switch  spring  itself  serves  by 
its  own  strength  to  raise  the  hook  lever  when  released  from  the 
weight  of  the  receiver. 

In  this  switch,  the  hook  lever,  and  in  fact  the  entire  exposed 
metal  portions  of  the  instrument,  are  insulated  from  all  of  the  contact 
springs  and,  therefore,  there  is  little  liability  of  shocks  on  the  part  of 
the  person  using  the  instrument. 


THE  HOOK  SWITCH  131 

Conventional  Symbols.  The  hook  switch  plays  a  very  impor- 
tant part  in  the  operation  of  telephone  circuits;  for  this  reason  read- 
ily understood  conventional  symbols,  by  which  they  may  be  conven- 
iently represented  in  drawings  of  circuits,  are  desirable.  In  Fig.  88 
are  shown  several  symbols  such  as  would  apply  to  almost  any  cir- 
cuit, regardless  of  the  actual  mechanical  details 
of  the  particular  hook  switch  which  happened 
to  be  employed.  Thus  diagram  A  in  Fig.  88 
shows  a  hook  switch  having  a  single  make  con- 
tact and  this  diagram  might  be  used  to  refer  to 

the  hook  switch  of  the  Dean  Electric  Company        Fte-  88.    Hook  Switch 

Symbols 

shown  in  Fig.  85,  in  which  only  a  single  con- 
tact is  made  when  the  receiver  is  removed,  and  none  is  made  when 
it  is  on  the  hook.  Similarly,  diagram  B  might  be  used  to  represent 
the  hook  switch  of  the  Kellogg  Company,  shown  in  Fig.  83,  the 
arrangement  being  for  two  make  and  two  break  contacts.  Likewise 
diagram  C  might  be  used  to  represent  the  hook  switch  of  the  Western 
Electric  Company,  shown  in  Fig.  84,  which,  as  before  stated,  has 
two  make  contacts  only.  Diagram  D  shows  another  modification 
in  which  contacts  made  by  the  hook  switch,  when  the  receiver  is 
removed,  control  two  separate  circuits.  Assuming  that  the  solid 
black  portion  represents  insulation,  it  is  obvious  that  the  contacts 
are  divided  into  two  groups,  one  insulated  from  the  other. 


CHAPTER  X 


Electromagnet.  The  physical  thing  which  we  call  an  electro- 
magnet, consisting  of  a  coil  or  helix  of  wire,  the  turns  of  which  are 
insulated  from  each  other,  and  within  which  is  usually  included  an 
iron  core,  is  by  far  the  most  useful  of  all  the  so-called  translating 
devices  employed  in  telephony.  In  performing  the  ordinary  functions 
of  an  electromagnet  it  translates  the  energy  of  an  electrical  current 
into  the  energy  of  mechanical  motion.  An  almost  equally  important 
function  is  the  converse  of  this,  that  is,  the  translation  of  the  energy 
of  mechanical  motion  into  that  of  an  electrical  current.  In  addition 
to  these  primary  functions  which  underlie  the  art  of  telephony,  the 
electromagnetic  coil  or  helix  serves  a  wide  field  of  usefulness  in  cases 
where  no  mechanical  motion  is  involved.  As  impedance  coils,  they 
serve  to  exert  important  influences  on  the  flow  of  currents  in  circuits, 
and  as  induction  coils,  they  serve  to  translate  the  energy  of  a  current 
flowing  in  one  circuit  into  the  energy  of  a  current  flowing  in  another 
circuit,  the  translation  usually,  but  not  always,  being  accompanied 
by  a  change  in  voltage. 

When  a  current  flows  through  the  convolutions  of  an  ordinary 
helix,  the  helix  will  exhibit  the  properties  of  a  magnet  even  though 
the  substance  forming  the  core  of  the  helix  is  of  non-magnetic  ma- 
terial, such  as  air,  or  wood,  or  brass.  If,  however,  a  mass  of  iron, 
such  as  a  rod  or  a  bundle  of  soft  iron  wires,  for  instance,  is  substi- 
tuted as  a  core,  the  magnetic  properties  will  be  enormously  in- 
creased. The  reason  for  this  is,  that  a  given  magnetizing  force  will 
set  up  in  iron  a  vastly  greater  number  of  lines  of  magnetic  force 
than  in  air  or  in  any  other  non-magnetic  material. 

Magnetizing  Force.  The  magnetizing  force  of  a  given  helix  is 
that  force  which  tends  to  drive  magnetic  lines  of  force  through  the 
magnetic  circuit  interlinked  with  the  helix.  It  is  called  magnetomotive 
force  and  is  analogous  to  electromotive  force,  that  is,  the  force  which 
tends  to  drive  an  electric  current  through  a  circuit. 


134  TELEPHONY 

The  magnetizing  force  of  a  given  helix  depends  on  the  product 
of  the  current  strength  and  the  number  of  turns  of  wire  in  the  helix. 
Thus,  when  the  current  strength  is  measured  in  amperes,  this  mag- 
netizing force  is  expressed  as  ampere-turns,  being  the  product  of  the 
number  of  amperes  flowing  by  the  number  of  turns.  The  magnetiz- 
ing force  exerted  by  a  given  current,  therefore,  is  independent  of  any- 
thing except  the  number  of  turns,  and  the  material  within  the  core 
or  the  shape  of  the  core  has  no  effect  upon  it. 

Magnetic  Flux.  The  total  magnetization  resulting  from  a  mag- 
netizing force  is  called  the  magnetic  flux,  and  is  analogous  to  current. 
The  intensity  of  a  magnetic  flux  is  expressed  by  the  number  of  mag- 
netic lines  of  force  in  a  square  centimeter  or  square  inch. 

While  the  magnetomotive  force  or  magnetizing  force  of  a  given 
helix  is  independent  of  the  material  of  the  core,  the  flux  which  it  sets 
up  is  largely  dependent  on  the  material  and  shape  of  the  core — not 
only  upon  this  but  on  the  material  that  lies  in  the  return  path  for  the 
flux  outside  of  the  core.  We  may  say,  therefore,  that  the  amount  of 
flux  set  up  by  a  given  current  in  a  given  coil  or  helix  is  dependent  on 
the  material  in  the  magnetic  path  or  magnetic  circuit,  and  on  the 
shape  and  length  of  that  circuit.  If  the  magnetic  circuit  be  of  air  or 
brass  or  wood  or  any  other  non-magnetic  material,  the  amount  of  flux 
set  up  by  a  given  magnetizing  force  will  be  relatively  small,  while  it  will 
be  very  much  greater  if  the  magnetic  circuit  be  composed  in  part  or 
wholly  of  iron  or  steel,  which  are  highly  magnetic  substances. 

Permeability.  The  quality  of  material,  which  permits  of  a  given 
magnetizing  force  setting  up  a  greater  or  less  number  of  lines  of  force 
within  it,  is  called  its  permeability.  More  accurately,  the  permea- 
bility is  the  ratio  existing  between  the  amount  of  magnetization  and 
the  magnetizing  force  which  produces  such  magnetization. 

The  permeability  of  a  substance  is  usually  represented  by  the 
Greek  letter  p.  (pronounced  mu}.  The  intensity  of  the  magnetizing 
force  is  commonly  symbolized  by  H,  and  since  the  permeability  of 
air  is  always  taken  as  unity,  we  may  express  the  intensity  of  magnet- 
izing force  by  the  number  of  lines  of  force  per  square  centimeter 
which  it  sets  up  in  air. 

Now,  if  the  space  on  which  the  given  magnetizing  force  H  were 
acting  were  filled  with  iron  instead  of  air,  then,  owing  to  the  greater 
permeability  of  iron,  there  would  be  set-up  a  very  much  greater  number 


ELECTROMAGNETS  AND  INDUCTIVE  COILS        135 

of  lines  of  force  per  square  centimeter,  'and  this  number  of  lines  of 
force  per  square  centimeter  in  the  iron  is  the  measure  of  the  magnet- 
ization produced  and  is  commonly  expressed  by  the  letter  B. 
From  this  we  have 


V-  = 


B 
H 


Thus,  when  we  say  that  the  permeability  of  a  given  specimen  of 
wrought  iron  under  given  conditions  is  2,000,  we  mean  that  2,000 
times  as  many  lines  of  force  would  be  induced  in  a  unit  cross-section 
of  this  sample  as  would  be  induced  by  the  same  magnetizing  force  in  a 
corresponding  unit  cross-section  of  air.  Evidently  for  air  B  =  H, 
hence  //  becomes  unity. 

The  permeability  of  air  is  always  a  constant.  This  means  that 
whether  the  magnetic  density  of  the  lines  of  force  through  the  air 
be  great  or  small  the  number  of  lines  will  always  be  proportional 
to  the  magnetizing  force.  Unfortunately  for  easy  calculations  in 
electromagnetic  work,  however,  this  is  not  true  of  the  permea- 
bility of  iron.  For  small  magnetic  densities  the  permeability  is  very 
great,  but  for  large  densities,  that  is,  under  conditions  where  the 
number  of  lines  of  force  existing  in  the  iron  is  great,  the  per- 
meability becomes  smaller,  and  an  increase  in  the  magnetizing 
force  does  not  produce  a  corresponding  increase  in  the  total  flux 
through  the  iron. 

Magnetization  Curves.  This  quality  of  iron  is  best  shown  by  the 
curves  of  Fig.  89,  which  illustrate  the  degree  of  magnetization  set  up 
in  various  kinds  of  iron  by  different  magnetizing  forces.  In  these 
curves  the  ordinates  represent  the  total  magnetization  B,  while  the 
abscissas  represent  the  magnetizing  force  H.  It  is  seen  from  an  in- 
spection of  these  curves  that  as  the  magnetizing  force  H  increases,  the 
intensity  of  flux  also  increases,  but  at  a  gradually  lessening  rate, 
indicating  a  reduction  in  permeability  at  the  higher  densities.  These 
curves  are  also  instructive  as  showing  the  great  differences  that  exist 
between  the  permeability  of  the  different  kinds  of  iron;  and  also  as 
showing  how,  when  the  magnetizing  force  becomes  very  great,  the 
iron  approaches  what  is  called  saturation,  that  is,  a  point  at  which  the 
further  increase  in  magnetizing  force  will  result  in  no  further  mag- 
aetization  of  the  core. 


136 


TELEPHONY 


From  the  data  of  the  curves  of  Fig.  89,  which  are  commonly 
called  magnetization  curves,  it  is  easy  to  determine  other  data  from 
which  so-called  permeability  curves  may  be  plotted.  In  permeability 
curves  the  total  magnetization  of  the  given  pieces  of  iron  are  plotted 
as  abscissas,  while  the  corresponding  permeabilities  are  plotted  as 
ordinates. 


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Fig.  89.     Magnetization  Curve 

Direction  of  Lines  of  Force.  The  lines  of  force  set  up  within  the 
core  of  a  helix  always  have  a  certain  direction.  This  direction  always 
depends  upon  the  direction  of  the  flow  of  current  around  the  core. 
An  easy  way  to  remember  the  direction  is  to  consider  the  helix  as 
grasped  in  the  right  hand  with  the  fingers  partially  encircling  it  and 
the  thumb  pointing  along  its  axis.  Then,  if  the  current  through  the 
convolutions  of  the  helix  be  in  the  direction  in  which  the  fingers  of  the 
hand  are  pointed  around  the  helix,  the  magnetic  lines  of  force  will 
proceed  through  the  core  of  the  helix  along  the  direction  in  which  the 
thumb  is'  pointed. 

In  the  case  of  a  simple  bar  electromagnet,  such  as  is  shown  in 
Fig.  90,  the  lines  of  force  emerging  from  one  end  of  the  bar  must  pass 
back  through  the  air  to  the  other  end  of  the  bar,  as  indicated  by  dot- 


ELECTROMAGNETS  AND  INDUCTIVE  COILS       137 

ted  lines  and  arrows.     The  path  followed  by  the  magnetic  lines  of 
force  is  called  the  magnetic  circuit,  and,  therefore,  the  magnetic  cir- 
cuit of  the  magnet  shown  in  Fig. 
90  is  composed  partly  of  iron  and 
partly  of  air.     From  what   has 
been  said  concerning  the  relative 
permeability  of  air  and  of  iron,  it 
will  be  obvious  that  the  presence 
of  such  a  long  air  path  in  the 
magnetic  circuit  will  greatly  re-  Fig.  90.    Bar  Electromagnet 

duce  the  number  of  lines  of  force 

that  a  given  magnetizing  force  can  set  up.  The  presence  of  an  air 
gap  in  a  magnetic  circuit  has  much  the  same  effect  on  the  total  flow 
of  lines  of  force  as  the  presence  of  a  piece  of  bad  conductor  in  a 
circuit  composed  otherwise  of  good  conductor,  in  the  case  of  the  flow 
of  electric  current. 

Reluctance.  As  the  property  which  opposes  the  flow  of  electric 
current  in  an  electrical  circuit  is  called  resistance,  so  the  property 
which  opposes  the  flow  of  magnetic  lines  of  force  in  a  magnetic  cir- 
cuit is  called  reluctance.  In  the  case  of  the  electric  circuit,  the  re- 
sistance is  the  reciprocal  of  the  conductivity;  in  the  case  of  the  mag- 
netic circuit,  the  reluctance  is  the  reciprocal  of  the  permeability.  As 
in  the  case  of  an  electrical  circuit,  the  amount  of  flow  of  current  is 
equal  to  the  electromotive  force  divided  by  the  resistance;  so  in  a 
magnetic  circuit,  the  magnetic  flux  is  equal  to  the  magnetizing  force 
or  magnetomotive  force  divided  by  the  reluctance. 

Types  of  Low=Reluctance  Circuits.  As  the  pull  of  an  electro- 
magnet upon  its  armature  depends  on  the  total  number  of  lines  of 
force  passing  from  the  core  to  the  armature — that  is,  on  the  total 
flux — and  as  the  total  flux  depends  for  a  given  magnetizing  force  on 
the  reluctance  of  the  magnetic  circuit,  it  is  obvious  that  the  design 
of  the  electromagnetic  circuit  is  of  great  importance  in  influencing  the 
action  of  the  magnet.  Obviously,  anything  that  will  reduce  the 
amount  of  air  or  other  non-magnetic  material  that  is  in  the  magnetic 
circuit  will  tend  to  reduce  the  reluctance,  and,  therefore,  to  increase 
the  total  magnetization  resulting  from  a  given  magnetizing  force. 

Horseshoe  Form.  One  of  the  easiest  and  most  common  ways 
of  reducing  reluctance  in  a  circuit  is  to  bend  the  ordinary  bar  electro- 


138 


TELEPHONY 


magnet   into  horseshoe  form.     In   order  to  make   clear  the   direc- 
tion of  current  flow,  attention  is  called  to  Fig.  91.     This  is  intended 

to  represent  a  simple  bar 
of  iron  with  a  winding  of 
one  direction  throughout 
its  length.  The  gap  in 
the  middle  of  the  bar, 
Fig.  91.  Bar  Electromagnet  which  divides  the  wind- 

ing into  two  parts,  is  in- 
tended merely  to  mark  the  fact  that  the  winding  need  not  cover  the 
whole  length  of  the  bar  and  still  will  be  able  to  magnetize  the  bar 
when  the  current  passes  through  it. 
In  Fig.  92  a  similar  bar  is  shown 
with  similar  winding  upon  it,  but 
bent  into  U-form,  exactly  as  if  it 
had  been  grasped  in  the  hand  and 
bent  without  further  change.  The 
magnetic  polarity  of  the  two  ends  of 
the  bar  remain  the  same  as  before 
for  the  same  direction  of  current, 
and  it  is  obvious  that  the  portion 
of  the  magnetic  circuit  which  ex- 
tends through  air  has  been  very 
greatly  shortened  by  the  bending. 
As  a  result,  the  magnetic  reluctance  of  the  circuit  has  been  greatly 
decreased  and  the  strength  of  the  magnet  correspondingly  increased. 

If  the  armature  of  the  electro- 
magnet shown  in  Fig.  92  is,  long 
enough  to  extend  entirely  across 
the  air  gap  from  the  south  to  the 
north  pole,  then  the  air  gap  in 
the  magnetic  circuit  is  still  fur- 
ther shortened,  and  is  now  repre- 
sented only  by  the  small  gap  be- 
tween the  ends  of  the  armature 
and  the  ends  of  the  core.  Such 
a  magnet,  with  an  armature 
closely  approaching  the  poles,  is  called  a  closed-circuit  magnet,  since 


Fig.  92.     Horseshoe  Electromagnet 


Fig.  93.     Horseshoe  Electromagnet 


ELECTROMAGNETS  AND  INDUCTIVE  COILS 


139 


the  only  gap  in  the  iron  of  the  magnetic  circuit  is  that  across  which 
the  magnet  pulls  in  attracting  its  armature. 

In  Fig.  93  is  shown  the  electrical  and  magnetic  counterpart  of 
Fig.  92.  The  fact  that  the  magnetic  circuit  is  not  a  single  iron  bar 
but  is  made  up  of  two  cores  and  one  backpiece  rigidly  secured  together, 
has  no  bearing  upon  the  principle,  but  only  shows  that  a  modification 
of  construction  is  possible.  In  the  construction  of  Fig.  93  the  armature 
1  is  shown  as  being  pulled  directly  against  the  two  cores  2  and  3, 
these  two  cores  being  joined  by  a  yoke  4>  which,  like  the  armature  and 
the  core,  is  of  magnetic  material.  The  path  of  the  lines  of  force  is 
indicated  by  dotted  lines.  This  is  a  very  important  form  of  elec- 
tromagnet and  is  largely  used  in  telephony. 

Iron-Clad  Form.  Another  way  of  forming  a  closed-circuit 
magnet  that  is  widely  used  in  telephony  is  to  enclose  the  helix  or 
winding  in  a  shell  of  magnetic  material  which 
joins  the  core  at  one  end.  This  construction 
results  in  what  is  known  as  the  tubular  or 
iron-clad  electromagnet,  which  is  shown  in 
section  and  in  end  view  in  Fig.  94.  In  this  the 
core  1  is  a  straight  bar  of  iron  and  it  lies 
centrally  within  a  cylindrical  shell  2,  also  of 
iron.  The  bar  is  usually  held  in  place  within 
the  shell  by  a  screw,  as  shown.  The  lines  of 
force  set  up  in  the  core  by  the  current  flowing 
through  the  coil,  pass  to  the  center  of  the 
bottom  of  the  iron  shell  and  thence  return 
through  the  metal  of  the  shell,  through  the  air 
gap  between  the  edges  of  the  shell  and  the 
armature,  and  then  concentrate  at  the  center 
of  the  armature  and  pass  back  to  the  end  of 
the  core.  This  is  a  highly  efficient  form  of 
closed-circuit  magnet,  since  the  magnetic  cir- 
cuit is  of  low  reluctance. 

Such  forms  of  magnets  are  frequently  used  where  it  is  necessary 
to  mount  a  large  number  of  them  closely  together  and  where  it  is 
desired  that  the  current  flowing  in  one  magnet  shall  produce  no  in- 
ductive effect  in  the  coils  of  the  adjacent  magnets.  The  reason  why 
mutual  induction  between  adjacent  magnets  is  obviated  in  the  case 


Fig.  94.     Iron-Clad  Elec- 
tromagnet 


140 


TELEPHONY 


of  the  iron-clad  or  tubular  magnet  is  that  practically  all  stray  field  is 
eliminated,  since  the  return  path  for  the  magnetic  lines  is  so  com- 
pletely provided  for  by  the  presence  of  the  iron  shell. 

Special  Horseshoe  Form.  In  Fig.  95  is  shown  a  type  of  relay 
commonly  employed  in  telephone  circuits.  The  purpose  of  illus- 
trating it  in  this  chapter  is  not  to  discuss  relays,  but  rather  to  show 
an  adaptation  of  an  electromagnet  wherein  low  reluctance  of  the 
magnetic  circuit  is  secured  by  providing  a  return  leg  for  the  magnetic 
lines  developed  in  the  core,  thus  forming  in  effect  a  horseshoe 
magnet  with  a  winding  on  one  of  its  limbs  only.  To  the  end  of  the 
core  1  there  is  secured  an  L-shaped  piece  of  soft  iron  2.  This  ex- 
tends upwardly  and  then  forwardly  throughout  the  entire  length  of 
the  magnet  core.  An  L-shaped  armature  3  rests  on  the  front  edge 


Fig.  95.     Electromagnet  of  Relay 

of  the  piece  2  so  that  a  slight  rocking  motion  wrill  be  permitted*  on  the 
"knife-edge"  bearing  thus  afforded.  It  is  seen  from  the  dotted  lines 
that  the  magnetic  circuit  is  almost  a  closed  one.  The  only  gap  is 
that  between  the  lower  end  of  the  armature  3  and  the  front  end  of  the 
core.  When  the  coil  is  energized,  this  gap  is  closed  by  the  attraction 
of  the  armature.  As  a  result,  the  rearwardly  projecting  end  of  the 
armature  3  is  raised  and  this  raises  the  spring  4  and  causes  it  to  break 
the  normally  existing  contact  with  the  spring  5  and  to  establish  an- 
other contact  with  the  spring  6.  Thus  the  energy  developed  within 
the  coil  of  the  magnet  is  made  to  move  certain  parts  which  in  turn 
operate  the  switching  devices  to  produce  changes  in  electrical  cir- 
cuits. These  relays  and  other  adaptations  of  the  electromagnet  will 
be  discussed  more  fully  later  on. 

There  are  almost  numberless  forms  of  electromagnets,  but  we 
have  illustrated  here  examples  of  the  principal  types  employed  in 
telephony,  and  the  modifications  of  these  types  will  be  readily  under- 
stood in  view  of  the  general  principles  laid  down. 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         141 

Direction  of  Armature  Motion.  It  may  be  said  in  general  that 
the  armature  of  an  electromagnet  always  moves  or  tends  to  move, 
when  the  coil  is  energized,  in  such  a  way  as  to  reduce  the  reluctance  of 
the  magnetic  circuit  through  the  coil.  Thus,  in  all  of  the  forms  of 
electromagnets  discussed,  the  armature,  when  attracted,  moves  in 
such  a  direction  as  to  shorten  the  air  gap  and  to  introduce  the  iron  of 
the  armature  as  much  as  possible  into  the  path  of  the  magnetic  lines, 
thus  reducing  the  reluctance.  In  the  case  of  a  solenoid  type  of  elec- 
tromagnet, or  the  coil  and  plunger  type,  which  is  a  better  name  than 
solenoid,  the  coil,  when  energized,  acts  in  effect  to  suck  the  iron  core 
or  plunger  within  itself  so  as  to  include  more  and  more  of  the  iron 
within  the  most  densely  occupied  portion  of  the  magnetic  circuit. 

Differential  Electromagnet.  Frequently  in  telephony,  the  elec- 
tromagnets are  provided  with  more  than  one  winding.  One  purpose 


Fig.  96.     Parallel  Differential  Electromagnet 


of  the  dxmble-wound'electromagnet  is  to  produce  the  so-called  differen- 
tial action  between  the  two  windings,  i.  e.,  making  one  of  the  windings 
develop  magnetization  in  the  opposite  direction  from  that  of  the  other, 
so  that  the  two  will  neutralize  each  other,  or  at  least  exert  different 
and  opposite  influences.  The  principle  of  the  differential  electro- 
magnet may  be  illustrated  in  connection  with  Fig.  96.  Here  two 
wires  1  and  2  are  shown  wrapped  in  the  same  direction  about  an  iron 
core,  the  ends  of  the  wire  being  joined  together  at  3.  Obviously,  if 
one  of  these  windings  only  is  employed  and  a  current  sent  through  it, 
as  by  connecting  the  terminals  of  a  battery  with  the  points  4  and  3, 
for  instance,  the  core  will  be  magnetized  as  in  an  ordinary  magnet. 
Likewise,  the  core  will  be  energized  if  a  current  be  sent  from  5  to  3. 
Assuming  that  the  two  windings  are  of  equal  resistance  and  number 
of  turns,  the  effects  so  produced,  when  either  the  coil  1  or  the  coil  2 
is  energized,  will  be  equal.  If  the  battery  be  connected  between  the 
terminals  4  and  5  with  the  positive  pole,  say,  at  5,  then  the  current 
will  proceed  through  the  winding  2  and  tend  to  generate  magnetism 


142 


TELEPHONY 


in  the  core  in  the  direction  of  the  arrow.  After  traversing  the  wind- 
ing 2,  however,  it  will  then  begin  to  traverse  the  other  winding  1  and 
will  pass  around  the  core  in  the  opposite  direction  throughout  the 
length  of  that  winding.  This  will  tend  to  set  up  magnetism  in  the 
core  in  the  opposite  direction  to  that  indicated  by  the  arrow.  Since  the 
two  currents  are  equal  and  also  the  number  of  turns  in  each  winding, 
it  is  obvious  that  the  two  magnetizing  influences  will  be  exactly  equal 
and  opposite  and  no  magnetic  effect  will  be  produced.  Such  a  wind- 
ing, as  is  shown  in  Fig.  96,  where  the  two  wires  are  laid  on  side  by 
side,  is  called  a  parallel  differential  winding. 

Another  way  of  winding  magnets  differentially  is  to  put  one 
winding  on  one  end  of  the  core  and  the  other  winding  on  the  other 
end  of  the  core  and  connect  these  so  as  to  cause  the  currents  through 
them  to  flow  around  the  core  in  opposite  directions.  Such  a  construc- 
tion is  shown  in  Fig.  97  and  is  called  a  tandem  differential  winding. 
The  tandem  arrangement,  while  often  good  enough  for  practical  pur- 
poses, cannot  result  in  the  complete  neutralization  of  magnetic  effect 
This  is  true  because  of  the  leakage  of  some  of  the  lines  of  force  from 


Tandem  Differential  Electromagnet 


intermediate  points  in  the  length  of  the  core  through  the  air,  result- 
ing in  some  of  the  lines  passing  through  more  of  the  turns  of  one  coil 
than  of  the  other.  Complete  neutralization  can  only  be  attained  by 
first  twisting  the  two  wires  together  with  a  uniform  lay  and  then 
winding  them  simultaneously  on  the  core. 

Mechanical  Details.  We  will  now  consider  the  actual  mechanical 
construction  of  the  electromagnet.  This  is  a  very  important  feature 
of  telephone  work,  because,  not  only  must  the  proper  electrical  and 
magnetic  effects  be  produced,  but  also  the  whole  structure  of  the 
magnet  must  be  such  that  it  will  not  easily  get  out  of  order  and  not 
be  affected  by  moisture,  heat,  careless  handling,  or  other  adverse 
conditions. 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         143 


The  most  usual  form  of    magnet    construction   employed    in 
telephony  is  shown  in  Fig.  98.     On  the  core,  which  is  of  soft  Nor- 
way iron,  usually  cylindrical  in  form,  are  forced  two  washers  of  either 
fiber  or  hard   rubber.     Fiber  is 
ordinarily  to  be  preferred  because 
it  is   tougher  and   less   liable  to 
breakage.     Around  the  core,  be- 
tween   the   two  heads,  are    then 
wrapped  several  layers  of  paper 
or    specially    prepared    cloth   in 
order  that  the  wire  forming  the      Fig.  98.    construction  of  Electromagnet 
winding  may  be  thoroughly  in- 
sulated from  the  core.     One  end  of  the  wire  is  then  passed  through 
a  hole  in  one  of   the  spool  heads  or  washers,  near  the  core,  and 
the  wire  is  then  wound  on  in  layers.      Sometimes  a  thickness  of  paper 
is  placed  around  each  layer  of  wire  in  order  to  further  guard  against 
the  breaking  down  of  the  insulation  between  layers.     When  the  last 
layer  is  wound  on,  the  end  of  the  wire  is  passed  out  through  a  hole  in 
the  head,  thus  leaving  both  ends  projecting. 

Magnet  Wire.  The  wire  used  in  winding  magnets  is,  of  course, 
an  important  part  of  the  electromagnet.  It  is  always  necessary  that 
the  adjacent  turns  of  the  wire  be  insulated  from  each  other  so  that  the 
current  shall  be  forced  to  pass  around  the  core  through  all  the  length 
of  wire  in  each  turn  rather  than  allowing  it  to  take  the  shorter  and 
easier  path  from  one  turn  to  the  next,  as  would  be  the  case  if  the  turns 
were  not  insulated.  For  this  purpose  the  wire  is  usually  covered  with 
a  coating  of  some  insulating  material.  There  are,  however,  methods 
of  winding  magnet  coils  with  bare  wire  and  taking  care  of  the  insula- 
tion between  the  turns  in  another  way,  as  will  be  pointed  out. 

Insulated  wire  for  the  purpose  of  winding  magnet  coils  is  termed 
magnet  wire.  Copper  is  the  material  almost  universally  employed 
for  the  conductor.  Its  high  conductivity,  great  ductility,  and  low 
cost  are  the  factors  which  make  it  superior  to  all  other  metals. 
However,  in  special  cases,  where  exceedingly  high  conductivity  is 
required  with  a  limited  winding  space,  silver  wire  is  sometimes  em- 
ployed, and  on  the  other  hand,  where  very  high  resistance  is  desired 
within  a  limited  winding  space,  either  iron  or  German  silver  or  some 
other  high-resistance  alloy  is  used. 


144 


TELEPHONY 


Wire  Gauges.  Wire  for  electrical  purposes  is  drawn  to  a  number 
of  different  standard  gauges.  Each  of  the  so-called  wire  gauges 
consists  of  a  series  of  graded  sizes  of  wire,  ranging  from  approximately 
one-half  an  inch  in  diameter  down  to  about  the  fineness  of  a  lady's 
hair.  In  certain  branches  of  telephone  work,  such  as  line  con- 
struction, the  existence  of  the  several  wire  gauges  or  standards  is  very 

TABLE  III 

Copper  Wire  Table 

Giving  weights,  lengths,  and  resistances  of  wire  @  68°  F.,  of  Matthiessen's 
Standard  Conductivity. 


« 

RESISTANCE 

LENGTH 

WEIGHT 

Oai 

W 
E-i    m 
W    d 

w  g  S 

co    a    0 

o 

a 

EH    D 

7. 

EH     K 

00     O 

o   o 

to    ^ 
o  a 

^  ^ 

s 

f    K    Z 
B    W    P 

K  " 

K    O 
W  p.) 

H  O 
H    _ 

S* 

P  ° 

<j 

i—  i 

5 

Oft    O 
fc 

O    K 
H 

fa    K 

fa    g 

0    « 
PH    H 

[V    H 

P 

a, 

H 

K 

* 

Pf 

^    CH 

0000 

460. 

211,600. 

0.00007639 

0.0000489 

1.561 

20,440. 

0.6405 

13,090. 

000 

409.6 

167,800. 

0.0001215 

0.0000617 

1.969 

16,210. 

0.5080 

8,232. 

00 

364.8 

133,100. 

0.0001931 

0.0000778 

2.482 

12,850. 

0.4028 

5,177. 

0 

324.9 

105,500. 

0.0003071 

0.0000981 

3.130 

10,190. 

0.3195 

3,256. 

1 

289.3 

83,690. 

0.0004883 

0.0001237 

3.947 

8,083. 

0.2533 

2,048. 

2 

257.6 

66,370. 

0.0007765 

0.0001560 

4.977 

6,410. 

0.2009 

1,288. 

3 

229.4 

52,630. 

0.001235 

0.0001967 

6.276 

5,084. 

0.1593 

810.0 

4 

204.3 

41,740. 

0.001963 

0.0002480 

7.914 

4,031. 

0.1264 

509.4 

5 

181.9 

33,100. 

0.003122 

0.0003128 

9.980 

3,197. 

0.1002 

320.4 

S 

162.0 

26,250. 

0.004963 

0.0003944 

12.58 

2,535. 

0.07946 

201.5 

7 

144.3 

20,820. 

0.007892 

0.0004973 

15.87 

2,011. 

0.06302 

126.7 

8 

128.5 

16,510. 

0.01255 

0.0006271 

20.01 

1,595. 

0.04998 

79.69 

9 

114.4 

13,090. 

0.01995 

0.0007908 

25.23 

1,265. 

0.03963 

50.12 

10 

101.9 

10,380. 

0.03173 

0.0009£72 

31.82 

1,003. 

0.03143 

31.52 

11 

90.74 

8,234. 

0.05045 

0.001257 

40.12 

795.3 

0.02493 

19.82 

12 

80.81 

6,530. 

0.08022 

0.001586 

50.59 

630.7 

0.01977 

12.47 

13 

71.96 

5,178. 

0.1276 

0.001999 

63.79 

500.1 

0.01568 

7.840 

14 

64.08 

"4,107. 

0.2028 

0.002521 

80.44 

396.6 

0.01243 

4.931 

15 

57.07 

3,257. 

0.3225 

0.003179 

101.4 

314.5 

0.009858 

3.101 

16 

50.82 

2,583. 

0.5128 

0.004009 

127.9 

249.4 

0.007818 

1.950 

17 

45.26 

2,048. 

0.8153 

0.005055 

161.3 

197.8 

0.006200 

1.226 

18 

40.30 

1,624. 

1.296 

0.006374 

203.4 

156.9 

0.004917 

0.7713 

19 

35.89 

1,288. 

2.061 

0.008038 

256.5 

124.4 

0.003899 

0.4851 

20 

31.96 

1,022. 

3.278 

0.01014 

323.4 

98.66 

0.003092 

0.3051 

21 

28.46 

810.1 

5.212 

0.01278 

407.8 

78.24 

0.002452 

0.1919 

22 

25.35 

642.4 

8.287 

0.01612 

514.2 

62.05 

0.001945 

0.1207 

23 

22.57 

509.5 

13.18 

0.02032 

648.4 

49.21 

0.001542 

0.07589 

24 

20.10 

404.0 

20.95 

0.02563 

817.6 

39.02 

0.001223 

0.04773 

25 

17.90 

320.4 

33.32 

0.03231 

1,031. 

30.95 

0.0003699 

0.03002 

26 

15.94 

254.1 

52.97 

0.04075 

1,300. 

24.54 

0.0007692 

0.1187 

27 

14.2 

201.5 

84.23 

0.05138 

1,639. 

19.46 

O.OOOG100 

0.01888 

28 

12.64 

159.8 

133.9 

0.06479 

2,067. 

15.43 

0.0004837 

0.007466 

29 

11.26 

126.7 

213.0 

0.08170 

2,607. 

12.24 

0.0003836 

0.004696 

30 

10.03 

100.5 

338.6 

0.1030 

3,287. 

9.707 

0.0003042 

0.002953 

31 

8.928 

79.70 

538.4 

0.1299 

4,145. 

7.698 

0.0002413 

0.001857 

32 

7.950 

63.21 

856.2 

0.1638 

5,227. 

6.105 

0.0001913 

0.001168 

33 

7.080 

50.13 

1,361. 

0.2066 

6,591. 

4.841 

0.0001517 

0.0007346 

34 

6.305 

39.75 

2,165. 

0.2605 

8,311. 

3.839 

0.0001203 

0.0004620 

35 

5.615 

31.52 

3,441. 

0.3284 

10,480. 

3.045 

0.00009543 

0.0002905 

36 

5.0 

25.0 

5,473. 

0.4142 

13,210. 

2.414 

0.00007568 

0.0001827 

37 

4.453 

19.83 

8,702. 

0.5222 

16,660. 

1.915 

0.00006001 

0.0001149 

38 

3.965 

15.72 

13,870. 

0.6585 

21,010. 

1.519 

0.00004759 

0.00007210 

39 

3.531 

12.47 

22,000. 

0.8304 

36,500. 

1.204 

0.00003774 

0.00004545 

40 

3.145 

9.888 

34,980. 

1.047 

33,410. 

0.9550 

0.00002993 

0.00002858 

ELECTROMAGNETS  AND  INDUCTIVE  COILS         145 

likely  to  lead  to  confusion.  Fortunately,  however,  so  far  as  magnet 
wire  is  concerned,  the  so-called  Brown  and  Sharpe,  or  American,  wire 
gauge  is  almost  universally  employed  in  this  country.  The  abbrevia- 
tions for  this  gauge  are  B.  &  S.  or  A.  W.  G. 

In  the  Brown  and  Sharpe  gauge  the  sizes,  beginning  with  the 
largest,  are  numbered  0000,  000,  00,  0,  1,  2,  and  so  on  up  to  40. 
Sizes  larger  than  about  No.  16  B.  &  S.  gauge  are  seldom  used  as 
magnet  wire  in  telephony,  but  for  the  purpose  of  making  the  list 
complete,  Table  III  is  given,  including  all  of  the  sizes  of  the  B.  &  S. 
gauge. 

In  Table  III  there  is  given  for  each  gauge  number  the  diameter 
of  the  wire  in  mils  (thousandths  of  an  inch);  the  cross-sectional  area 
in  circular  mils  (a  unit  area  equal  to  that  of  a  circle  having  a  diameter 
of  one  one-thousandth  of  an  inch) ;  the  resistance  of  the  wire  in  various 
units  of  length  and  weight;  the  length  of  the  wire  in  terms  of  resistance 
and  of  weight;  and  the  weight  of  the  wire  in  terms  of  its  length  and 
resistance. 

It  is  to  be  understood  that  in  Table  III  the  wire  referred  to  is 
bare  wire  and  is  of  pure  copper.  It  is  not  commercially  practicable 
to  use  absolutely  pure  copper,  and  the  ordinary  magnet  wire  has  a 
conductivity  equal  to  about  98  per  cent  of  that  of  pure  copper.  The 
figures  given  in  this  table  are  sufficiently  accurate  for  all  ordinary 
practical  purposes. 

Silk  and  Cotton  Insulation.  The  insulating  material  usually  em- 
ployed for  covering  magnet  wire  is  of  silk  or  cotton.  Of  these,  silk 
is  by  far  the  better  material  for  all  ordinary  purposes,  since  it  has  a 
much  higher  insulating  property  than  cotton,  and  is  very  much  thinner. 
Cotton,  however,  is  largely  employed,  particularly  in  the  larger  sizes 
of  magnet  wire.  Both  of  these  materials  possess  the  disadvantage 
of  being  hygroscopic,  that  is,  of  readily  absorbing  moisture.  This 
disadvantage  is  overcome  in  many  cases  by  saturating  the  coil  after 
it  is  wound  in  some  melted  insulating  compound,  such  as  wax  or  var- 
nish or  asphaltum,  which  will  solidify  on  cooling.  Where  the  coils 
are  to  be  so  saturated  the  best  practice  is  to  place  them  in  a  vacuum 
chamber  and  exhaust  the  air,  after  which  the  hot  insulating  compound 
is  admitted  and  is  thus  drawn  into  the  innermost  recesses  of  the  wind- 
ing space. 

Silk-insulated  wire,  as  regularly  produced,  has  either  one  or  two 


146  TELEPHONY 

layers  of  silk.  This  is  referred  to  commercially  as  single  silk  wire  or 
as  double  silk  wire.  The  single  silk  has  a  single  layer  of  silk  fibers 
wrapped  about  it,  while  the  double  silk  has  a  double  layer,  the  two 
layers  being  put  on  in  reverse  direction.  The  same  holds  true  of 
cotton  insulated  wire.  Frequently,  also,  there  is  a  combination  of  the 
two,  consisting  of  a  single  or  a  double  wrapping  of  silk  next  to  the 
wire  with  an  outer  wrapping  of  cotton.  Where  this  is  done  the  cotton 
serves  principally  as  a  mechanical  protection  for  the  silk,  the  principal 
insulating  properties  residing  in  the  silk. 

Enamel.  A  later  development  in  the  insulation  of  magnet  wire 
has  resulted  in  the  so-called  enamel  wire.  In  this,  instead  of  coating 
the  wire  with  some  fibrous  material  such  as  silk  or  cotton,  the  wire  is 
heated  and  run  through  a  bath  of  fluid  insulating  material  or  liquid 
enamel,  which  adheres  to  the  wire  in  a  very  thin  coating.  The  wire 
is  then  run  through  baking  ovens,  so  that  the  enamel  is  baked  on. 
This  process  is  repeated  several  times  so  that  a  number  of  these  thin 
layers  of  the  enamel  are  laid  on  and  baked  in  succession. 

The  characteristics  sought  in  good  enamel  insulation  for  mag- 
net wire  may  be  thus  briefly  set  forth :  It  is  desirable  for  the  insu- 
lation to  possess  the  highest  insulating  qualities;  to  have  a  glossy, 
flawless  surface;  to  be  hard  without  being  brittle;  to  adhere  tena- 
ciously and  stand  all  reasonable  handling  without  cracking  or  flak- 
ing; to  have  a  coefficient  of  elasticity  greater  than  the  wire  itself; 
to  withstand  high  temperatures;  to  be  moisture-proof  and  inert  to 
corrosive  agencies;  and  not  to  "dry  out"  or  become  brittle  over  a  long 
period  of  time. 

Space  Utilization.  The  utilization  of  the  winding  space  in  an 
electromagnet  is  an  important  factor  in  design,  since  obviously  the 
copper  or  other  conductor  is  the  only  part  of  the  winding  that  is  ef- 
fective in  setting  up  magnetizing  force.  The  space  occupied  by  the 
insulation  is,  in  this  sense,  waste  space.  An  ideally  perfect  winding 
may  be  conceived  as  one  in  which  the  space  is  all  occupied  by 
wire;  and  this  would  necessarily  involve  the  conception  of  wire  of 
square  cross-section  and  insulation  of  infinite  thinness.  In  such  a 
winding  there  would  be  no  waste  of  space  and  a  maximum  amount  of 
metal  employed  as  a  conductor.  Of  course,  such  a  condition  is  not 
possible  to  attain  and  in  practice  some  insulating  material  must  be 
introduced  between  the  layers  of  wire  and  between  the  adjacent 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         147 

convolutions  of  wire.  The  ratio  of  the  space  occupied  by  the  con- 
ductor to  the  total  space  occupied  by  the  winding,  that  is,  by  the  con- 
ductor and  the  insulation,  is  called  the  coefficient  of  space  utilization 
of  the  coil.  For  the  ideal  coil  just  conceived  the  coefficient  of  space 
utilization  would  be  1.  Ordinarily  the  coefficient  of  space  utiliza- 
tion is  greater  for  coarse  wire  than  for  fine  wire,  since  obviously  the 
ratio  of  the  diameter  of  the  wire  to  the  thickness  of  the  insulation 
increases  as  the  size  of  the  wire  grows  larger. 

The  chief  advantage  of  enamel  insulation  for  magnet  wire  is  its 
thinness,  and  the  high  coefficient  of  space  utilization  which  may  be 
secured  by  its  use.  In  good  enamel  wire  the  insulation  will  average 
about  one-quarter  the  thickness  of  the  standard  single  silk  insula- 
tion, and  the  dielectric  strength  is  equal  or  greater.  Where  economy 
of  winding  space  is  desirable  the  advantages  of  this  may  readily  be 
seen.  For  instance,  in  a  given  coil  wound  with  No.  36  single  silk 
wire  about  one-half  of  the  winding  space  is  taken  up  with  the  insula- 
tion, whereas  when  the  same  coil  is  wound  with  No.  36  enameled 
wire  only  about  one-fifth  of  the  winding  space  is  taken  up  by  the 
insulation.  Thus  the  coefficient  of  space  utilization  is  increased 
from  .50  to  .80.  The  practical  result  of  this  is  that,  in  the  case  of 
any  given  winding  space  where  No.  36  wire  is  used,  about  60  per 
cent  more  turns  can  be  put  on  with  enameled  wire  than  with  single 
silk  insulation,  and  of  course  this  ratio  greatly  increases  when  the 
comparison  is  made  with  double  silk  insulation  or  with  cotton  in- 
sulation. Again,  where  it  is  desired  to  reduce  the  winding  space  and 
keep  the  same  number  of  turns,  an  equal  number  of  turns  may  be 
had  with  a  corresponding  reduction  of  winding  space  where  enam- 
eled wire  is  used  in  place  of  silk  or  cotton. 

In  the  matter  of  heat-resisting  properties  the  enameled  wire 
possesses  a  great  advantage  over  silk  and  cotton.  Cotton  or  silk 
insulation  will  char  at  about  260°  Fahrenheit,  while  good  enameled 
wire  will  stand  400°  to  500°  Fahrenheit  without  deterioration  of  the 
insulation.  It  is  in  the  matter  of  liability  to  injury  in  rough  or  careless 
handling,  or  in  winding  coils  having  irregular  shapes,  that  enamel  wire 
is  decidedly  inferior  to  silk  or  cotton-covered  wire.  It  is  likely  to  be 
damaged  if  it  is  allowed  to  strike  against  the  sharp  corners  of  the 
magnet  spool  during  winding,  or  run  over  the  edge  of  a  hard  sur- 
face while  it  is  being  fed  on  to  the  spool.  Coils  having  other  than 


148 


TELEPHONY 


round  cores,  or  having  sharp  corners  on  their  spool  heads,  should 
not  ordinarily  be  wound  with  enamel  wire. 

The  dielectric  strength  of  enamel  insulation  is  much  greater 
than  that  of  either  silk  or  cotton  insulation  of  equal  thickness.  This 
is  a  distinct  advantage  and  frequently  a  combination  of  the  two 
kinds  of  insulation  results  in  a  superior  wire.  If  wire  insulated 
with  enamel  is  given  a  single  wrapping  of  silk  or  of  cotton,  the  insu- 
lating and  dielectric  properties  of  the  enamel  is  secured,  while  the 
presence  of  the  silk  and  cotton  affords  not  only  an  additional  safe- 
guard against  bare  spots  in  the  enamel  but  also  a  certain  degree  of 
mechanical  protection  to  the  enamel. 

Winding  Methods.  In  winding  a  coil,  the  spool,  after  being 
properly  prepared,  is  placed  upon  a  spindle  which  may  be  made  to 

revolve  rapidly.  Ordinarily  the 
wire  is  guided  on  by  hand;  some- 
times, however,  machinery  is 
used,  the  wire  being  run  over  a 
tool  which  moves  tc  and  fro 
along  the  length  of  the  spool, 
just  fast  enough  to  lay  the  wire 
on  at  the  proper  rate.  The 
movement  of  this  tool  is  much 
Fig.  99,  Electromagnet  with  Bare  Wire  the  Same  as  that  of  the  tool 

in  a  screw  cutting  lathe. 

Unless  high  voltages  are  to  be  encountered,  it  is  ordinarily  not 
necessary  to  separate  the  layers  of  wire  with  paper,  in  the  case  of 
silk-  or  cotton-insulated  magnet  wire;  although  where  especially  high 
insulation  resistance  is  needed  this  is  often  done.  It  is  necessary 
to  separate  the  successive  layers  of  a  magnet  that  is  wound  with  en- 
amel wire,  by  sheets  of  paper  or  thin  oiled  cloth. 

In  Fig.  99  is  shown  a  method,  that  has  been  used  with  some  suc- 
cess, of  winding  magnets  with  bare  wire.  In  this  the  various  adjacent 
turns  are  separated  from  each  other  by  a  fine  thread  of  silk  or  cotton 
wound  on  beside  the  wire.  Each  layer  of  wire  and  thread  as  it  is  placed 
on  the  core  is  completely  insulated  from  the  subsequent  layer  by  a  layer 
of  paper.  This  is  essentially  a  machine-wound  coil,  and  machines  for 
winding  it  have  been  so  perfected  that  several  coils  are  wound  simul- 
taneously, the  paper  being  fed  in  automatically  at  the  end  of  each  layer. 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         149 

Another  method  of  winding  the  bare  wire  omits  the  silk  thread 
and  depends  on  the  permanent  positioning  of  the  wire  as  it  is  placed 
on  the  coil,  due  to  the  slight  sinking  into  the  layer  of  paper  on  which 
it  is  wound.  In  this  case  the  feed  of  the  wire  at  each  turn  of  the 
spool  is  slightly  greater  than  the  diameter  of  the  wire,  so  that  a  small 
distance  will  be  left  between  each  pair  of  adjacent  turns. 

Upon  the  completion  of  the  winding  of  a  coil,  regardless  of 
what  method  is  used,  it  is  customary  to  place  a  layer  of  bookbinders' 
cloth  over  the  coil  so  as  to  afford  a  certain  mechanical  protection  for 
the  insulated  wire. 

Winding  Terminals.  The  matter  of  bringing  out  the  ter- 
minal ends  of  the  winding  is  one  that  has  received  a  great  deal  of 
attention  in  the  construction  of  electromagnets  and  coils  for  various 
purposes.  Where  the  winding  is  of  fine  wire,  it  is  always  well  to  re- 
inforce its  ends  by  a  short  piece  of  larger  wire.  Where  this  is  done 
the  larger  wire  is  given  several  turns  around  the  body  of  the  coil,  so 
that  the  finer  wire  with  which  it  connects  may  be  relieved  of  all 
strain  which  may  be  exerted  upon  it  from  the  protruding  ends  of  the 
wire.  Great  care  is  necessary  in  the  bringing  out  of  the  inner 
terminal — i.  e.,  the  terminal  which  connects  with  the  inner  layer — 
that  the  terminal  wire  shall  not  come  in  contact  with  any  of  the 
subsequent  layers  that  are  wound  on. 

Where  economy  of  space  is  necessary,  a  convenient  method  of 
terminating  the  winding  of  the  coil  consists  in  fastening  rigid  ter- 
minals to  the  spool  head.     This, 
in  the  case  of  a  fiber  spool  head, 
may    be  done  by  driving  heavy 
metal    terminals   into    the  fiber. 
The  connections  of  the  two  wires       FJS.  100.     Electromagnet  with  Terminals 
leading  from  the  winding  are  then 

made  with  these  heavy  rigid  terminals  by  means  of  solder.     A  coil 
having  such  terminals  is  shown  in  its  finished  condition  in  Fig.  100. 

Winding  Data.  The  two  things  principally  affecting  the  man- 
ufacture of  electromagnets  for  telephone  purposes  are  the  number  oj 
turns  in  a  winding  and  the  resistance  of  the  wound  wire.  The  lattei 
governs  the  amount  of  current  which  may  flow  through  the  coil  with 
a  given  difference  of  potential  at  its  end,  while  the  former  controls 
the  amount  of  magnetism  produced  in  the  core  by  the  current  flow- 


150 


TELEPHONY 


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TURNS  PER 
QUARE  INCH 


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ELECTROMAGNETS  AND  INDUCTIVE  COILS         151 

ing.  While  a  coil  is  being  wound,  it  is  a  simple  matter  to  count  the 
turns  by  any  simple  form  of  revolution  counter.  When  the  coil  has 
been  completed  it  is  a  simple  matter  to  measure  its  resistance.  But 
it  is  not  so  simple  to  determine  in  advance  how  many  turns  of  a  given 
size  wire  may  be  placed  on  a  given  spool,  and  still  less  simple  to 
know  what  the  resistance  of  the  wire  on  that  spool  will  be  when  the 
desired  turns  shall  have  been  wound. 

If  the  length  and  the  depth  of  the  winding  space  of  the  coil  as 
well  as  the  diameter  of  the  core  are  known,  it  is  not  difficult  to  deter- 
mine how  much  bare  copper  wire  of  a  given  size  may  be  wound 
on  it,  but  it  is  more  difficult  to  know  these  facts  concerning  copper 
wire  which  has  been  covered  with  cotton  or  silk.  Yet  something 
may  be  done,  and  tables  have  been  prepared  for  standard  wire  sizes 
with  definite  thicknesses  of  silk  and  cotton  insulation.  As  a  result 
of  facts  collected  from  a  large  number  of  actually  wound  coils,  the 
number  of  turns  per  linear  inch  and  per  square  inch  of  B.  &  S. 
gauge  wires  from  No.  20  to  No.  40  have  been  tabulated,  and  these, 
supplemented  by  a  tabulation  of  the  number  of  ohms  per  cubic  inch 
of  winding  space  for  wires  of  three  different  kinds  of  insulation,  are 
given  in  Table  IV. 

Bearing  in  mind  that  the  calculations  of  Table  IV  are  all  based 
upon  the  "diameter  over  insulation,"  which  it  states  at  the  outset 
for  each  of  four  different  kinds  of  covering,  it  is  evident  what  is  meant 
by  "turns  per  linear  inch."  The  columns  referring  to  "turns  per 
square  inch"  mean  the  number  of  turns,  the  ends  of  which  would 
be  exposed  in  one  square  inch  if  the  wound  coil  were  cut  in  a  plane 
passing  through  the  axis  of  the  core.  Knowing  the  distance  between 
the  head,  and  the  depth  to  which  the  coil  is  to  be  wound,  it  is  easy  to 
select  a  size  of  wire  which  will  give  the  required  number  of  turns  in 
the  provided  space.  It  is  to  be  noted  that  the  depth  of  winding  space 
is  one-half  of  the  difference  between  the  core  diameter  and  the  com- 
plete diameter  of  the  wound  coil.  The  resistance  of  the  entire  vol- 
ume of  wound  wire  may  be  determined  in  advance  by  knowing  the 
total  cubic  contents  of  the  winding  space  and  multiplying  this  by 
the  ohms  per  cubic  inch  of  the  selected  wire;  that  is,  one  must  multi- 
ply in  inches  the  distance  between  the  heads  of  the  spool  by  the  differ- 
ence between  the  squares  of  the  diameters  of  the  core  and  the  wind- 
ing space,  and  this  in  turn  by  .7854.  This  result,  times  the  ohms 


152  TELEPHONY 

per  cubic  inch,  as  given  in  the  table,  gives  the  resistance  of  the 
winding. 

There  is  a  considerable  variation  in  the  method  of  applying  silk 
insulation  to  the  finer  wires,  and  it  is  in  the  finer  sizes  that  the  errors, 
if  any,  pile  up  most  rapidly.  Yet  the  table  throughout  is  based  on 
data  taken  from  many  samples  of  actual  coil  winding  by  the  present 
process  of  winding  small  coils.  It  should  be  said  further  that  the 
table  does  not  take  into  account  the  placing  of  any  layers  of  paper 
between  the  successive  layers  of  the  wires.  This  table  has  been 
compared  with  many  examples  and  has  been  used  in  calculating 
windings  in  advance,  and  is  found  to  be  as  close  an  approximation 
as  is  afforded  by  any  of  the  formulas  on  the  subject,  and  with  the 
further  advantage  that  it  is  not  so  cumbersome  to  apply. 

Winding  Calculations.  In  experimental  work,  involving  the 
winding  of  coils,  it  is  frequently  necessary  to  try  one  winding  to  de- 
termine its  effect  in  a  given  circuit  arrangement,  and  from  the  knowl- 
edge so  gained  to  substitute  another  just  fitted  to  the  conditions.  It 
is  in  such  a  substitution  that  the  table  is  of  most  value.  Assume  a 
case  in  which  are  required  a  spool  and  core  of  a  given  size  with  a 
winding  of,  say  No.  25  single  silk-covered  wire,  of  a  resistance  of 
50  ohms.  Assume  also  that  the  circuit  regulations  required  that 
this  spool  should  be  rewound  so  as  to  have  a  resistance  of,  say 
1,000  ohms.  What  size  single  silk-covered  wire  shall  be  used? 
Manifestly,  the  winding  space  remains  the  same,  or  nearly  so.  The 
resistance  is  to  be  increased  from  50  to  1,000  ohms,  or  twenty  times 
its  first  value.  Therefore,  the  wire  to  be  used  must  show  in  the  table 
twenty  times  as  many  ohms  per  cubic  inch  as  are  shown  in  No.  25, 
the  known  first  size.  This  amount  would  be  twenty  times  7.489, 
which  is  149.8,  but  there  is  no  size  giving  this  exact  resistance.  No. 
32,  however,  is  very  nearly  of  that  resistance  and  if  wound  to  exactly 
the  same  depth  would  give  about  970  ohms.  A  few  turns  more 
would  provide  the  additional  thirty  ohms. 

Similarly,  in  a  coil  known  to  possess  a  certain  number  of  turns, 
the  table  will  give  the  size  to  be  selected  for  rewinding  to  a  greater 
or  smaller  number  of  turns.  In  this  case,  as  in  the  case  of  sub- 
stituting a  winding  of  different  resistance,  it  is  unnecessary  to  meas- 
ure and  calculate  upon  the  dimensions  of  the  spool  and  core.  Assume 
a  spool  wound  with  No.  30  double  silk-covered  wire,  which  requires 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         153 

to  be  wound  with  a  size  to  double  the  number  of  turns.  The  exact 
size  to  do  this  would  have  8922.  turns  per  square  inch  and  would 
be  between  No.  34  and  No.  35.  A  choice  of  these  two  wires  may 
be  made,  using  an  increased  winding  depth  with  the  smaller  wire 
and  a  shallower  winding  depth  for  the  larger  wire. 

Impedance  Coils.  In  telephony  electromagnets  frequently  serve, 
as  already  stated,  to  perform  other  functions  than  the  producing 
of  motion  by  attracting  or  releasing  their  armatures.  They  are 
required  to  act  as  impedance  coils  to  present  a  barrier  to  the  pas- 
sage of  alternating  or  other  rapidly  fluctuating  currents,  and  at 
the  same  time  to  allow  the  comparatively  free  passage  of  steady  cur- 
rents. Where  it  is  desired  that  an  electromagnet  coil  shall  possess 
high  impedance,  it  is  usual  to  employ  a  laminated  instead  of  a  solid 
core.  This  is  done  by  building  up  a  core  of  suitable  size  by  laying 
together  thin  sheets  of  soft  iron,  or  by  forming  a  bundle  of  soft  iron 
wires.  The  use  of  laminated  cores  is  for  the  purpose  of  preventing 
eddy  currents,  which,  if  allowed  to  flow,  would  not  only  be  wasteful 
of  energy  but  would  also  tend  to  defeat  the  desired  high  impedance. 
Sometimes  in  iron-clad  impedance  coils,  the  iron  shell  is  slotted  lon- 
gitudinally to  break  up  the  flow  of  eddy  currents  in  the  shell. 

Frequently  electromagnetic  coils  have  only  the  function  of 
offering  impedance,  where  no  requirements  exist  for  converting  any 
part  of  the  electric  energy  into  mechanical  work.  Where  this  is 
the  case,  such  coils  are  termed  impedance,  or  retardation,  or  choke 
coils,  since  they  are  employed  to  impede  or  to  retard  or  to  choke 
back  the  flow  of  rapidly  varying  current.  The  distinction,  therefore, 
between  an  impedance  coil  and  the  coil  of  an  ordinary  electro- 
magnet is  one  of  function,  since  structurally  they  may  be  the  same, 
and  the  same  principles  of  design  and  construction  apply  largely 
to  each. 

Number  of  Turns.  It  should  be  remembered  that  an  impe- 
dance coil  obstructs  the  passage  of  fluctuating  current,  not  so  much 
by  ohmic  resistance  as  by  offering  an  opposing  or  counter-electro- 
motive force.  Other  things  being  equal,  the  counter-electromotive 
force  of  self-induction  increases  directly  as  the  number  of  turns  on 
a  coil  and  directly  as  the  number  of  lines  of  force  threading  the  coil, 
and  this  latter  factor  depends  also  on  the  reluctance  of  the  mag- 
netic circuit.  Therefore,  to  secure  high  impedance  we  need  many 


154 


TELEPHONY 


turns  or  low  reluctance,  or  both.  Often,  owing  to  requirements  for 
direct-current  carrying  capacity  and  limitations  of  space,  a  very 
large  number  of  turns  is  not  permissible,  in  which  case  sufficiently 
high  impedance  to  such  rapid  fluctuations  as  those  of  voice  currents 
may  be  had  by  employing  a  magnetic  circuit  of  very  low  reluctance, 
usually  a  completely  closed  circuit. 

Kind  of  Iron.  An  important  factor  in  the  design  of  impedance 
coils  is  the  grade  of  iron  used  in  the  magnetic  circuit.  Obviously, 
it  should  be  of  the  highest  permeability  and,  furthermore,  there 
should  be  ample  cross-section  of  core  to  prevent  even  an  approach 
to  saturation.  The  iron  should,  if  possible,  be  worked  at  that 
density  of  magnetization  at  which  it  has  the  highest  permeability 
in  order  to  obtain  the  maximum  impedance  effects. 

Types.  Open-Circuit: — Where  very  feeble  currents  are  being 
dealt  with,  and  particularly  where  there  is  no  flow  of  direct  cur- 


Fig   101.     Section  of  Open-Circuit  Im- 
pedance Coil 


Fig.  102.     Open-Circuit  Impedance  Coil 


rent,  an  open  magnetic  circuit  is  much  used.  An  impedance  coil 
having  an  open  magnetic  circuit  is  shown  in  section  in  Fig.  101, 
Fig.  102  showing  its  external  appearance  and  illustrating  particularly 
the  method  of  bringing  out  the  terminals  of  the  winding. 

Closed-Circuit: — A  type   of   retardation    coil  which    is  largely 

used  in  systems  of  simultaneous 
telegraphy  and  telephony,  known 
as  composite  systems,  is  shown  in 
Fig.  103.  In  the  construction  of 
this  coil  the  core  is  made  of  a 
bundle  of  fine  iron  wires  first 
bent  into  U-shape,  and  then  after 
the  coils  are  in  place,  the  free 
ends  of  the  core  are  brought 
together  to  form  a  closed  mag- 
netic circuit.  The  coils  have  a 
Fig.  103.  Closed-circuit  impedance  Coil  large  number  of  turns  of  rather 


ELECTROMAGNETS  AND  INDUCTIVE  COILS         155 


Fig.  104.     Symbol  of 

Toroidal  Impedance 

Coil 


coarse  wire.  The  conditions  surrounding  the  use  of  this  coil  are 
those  which  require  very  high  impedance  and  rather  large  current- 
carrying  capacity,  and  fortunately  the  added  requirement,  that  it 
shall  be  placed  in  a  very  small  space,  does  not  exist. 

Toroidal: — Another  type  of  retardation  coil,  called  the  toroidal 
type  due  to  the  fact  that  its  core  is  a  torus  formed 
by  winding  a  continuous  length  of  fine  iron  wire, 
is  shown  in  diagram  in  Fig.  104.  The  two 
windings  of  this  coil  may  be  connected  in  series 
to  form  in  effect  a  single  winding,  or  it  may  be 
used  as  a  "split-winding"  coil,  the  two  windings 
being  in  series  but  having  some  other  element, 
such  as  a  battery,  connected  between  them  in  the 
circuit.  Evidently  such  a  coil,  however  con- 
nected, is  well  adapted  for  high  impedance,  on  account  of  the  low 
reluctance  of  its  core. 

This  coil  is  usually  mounted  on  a  base-board,  the  coil  being  en- 
closed in  a  protecting  iron  case, 
as  shown  in  Fig.  105.  The  ter- 
minal wires  of  both  windings  of 
each  coil  are  brought  out  to 
terminal  punchings  on  one  end 
of  the  base-board  to  facilitate 
the  making  of  the  necessary  cir- 
cuit connections. 

The  usual  diagrammatic  sym- 
bol for  an  impedance  coil  is  shown  in  Fig.  106.  This  is  the  same  as 
for  an  ordinary  bar  magnet,  except  that  the  parallel  lines  through 
the  core  may  be  taken  as  indicating  that  the  core 
is  laminated,  thus  conveying  the  idea  of  high  im- 
pedance. The  symbol  of  Fig.  104  is  a  good  one 
for  the  toroidal  type  of  impedance  coil. 

Induction  Coil.  An  induction  coil  consists  of  two  or  more 
windings  of  wire  interlinked  by  a  common  magnetic  circuit.  In 
an  induction  coil  having  two  windings,  any  change  in  the  strength 
of  the  current  flowing  in  one  of  the  windings,  called  the  primary, 
will  cause  corresponding  changes  in  the  magnetic  flux  threading 
the  magnetic  circuit,  and,  therefore,  changes  in  flux  through  the 


Fig.  105.     Toroidal  Impedance  Coil 


Fig.  106.     Symbol  of 
Impedance  Coil 


156  TELEPHONY 

other  winding,  called  the  secondary.  This,  by  the  laws  of  electro- 
magnetic induction,  will  produce  corresponding  electromotive  forces 
in  the  secondary  winding  and,  therefore,  corresponding  currents  in 
that  winding  if  its  circuit  be  closed. 

Current  and  Voltage  Ratios.  In  a  well-designed  induction  coil 
the  energy  in  the  secondary,  i.  e.,  the  induced  current,  is  for  all 
practical  purposes  equal  to  that  of  the  primary  current,  yet  the  values 
of  the  voltage  and  the  amperage  of  the  induced  current  may  vary 
widely  from  the  values  of  the  voltage  and  the  amperage  of  the  primary 
current.  With  simple  periodic  currents,  such  as  the  commercial 
alternating  lighting  currents,  the  ratio  between  the  voltage  in  the 
primary  and  that  in  the  secondary  will  be  equal  to  the  ratio  of  the 
number  of  turns  in  the  primary  to  the  number  of  turns  in  the  sec- 
ondary. Since  the  energy  in  the  two  circuits  will  be  practically 
the  same,  it  follows  that  the  ratio  between  the  current  in  the  primary 
and  that  in  the  secondary  will  be  equal  to  the  ratio  of  the  number  of 
turns  in  the  secondary  to  the  number  of  turns  in  the  primary.  In 
telephony,  where  the  currents  are  not  simple  periodic  currents,  and 
where  the  variations  in  current  strength  take  place  at  different  rates, 
such  a  law  as  that  just  stated  does  not  hold  for  all  cases;  but  it  may 
be  stated  in  general  that  the  induced  currents  will  be  of  higher  volt- 
age and  smaller  current  strength  than  those  of  the  primary  in  all 
coils  where  the  secondary  winding  has  a  greater  number  of  turns 
than  the  primary,  and  vice  versa. 

Functions.  The  function  of  the  induction  coil  in  telephony 
is,  therefore,  mainly  one  of  transformation,  that  is,  either  of  step- 
ping up  the  voltage  of  a  current,  or  in  other  cases  stepping  it  down. 
The  induction  coil,  however,  does  serve  another  purpose  in  cases 
where  no  change  in  voltage  and  current  strength  is  desired,  that  is, 
it  serves  as  a  means  for  electrically  separating  two  circuits  so  far 
as  any  conductive  relation  exists,  and  yet  of  allowing  the  free  trans- 
mission by  induction  from  one  of  these  circuits  to  the  other.  This 
is  a  function  that  in  telephony  is  scarcely  of  less  importance  than 
the  purely  transforming  function. 

Design.  Induction  coils,  as  employed  in  telephony,  may  be 
divided  into  two  general  types:  first,  those  having  an  open  magnetic 
circuit;  and,  second,  those  having  a  closed  magnetic  circuit.  In 
the  design  of  either  type  it  is  important  that  the  core  should  be 


ELECTROMAGNETS  AND  INDUCTIVE  COILS        157 


Fig.  107.    Induction  Coil 


thoroughly  laminated,  and  this  is  done  usually  by  forming  it  of  a 
bundle  of  soft  Swedish  or  Norway  iron  wire  about  .02  of  an  inch 
in  diameter.  The  diameter  and  the  length  of  the  coil,  and  the 
relation  between  the  number  of  turns  in  the  primary  and  in  the  sec- 
ondary, and  the  mechanical  construction  of  the  coil,  are  all  matters 
which  are  subject  to  very  wide 
variation  in  practice.  While  the 
proper  relationship  of  these  fac- 
tors is  of  great  importance,  yet 
they  may  not  be  readily  deter- 
mined except  by  actual  experi- 
ment with  various  coils,  owing  to 

the  extreme  complexity  of  the  action  which  takes  place  in  them  and 
to  the  difficulty  of  obtaining  fundamental  data  as  to  the  existing 
facts.  It  may  be  stated,  therefore,  that  the  design  of  induction  coils 
is  nearly  always  carried  out  by  "cut-and-try"  methods,  bringing  to 
bear,  of  course,  such  scientific  and  practical  knowledge  as  the  ex- 
perimenter may  possess. 

Use  and  Advantage.  The  use  and  advantages  of  the  induction 
coil  in  so-called  local-battery  telephone  sets  have  already  been  ex- 
plained in  previous  chapters.  Such  induction  coils  are  nearly  always 
of  the  open  magnetic  circuit  type,  consisting  of  a  long,  straight  core 
comprised  of  a  bundle  of  small  annealed  iron  wires,  on  which  is 
wound  a  primary  of  comparatively  coarse  wire  and  having  a  small 


Fig.  108.     Section  of  Induction  Coil 

number  of  turns,  and  over  which  is  wound  a  secondary  of  com- 
paratively fine  wire  and  having  a  very  much  larger  number  of  turns. 
A  view  of  such  a  coil  mounted  on  a  base  is  shown  in  Fig.  107,  and 
a  sectional  view  of  a  similar  coil  is  shown  in  Fig.  108.  The  method 
of  bringing  out  the  winding  terminals  is  clearly  indicated  in  this 
figure,  the  terminal  wires  2  and  4  being  those  of  the  primary  wind- 
ing and  1  and  3  those  of  the  secondary  winding.  It  is  customary 
to  bring  out  these  wires  and  attach  them  by  solder  to  suitable  ter- 


158  TELEPHONY 

minal  clips.  In  the  case  of  the  coil  shown  in  Fig.  108  these  clips  are 
mounted  on  the  wooden  heads  of  the  coil,  while  in  the  design  shown 
in  Fig.  107  they  are  mounted  on  the  base,  as  is  clearly  indicated. 

Repeating  Coil.  The  so-called  repeating  coil  used  in  teleph- 
ony is  really  nothing  but  an  induction  coil.  It  is  used  in  a  variety 
of  ways  and  usually  has  for  its  purpose  the  inductive  association 
of  two  circuits  that  are  conductively  separated.  Usually  the  re- 
peating coil  has  a  one  to  one  ratio  of  turns,  that  is,  there  are  the 
same  number  of  turns  in  the  primary  as  in  the  secondary.  How- 
ever, this  is  not  always  the  case,  since  sometimes  they  are  made  to 
have  an  unequal  number  of  turns,  in  which  case  they  are  called 
step-up  or  step-down  repeating  coils,  according  to  whether  the  pri- 
mary has  a  smaller  or  a  greater  number  of  turns  than  the  secondary. 
Repeating  coils  are  almost  universally  of  the  closed  magnetic  circuit 
type. 

Ringing  and  Talking  Considerations.  Since  repeating  coils 
often  serve  to  connect  two  telephones,  it  follows  that  it  is  some- 
times necessary  to  ring  through  them  as  well  as  talk  through  them. 
By  this  is  meant  that  it  is  necessary  that  the  coil  shall  be  so  designed 
as  to  be  capable  of  transforming  the  heavy  ringing  currents  as  well 
as  the  very  much  smaller  telephone  or  voice  currents.  Ringing 
currents  ordinarily  have  a  frequency  ranging  from  about  16  to  75 
cycles  per  second,  while  voice  currents  have  frequencies  ranging 
from  a  few  hundred  up  to  perhaps  ten  thousand  per  second.  Ordi- 
narily, therefore,  the  best  form  of  repeating  coil  for  transforming 
voice  currents  is  not  the  best  for  transforming  the  heavy  ringing 
currents  and  vice  versa.  If  the  comparatively  heavy  ringing  currents 
alone  were  to  be  considered,  the  repeating  coil  might  well  be  of 
heavy  construction  with  a  large  amount  of  iron  in  its  magnetic  cir- 
cuit. On  the  other  hand,  for  carrying  voice  currents  alone  it  is 
usually  made  with  a  small  amount  of  iron  and  with  small  windings, 
in  order  to  prevent  waste  of  energy  in  the  core,  and  to  give  a  high 
degree  of  responsiveness  with  the  least  amount  of  distortion  of  wave 
form,  so  that  the  voice  currents  will  retain  as  far  as  possible  their 
original  characteristics.  When,  therefore,  a  coil  is  required  to  carry 
both  ringing  and  talking  currents,  a  compromise  must  be  effected. 

Types.  The  form  of  repeating  coil  largely  used  for  both  ring- 
ing and  talking  through  is  shown  in  Fig.  109.  This  coil  comprises  a 


ELECTROMAGNETS  AND  INDUCTIVE  COILS 


159 


soft  iron  core  made  up  of  a  bundle  of  wires  about  .02  inch  in  diameter, 
the  ends  of  which  are  left  of  sufficient  length  to  be  bent  back  around 
the  windings  after  they  are  in  place  and  thus  form  a  completely 


Fig.  109.    Repeating  Coil 

closed  magnetic  path  for  the  core.  The  windings  of  this  particular 
coil  are  four  in  number,  and  contain  about  2,400'"  turns  each,  and 
have  a  resistance  of  about  60  ohms.  In  this  coil,  when  connected 
for  local  battery  work,  the  windings  are  connected  in  pairs  in  series, 
thus  forming  effectively  two  windings  having  about  120  ohms  re- 
sistance each.  The  whole  coil  is  enclosed  in  a  protecting  case  of 
iron.  The  terminals  are  brought  out  to  suitable  clips  on  the  wooden 
base,  as  shown.  An  external  perspective  view  of  this  coil  is  shown 
in  Fig.  110.  By  bringing  out  each  terminal  of  each  winding,  eight 


Fig.  110.    Repeating  Coil 

in  all,  as  shown  in  this  figure,  great  latitude  of  connection  is  provided 
for,  since  the  windings  may  be  connected  in  circuit  in  any  desirable 
way,  either  by  connecting  them  together  in  pairs  to  form  virtually  a 
primary  and  a  secondary,  or,  as  is  frequently  the  case,  to  split  the 
primary  and  the  secondary,  connecting  a  battery  between  each  pair 
of  windings. 


160 


TELEPHONY 


f\ 


Fig.   Ill  illustrates  in  section  a  commercial  type  of  coil  de- 
signed for  talking  through  only.     This  coil  is  provided  with  four 

windings  of  1,357  turns  each, 
and  when  used  for  local  bat- 
tery work  the  coils  are  con- 
nected in  pairs  in  series,  thus 
giving  a  resistance  of  about 
190  ohms  in  each  half  of  the 
repeating  coil.  The  core  of 
this  coil  consists  of  a  bundle 
of  soft  iron  wires,  and  the 
shell  which  forms  the  return 
path  for  the  magnetic  lines 
is  of  very  soft  sheet  iron. 

Repeating  Coil  .     This  shell  is  drawn  into  cup 

shape  and    its   open  end   is 

closed,  after  the  coil  is  inserted,  by  the  insertion  of  a  soft  iron  head, 
as  indicated.     As  in  the  case  of  the  coil  shown  in  Figs.  109  and  110, 
eight  terminals  are  brought  out  on  this  coil,  thus 
providing  the  necessary  flexibility  of  connection. 
Still  another  type  of  repeating  coil  is  illus- 
trated  in  diagram  in  Fig.  112,  and  in  view  in 
Fig.   113.     This    coil,    like    the   impedance   coil 
shown  in  Fig.  104,  comprises  a  core  made  up  of  a 
bundle  of  soft  iron  wires  wound  into  the  form  of 
a  ring.     It  is  usually  provided  with  two  primary 
windings   placed   opposite  each  other  upon  the 
core,  and  with  two  secondary  windings,  one  over  each  primary.     In 
practice  these  two  primary  windings  are  connected  in  one  circuit  and 


Fit 


Fig.  112.  Diagram  of 

Toroidal  Repeating 

Coil 


Fig.  113.     Toroidal  Repeating  Coil 


the  two  secondaries  in  another.    This  is  the  standard  repeating  coil 
now  used  by  the  Bell  companies  in  their  common-battery  cord  circuits. 


ELECTROMAGNETS  AND  INDUCTIVE  COILS '       161 

Conventional  Symbols.  The  ordinary  symbol  for  the  induc- 
tion coil  used  in  local  battery  work  is  shown  in  Fig.  1 14.  This  con- 
sists merely  of  a  pair  of  parallel  zig-zag  lines. 

The   primary   winding   is    usually  indicated  by  a     \ .  .  / 

u          r       u     •         *  A      VWWWWW 

heavy  line  having  a  fewer  number  ol  zig-zags,  and       A A A A AA 

the   secondary   by   a   finer   line   having  a  greater      /VVV"V\ 
number  of  zig-zags.     In  this  way  the  fact  that  the    Fig.  114.  Symbol  of 

.        „,  .  ,      /  ,.      ,      ,  Induction  Coil 

primary  is  01  large  wire  and  or  comparatively  lew 
turns  is  indicated.      This  diagrammatic  symbol  may  be  modified 
to   suit   almost  any  conditions,  and  where  a  tertiary  as  well  as  a 
secondary  winding  is  provided  it  may  be  shown  by  merely  adding 
another  zig-zag  line. 

The  repeating  coil  is  indicated  symbolically  in  the  two  diagrams 
of  Fig.  115.  Where  there  is  no  necessity  for  indicating  the  internal 
connections  of  the  coil,  the  symbol  shown  in 
the  left  of  this  figure  is  usually  employed. 
Where,  however,  the  coil  consists  of  four 
windings  rather  than  two  and  the  method 
of  connecting  them  is  to  be  indicated,  the 

Fig'  115'sy5fboeif ing"°oil  symkol  at  the  right  hand  is  employed.  In 
Fig.  116  another  way  of  indicating  a  four- 
winding  repeating  coil  or  induction  coil  is  shown.  Sometimes  such 
windings  may  be  combined  by  connection  to 
form  merely  a  primary  and  a  secondary  winding, 
and  in  other  cases  the  four  windings  all  act  sep- 
arately, in  which  case  one  may  be  considered 
the  primary  and  the  others,  respectively,  the  sec- 
ondary, tertiary,  and  quaternary.  FFour-winSbRe-0f 

Where  the  toroidal  type  of  repeating  coil  is 

employed,  the  diagram  of   Fig.  112,  already  referred  to,  is  a  good 
symbolic  representation. 


WWWW/ 
MMMMA 


CHAPTER   XI 


NON=INDUCTIVE  RESISTANCE  DEVICES 

It  is  often  desired  to  introduce  simple  ohmic  resistance  into 
telephone  circuits,  in  order  to  limit  the  current  flow,  or  to  create 
specific  differences  of  potential  at  given  points  in  the  circuit. 

Temperature  Coefficient.  The  design  or  selection  of  resistance 
devices  for  various  purposes  frequently  involves  the  consideration  of 
the  effect  of  temperature  on  the  resistance  of  the  conductor  em- 
ployed. The  resistance  of  conductors  is  subject  to  change  by  changes 
in  temperature.  While  nearly  all  metals  show  an  increase,  carbon 
shows  a  decrease  in  its  resistance  when  heated. 

The  temperature  coefficient  of  a  conductor  is  a  factor  by  which 
the  resistance  of  the  conductor  at  a  given  temperature  must  be  mul- 
tiplied in  order  to  determine  the  change  in  resistance  of  that  conduc- 
tor brought  about  by  a  rise  in  temperature  of  one  degree. 

TABLE  V 
Temperature  Coefficients 


PURE  METALS 

TEMPERATURE   COEFFICIENTS 

CENTIGRADE 

FAHRENHEIT 

Silver  (annealed) 
Copper  (annealed) 
Gold  (99.9%) 
Aluminum  (99%) 
Zinc 

0.00400 
0.00428 
0.00377 
0.00423 
0.00406 

0.00222 
0.00242 
0.00210 
0.00235 
0.00226 

Platinum  (annealed) 
Iron 

0.00247 
0.00625 

0.00137 
0.00347 

Nickel 

0.0062 

0.00345 

Tin 

0.00440 

0.00245 

Lead 

0.00411 

0.00228 

Antimony 
Mercury 
Bismuth 

0.00389 
0.00072 
0.00354 

0.00216 
0.00044 
0.00197 

NON-INDUCTIVE  RESISTANCE  DEVICES  163 

Positive  and  Negative  Coefficients,  Those  conductors,  in  which 
a  rise  in  temperature  produces  an  increase  in  resistance,  are  said  to 
have  positive  temperature  coefficients,  while  those  in  which  a  rise 
in  temperature  produces  a  lowering  of  resistance  are  said  to  have 
negative  temperature  coefficients. 

The  temperature  coefficients  of  pure  metals  are  always  positive 
and  for  some  of  the  more  familiar  metals,  have  values,  according  to 
Foster,  as  in  Table  V. 

Iron,  it  will  be  noticed,  has  the  highest  temperature  coefficient 
of  all.  Carbon,  on  the  other  hand,  has  a  large  negative  coefficient,  as 
proved  by  the  fact  that  the  filament  of  an  ordinary  incandescent 
lamp  has  nearly  twice  the  resistance  when  cold  as  when  heated  to 
full  candle-power. 

Certain  alloys  have  been  produced  which  have  very  low  tem- 
perature coefficients,  and  these  are  of  value  in  producing  resistance 
units  which  have  practically  the  same  resistance  for  all  ordinary  tem- 
peratures. Some  of  these  alloys  also  have  very  high  resistance  as 
compared  with  copper  and  are  of  value  in  enabling  one  to  obtain  a 
high  resistance  in  small  space. 

One  of  the  most  valuable  resistance  wires  is  of  an  alloy 
known  as  German  silver.  The  so-called  eighteen  per  cent  alloy 
has  approximately  18.3  times  the  resistance  of  copper  and  a 
temperature  coefficient  of  .00016  per  degree  Fahrenheit.  The 
thirty  per  cent  alloy  has  approximately  28  times  the  resistance 
of  copper  and  a  temperature  coefficient  of  .00024  per  degree 
Fahrenheit. 

For  facilitating  the  design  of  resistance  coils  of  German  silver 
wire,  Tables  VI  and  VII  are  given,  containing  information  as  to 
length,  resistance,  and  weight  of  the  eighteen  per  cent  and  the  thirty 
per  cent  alloys,  respectively,  for  all  sizes  of  wire  smaller  than  No. 
20  B.  &  S.  gauge. 

Special  resistance  alloys  may  be  obtained  having  temperature 
coefficients  as  low  as  .000003  per  degree  Fahrenheit.  Other  alloys 
of  nickel  and  steel  are  adapted  for  use  where  the  wire  must  carry 
heavy  currents  and  be  raised  to  comparatively  high  temperatures 
thereby ;  for  such  use  non-corrosive  properties  are  specially  to  be  de- 
sired. Such  wire  may  be  obtained  having  a  resistance  of  about  fifty 
times  that  of  copper. 


164 


TELEPHONY 


TABLE  VI 
18  Per  Cent  German  Silver  Wire 


No. 
B.  &  S. 
GAUGE 

DIAMETER 
INCHES 

WEIGHT 
POUNDS  PER  FOOT 

LENGTH 
FEET  PER  POUND 

RESISTANCE 
OHMS  PE  it  FOOT 

21 

.02846 

.002389 

418.6 

.2333 

22 

.02535 

.001894 

527.9 

.2941 

23 

.02257 

.001502 

665.8 

.3710 

24 

.02010 

.001191 

839.5 

.4678 

25 

.01790 

.0009449 

1058. 

.5899 

26 

.01594 

.0007493 

1335. 

.7438 

27 

.01419 

.0005943 

1683. 

.9386 

28 

.01264 

.0004711 

2123. 

1.183 

29 

.01126 

.0003735 

2677. 

1.491 

30 

.01003 

.0002962 

3376. 

1.879 

31 

.008928 

.0002350 

4255. 

2.371 

32 

.007950 

.0001864 

5366. 

2.990 

33 

.007080 

.0001478 

6766. 

3.771 

34 

.006304 

.0001172 

8532. 

4.756 

35 

.005614 

.00009295 

10758. 

5.997 

36 

.005000 

.00007369 

13569  . 

7.560 

37 

.004453 

.00005845 

17108. 

9.532 

38 

.003965 

.00004636 

21569. 

12.02 

39 

.003531 

.00003675 

27209  . 

15.16 

40 

.003145 

.00002917 

34282  . 

19.11 

Inductive  Neutrality.  Where  the  resistance  unit  is  required  to 
be  strictly  non-inductive,  and  is  to  be  in  the  form  of  a  coil,  special 
designs  must  be  employed  to  give  the  desired  inductive  neutrality. 

Provisions  Against  Heating.  In  cases  where  a  considerable 
amount  of  heat  is  to  be  generated  in  the  resistance,  due  to  the  ne- 
cessity of  carrying  large  currents,  special  precautions  must  be  taken 
as  to  the  heat-resisting  properties  of  the  structure,  and  also  as  to  the 
provision  of  sufficient  radiating  surface  or  its  equivalent  to  provide 
for  the  dissipation  of  the  heat  generated. 

Types.  Mica  Card  Unit.  One  of  the  most  common  resistance 
coils  used  in  practice  is  shown  in  Fig.  117.  This  comprises  a  coil 
of  fine,  bare  German  silver  wire  wound  on  a  card  of  mica,  the  wind- 
ings being  so  spaced  that  the  loops  are  not  in  contact  with  each 
other.  The  winding  is  protected  by  two  cards  of  mica  and  the  whole 
is  bound  in  place  by  metal  strips,  to  which  the  ends  of  the  winding 


NON-INDUCTIVE  RESISTANCE  DEVICES 


165 


TABLE  VII 
30  Per  Cent  German  Silver  Wire 


No. 
B.  &  S. 
GAUGE 

DIAMETER 
INCHES 

WEIGHT 
POUNDS  PER  FOOT 

LENGTH 
FEET  PER  POUND 

RESISTANCE 
OHMSPERFOOT 

21 

.02846 

.002405 

415.8 

.3581 

22 

.02535 

.001907 

524.4 

.4513 

23 

.02257 

.001512 

661.3 

.5693 

24 

.02010 

.001199 

833.9 

.7178 

25 

.01790 

.0009513 

1051. 

.9051 

26 

.01594 

.0007544 

1326. 

1.141 

27 

.01419 

.0005983 

1671. 

1.440 

28 

.01264 

.0004743 

,  2108. 

1.815 

29 

.01126 

.0003761 

2659. 

2.287 

30 

.01003 

.0002982 

3353. 

2.883 

31 

.008928 

.0002366 

4227. 

3.638 

32 

.007950 

.0001876 

5330. 

4.588 

33 

.007080 

.0001488 

6721. 

5.786 

34 

.006304 

.0001180 

8475. 

7.297 

35 

.005614 

.00009358 

10686  . 

9.201 

36 

.005000 

.00007419 

13478. 

11.60 

37 

.004453 

.00005885 

16994. 

14.63 

38 

.003965 

.00004668 

21424. 

18.45 

39 

.003531 

.00003700 

27026  . 

23.26 

40 

.003145 

.00002937 

34053  . 

29.32 

are  attached.  Binding  posts  are  provided  on  the  extended  por- 
tions of  the  terminals  to  assist  in  mounting  the  resistance  on  a  sup- 
porting frame,  and  the  posts  terminate  in  soldering  terminals  by 
which  the  resistance  is  connected  into  the  circuit. 

Differentially-Wound  Unit.  Another  type  of  resistance  coil  is 
that  in  which  the  winding  is  placed  upon  an  insulating  core  of  heat- 
resisting  material  and  wound  so  as  to  overcome  inductive  effects. 
In  order  to  accomplish  this,  the  wire  to  be  bound  on  the  core  is 
doubled  back  on  itself  at  its  middle  portion  to  form  two  strands,  and 
these  are  wound  simultaneously  on  the  core,  thus  forming  two 
spirals  of  equal  number  of  turns.  The  current  in  traversing  the 
entire  coil  must  flow  through  one  spiral  in  one  direction  with  re- 
lation to  the  core,  and  in  the  opposite  direction  in  the  other  spiral, 
thereby  nullifying  the  inductive  effects  of  one  spiral  by  those  of  the 
other.  This  is  called  a  non-inductive  winding  and  is  in  reality  an 
example  of  differential  winding. 


166 


TELEPHONY 


Lamp  Filament.  An  excellent  type  of  non-inductive  resistance 
is  the  ordinary  carbon-filament  incandescent  lamp.  This  is  used 
largely  in  the  circuits  of  batteries,  generators,  and  other  sources  of 
supply  to  prevent  overload  in  case  of  short  circuits  on  the  line.  These 
are  cheap,  durable,  have  large  current-carrying  capacities,  and  are  not 
likely  to  set  things  afire  when  overheated.  An  additional  advantage 
incident  to  their  use  for  this  purpose  is  that  an  overload  on  a  circuit  in 
which  they  are  placed  is  visibly  indicated  by  the  glowing  of  the  lamp 


Fig.  117.     Mica  Card 
Resistance 


Fig.  118.     Iron- 
Wire  Ballast 


Obviously,  the  carbon-filament  incandescent  lamp,  when  used 
as  a  resistance,  has,  on  account  of  the  negative  temperature  coeffi- 
cient of  carbon,  the  property  of  presenting  the  highest  resistance  to  the 
circuit  when  carrying  no  current,  and  of  presenting  a  lower  and 
lower  resistance  as  the  current  and  consequent  heating  increases. 
For  some  conditions  of  practice  this  is  not  to  be  desired,  and  the 
opposite  characteristic  of  presenting  low  resistance  to  small  currents 
and  comparatively  high  resistance  to  large  currents  would  best  meet 
the  conditions  of  practice. 


NON-INDUCTIVE  RESISTANCE  DEVICES  167 

Iron-Wire  Ballast.  Claude  D.  Enochs  took  advantage  of  the 
very  high  positive  temperature  coefficient  of  iron  to  produce  a  resist- 
ance device  having  these  characteristics.  His  arrangement  possesses 
the  compactness  of  the  carbon-filament  lamp  and  is  shown  in  Fig.  1 18. 
The  resistance  element  proper  is  an  iron  wire,  wound  on  a  central 
stem  of  glass,  and  this  is  included  in  an  exhausted  bulb  so  as  to 
avoid  oxidation.  Such  a  resistance  is  comparatively  low  when  cold, 
but  when  traversed  by  currents  sufficient  to  heat  it  considerably  will 
offer  a  very  large  increase  of  resistance  to  oppose  the  further  in- 
crease of  current.  In  a  sense,  it  is  a  self-adjusting  resistance,  tend- 
ing towards  the  equalization  of  the  flow  of  current  in  the  circuit  in 
which  it  is  placed. 


CHAPTER   XII 
CONDENSERS 

Charge.  A  conducting  body  insulated  from  all  other  bodies 
will  receive  and  hold  a  certain  amount  of  electricity  (a  charge),  if 
subjected  to  an  electrical  potential.  Thus,  referring  to  Fig.  119, 
if  a  metal  plate,  insulated  from  other  bodies,  be  connected  with,  say, 
the  positive  pole  of  a  battery,  the  negative  pole  of  which  is  grounded, 
a  current  will  flow  into  the  plate  until  the  plate  is  raised  to  the  same 
potential  as  that  of  the  battery  pole  to  which  it  is  connected.  The 
amount  of  electricity  that  will  flow  into  the  plate  will  depend,  other 
things  being  equal,  on  the  potential  of  the  source  from  which  it  is 
charged;  in  fact,  it  is  proportional  to  the  potential  of  the  source  from 
which  it  is  charged.  This  amount  of  electricity  is  a  measure  of  the 
capacity  of  the  plate,  just  as  the  amount  of  water  that  a  bath-tub 
will  hold  is  a  measure  of  the  capacity  of  the  bath-tub. 

Capacity.  Instead  of  measuring  the  amount  of  electricity  by  the 
quart  or  pound,  as  in  the  case  of  material  things,  the  unit  of  electrical 
quantity  is  the  coulomb.  The  unit  of  capacity  of  an  insulated  conduc- 
tor is  the  farad,  and  a  given  insulated  conductor  is  said  to  have  unit 
capacity,  that  is,  the  capacity  of  one  farad,  when  it  will  receive  a 
charge  of  one  coulomb  of  electricity  at  a  potential  of  one  volt. 

Referring  to  Fig.  119,  the  potential  of  the  negative  terminal  of  the 
battery  may  be  said  to  be  zero,  since  it  is  connected  to  the  earth. 
If  the  battery  shown  be  supposed  to  have  exactly  one  volt  potential, 
then  the  plate  would  be  said  to  have  the  capacity  of  one  farad  if  one 
coulomb  of  electricity  flowed  from  the  battery  to  the  plate  before 
the  plate  was  raised  to  the  same  potential  as  that  of  the  positive 
pole,  that  is,  to  a  potential  of  one  volt  above  the  potential  of  the  earth; 
it  being  assumed  that  the  plate  was  also  at  zero  potential  before  the 
connection  was  made.  Another  conception  of  this  quantity  may  be 
had  by  remembering  that  a  coulomb  is  such  a  quantity  of  current  as 
will  result  from  one  ampere  flowing  one  second. 


CONDENSERS 


169 


The  capacity  of  a  conductor  depends,  among  other  things,  on 
its  area.  If  the  plate  of  Fig.  119  should  be  made  twice  as  large  in 
area,  other  things  remaining  the  same,  it  would  have  twice  the  capac- 
ity. But  there  are  other  factors  governing 
the  capacity  of  a  conductor.  Consider  the 
diagram  of  Fig.  120,  which  is  supposed  to 
represent  two  such  plates  as  are  shown  in 
Fig.  119,  placed  opposite  each  other  and  con- 
nected respectively  with  the  positive  and  the 
negative  poles  of  the  battery.  When  the  con- 
nection between  the  plates  and  the  battery 


Fig.  119.     Condenser  Plate 


is  made,  the  two  plates  become  charged  to 
a  difference  of  potential  equal  to  the  elec- 
tromotive force  of  the  battery.  In  order  to 
obtain  these  charges,  assume  that  the  plates 
were  each  at  zero  potential  before  the  connection  was  made;  then 
current  flows  from  the  battery  into  the  plates  until  they  each  assume 
the  potential  of  the  corresponding  battery  terminal.  If  the  two  plates 
be  brought  closer  together,  it  will  be  found  that  more  current  will  now 
flow  into  each  of  them,  although  the  difference  of  potential  between 
the  two  plates  must  obviously  remain  the  same,  since  each  of  them 
is  still  connected  to  the  battery. 

Theory.  Due  to  the  proximity  of  the 
plates,  the  positive  electricity  on  plate  A  is 
drawn  by  the  negative  charge  on  plate  B  to- 
wards plate  B,  and  likewise  the  negative 
electricity  on  plate  B  is  drawn  to  the  side 
towards  plate  A  by  the  positive  charge  on  that 
plate.  These  two  charges  so  drawn  towards 
each  other  will,  so  to  speak,  bind  each  other, 
and  they  are  referred  to  as  bound  charges. 
The  charge  on  the  right-hand  side  of  plate  A 
and  on  the  left-hand  side  of  plate  B  will, 
however,  be  free  charges,  since  there  is  nothing 
to  attract  them,  and  these  are,  therefore,  neu- 
tralized by  a  further  flow  of  electricity  from  the  battery  to  the  plate. 

Obviously,  the  closer  together  the  plates  are  the  stronger  will  be 
the  attractive  influence  of  the  two  charges  on  each  other.    From  this 


Fig.  120.     Theory  of 
Condenser 


170  TELEPHONY 

it  follows  that  in  the  case  of  plate  A,  when  the  two  plates  are  being 
moved  closer  together,  more  positive  electricity  will  flow  into  plate 
A  to  neutralize  the  increasing  free  negative  charges  on  the  right- 
hand  side  of  the  plate.  As  the  plates  are  moved  closer  together 
still,  a  new  distribution  of  charges  will  take  place,  resulting  in  more 
positive  electricity  flowing  into  plate  A  and  more  negative  electric- 
ity flowing  into  plate  B.  The  closer  proximity  of  the  plates,  there- 
fore, increases  the  capacity  of  the  plates  for  holding  charges,  due  to 
the  increased  inductive  action  across  the  dielectric  separating  the 
plates. 

Condenser  Defined.  A  condenser  is  a  device  consisting  of  two 
adjacent  plates  of  conducting  material,  separated  by  an  insulating 
material,  called  a  dielectric.  The  purpose  is  to  increase  by  the 
proximity  of  the  plates,  each  to  the  other,  the  amount  of  elec- 
tricity which  each  plate  will  receive  and  hold  when  subjected  to 
a  given  potential. 

Dielectric.  We  have  already  seen  that  the  capacity  of  a  con- 
denser depends  upon  the  area  of  its  plates,  and  also  upon  their 
distance  apart.  There  is  still  another  factor  on  which  the  capacity 
of  a  condenser  depends,  i.  e.,  on  the  character  of  the  insulating 
medium  separating  its  plates.  The  inductive  action  which  takes 
place  between  a  charged  conductor  and  other  conductors  nearby  it, 
as  between  plate  A  and  plate  B  of  Fig.  120,  is  called  electrostatic 
induction,  and  it  plays  an  important  part  in  telephony.  It  is  found 
that  the  ability  of  a  given  charged  conductor  to  induce  charges 
on  other  neighboring  conductors  varies  largely  with  the  insulating 
medium  or  dielectric  that  separates  them.  This  quality  of  a  di- 
electric, by  which  it  enables  inductive  action  to  take  place  between 
two  separated  conductors,  is  called  inductive  capacity.  Usually 
this  quality  of  dielectrics  is  measured  in  terms  of  the  same  quality 
in  dry  air,  this  being  taken  as  unity.  When  so  expressed,  it  is 
termed  specific  inductive  capacity.  To  be  more  accurate  the  specific 
inductive  capacity  of  a  dielectric  is  the  ratio  between  the  capacity 
of  a  condenser  having  that  substance  as  a  dielectric,  to  the  capacity 
of  the  same  condenser  using  dry  air  at  zero  degrees  Centigrade  and 
at  a  pressure  of  14.7  pounds  per  square  inch  as  the  dielectric. 
To  illustrate,  if  two  condensers  having  plates  of  equal  size  and 
equal  distance  apart  are  constructed,  one  using  air  as  the  dielectric 


CONDENSERS 


171 


and  the  other  using  hard  crown  glass  as  the  dielectric,  the 'one 
using  glass  will  have  a  capacity  of  6.96  times  that  of  the  one  using 
air.  From  this  we  say  that  crown  glass  has  a  specific  inductive 
capacity  of  6.96. 

Various  authorities  differ  rather  widely  as  to  the  specific  induc- 
tive capacity  of  many  common  substances.  The  values  given  in 
Table  VIII  have  been  chosen  from  the  Smithsonian  Physical  Tables. 


DIELECTRIC 

REFERRED  TO  AIR  AS  1 

Vacuum 

.99941 

Hydrogen 
Carbonic  Acid 

.99967 
1.00036 

Dry  Paper 
Paraffin 

1.25  to  1.75 
1  .  95  to  2  .  32 

Ebonite 

1.9  to3.48 

Sulphur 
Shellac 

2  .  24  to  3  .  90 
2  .  95  to  3  .  73 

Gutta-percha 
Plate  Glass 

3.3  to4.9 
3.31  to  7.  5 

Porcelain 

4.38 

Mica 

4.6  to  8.0 

Glass—  Light  Flint 
Glass  —  Hard  Crown 

6.61 
6.96 

Selenium 

10.2 

This  data  is  interesting  as  showing  the  wide  divergence  in 
specific  inductive  capacities  of  various  materials,  and  also  showing 
the  wide  divergence  in  different  observations  of  the  same  material. 
Undoubtedly,  this  latter  is  due  mainly  to  the  fact  that  various 
materials  differ  largely  in  themselves,  as  in  the  case  of  paraffin,  for 
instance,  which  exhibits  widely  different  specific  inductive  capacities 
according  to  the  difference  in  rapidity  with  which  it  is  cooled  in 
changing  from  a  liquid  to  a  solid  state. 

We  see  then  that  the  capacity  of  a  condenser  varies  as  the  area 
of  its  plates,  as  the  specific  inductive  capacity  of  the  dielectric  em- 
ployed, and  also  inversely  as  the  distance  between  the  plates. 

Obviously,  therefore,  in  making  a  condenser  of  large  capacity, 
it  is  important  to  have  as  large  an  area  of  the  plate  as  possible;  to 


172 


TELEPHONY 


have  them  as  close  together  as  possible;  to  have  the  dielectric  a 
good  insulating  medium  so  that  there  will  be  practically  no  leakage 
between  the  plates;  and  to  have  the  dielectric  of  as  high  a  specific 
inductive  capacity  as  economy  and  suitability  of  material  in  other 
respects  will  permit. 

Dielectric  Materials.  Mica.  Of  all  dielectrics  mica  is  the  most 
suitable  for  condensers,  since  it  has  very  high  insulation  resistance 
and  also  high  specific  inductive  capacity,  and  furthermore  may  be 
obtained  in  very  thin  sheets.  High-grade  condensers,  such  as  are 
used  for  measurements  and  standardization  purposes,  usually  have 
mica  for  the  dielectric.  \ 

Dry  Paper.  The  demands  of  telephonic  practice  are,  however, 
such  as  to  require  condensers  of  very  cheap  construction  with  large 
capacity  in  a  small  space.  For  this  purpose  thin  bond  paper,  sat- 


Fig.  121.     Rolled  Condenser 

urated  with  paraffin,  has  been  founa  to  be  the  best  dielectric.  The 
conductors  in  condensers  are  almost  always  of  tinfoil,  this  being  an 
ideal  material  on  account  of  its  cheapness  and  its  thinness.  Before 
telephony  made  such  urgent  demands  for  a  cheap  compact  conden- 
ser, the  customary  way  of  making  them  was  to  lay  up  alternate 
sheets  of  dielectric  material,  either  of  oiled  paper  or  mica  and  tin- 
foil, the  sheets  of  tinfoil  being  cut  somewhat  smaller  than  the  sheets 
of  dielectric  material  in  order  that  the  proper  insulation  might  be 
secured  at  the  edges.  After  a  sufficient  number  of  such  plates  were 
built  up  the  alternate  sheets  of  tinfoil  were  connected  together  to 
form  one  composite  plate  of  the  condenser,  while  the  other  sheets 
were  similarly  connected  together  to  form  the  other  plate.  Ob- 
viously, in  this  way  a  very  large  area  of  plates  could  be  secured  with 
a  minimum  degree  of  separation- 


CONDENSERS 


173 


Fig.  122.    Rolled  Condenser 


There  has  been  developed  for  use  in  telephony,  however,  and  its 
use  has  since  extended  into  other  arts  requiring  condensers,  what  is 
called  the  rolled  condenser.  This  is  formed  by  rolling  together  in  a 
flat  roll  four  sheets  of  thin  bond  paper,  1,  2,  3,  and  4)  and  two  some- 
what narrower  strips  of  tinfoil, 
5  and  6,  Fig.  121.  The  strips 
of  tinfoil  and  paper  are  fed  on 
to  the  roll  in  continuous  lengths 
and  in  such  manner  that  two 
sheets  of  paper  will  lie  between 
the  two  strips  of  tinfoil  in  all 
cases.  Thin  sheet  metal  ter- 
minals 7  and  8  are  rolled  into 

the  condenser  as  it  is  being  wound,  and  as  these  project  beyond 
the  edges  of  the  paper  they  form  convenient  terminals  for  the  con- 
denser after  it  is  finished.  After  it  is  rolled,  the  roll  is  boiled  in  hot 
paraffin  so  as  to  thoroughly  impregnate  it  and  expel  all  moisture.  It  is 
then  squeezed  in  a  press  and  allowed  to  cool  while  under  pressure. 
In  this  way  the  surplus  paraffin  is  expelled  and  the  plates  are  brought 
very  close  together.  It  then  appears  as  in  Fig.  122.  The  conden- 
ser is  now  sealed  in  a  metallic  case,  usually 
rectangular  in  form,  and  presents  the  appear- 
ance shown  in  Fig.  123. 

A  later  method  of  condenser  making  which 
has  not  yet  been  thoroughly  proven  in  practice, 
but  which  bids  fair  to  produce  good  results, 
varies  from  the  method  just  described  in  that  a 
paper  is  used  which  in  itself  is  coated  with  a 
very  thin  conducting  material.  This  conducting 
material  is  of  metallic  nature  and  in  reality  forms 
a  part  of  the  paper.  To  form  a  condenser  of 
this  the  sheets  are  merely  rolled  together  and 
then  boiled  in  paraffin  and  compressed  as 
before. 

Sizes.    The  condensers  ordinarily  used  in 
telephone  practice  range  in  capacity  from  about 
microfarads.     When   larger  capacities  than  2  microfarads  are  de- 
sired, they  may  be  obtained  by  connecting  several  of  the  smaller  size 


Fig.  123.    Rolled 
Condenser 


microfarad  to  2 


174 


TELEPHONY 


TABLE  IX 

Condenser  Data 


DIMENSIONS  IN  INCHES 

Height 

Width 

Thickness 

2  m.  f. 

Rectangular 

9K 

4M 

% 

1  m.  f. 

9K 

4% 

!Y6 

1  m.  f. 

4M 

2/5 

% 

Y2  m.  f. 

2M 

1M 

M 

1  m.  f. 

4tt 

2A 

If 

X  m.  f. 

4M 

2A 

% 

T3o  m.  f. 

4M 

2/2 

% 

1  m.  f. 

2M 

3 

1 

Pig.  124.     Condenser  Symbols 


condensers  in  multiple.  Table  IX  gives  the  capacity,  shape,  and 
dimensions  of  a  variety  of  condensers  selected  from  those  regularly 
on  the  market. 

Conventional  Symbols.  The  conventional  symbols  usually 
employed  to  represent  condensers  in  telephone  diagrams  are  shown 

in  Fig.  124.  These  all  convey 
the  idea  of  the  adjacent  conduct- 
ing plates  separated  by  insulat- 
ing material. 

Functions.  Obviously,  when  placed  in  a  circuit  a  condenser 
offers  a  complete  barrier  to  the  flow  of  direct  current,  since  no  con- 
ducting path  exists  -between  its  terminals,  the  dielectric  offering  a 
very  high  insulation  resistance.  If,  however,  the  condenser  is  con- 
nected across  the  terminals  of  a  source  of  alternating  current, 
this  current  flows  first  in  one  direction  and  then  in  the  other,  the 
electromotive  force  in  the  circuit  increasing  from  zero  to  a  max- 
imum in  one  direction,  and  then  decreasing  back  to  zero  and  to 
a  maximum  in  the  other  direction,  and  so  on.  With  a  condenser 
connected  so  as  to  be  subjected  to  such  alternating  electromotive 
forces,  as  the  electromotive  force  begins  to  rise  the  electromotive 
force  at  the  condenser  terminals  will  also  rise  and  a  current  will, 
therefore,  flow  into  the  condenser.  When  the  electromotive  force 
reaches  its  maximum,  the  condenser  will  have  received  its  full  charge 
for  that  potential,  and  the  current  flow  into  it  will  cease.  When  the 
electromotive  force  begins  to  fall,  the  condenser  can  no  longer  re- 


CONDENSERS  175 

tain  its  charge  and  a  current  will,  therefore,  flow  out  of  it.  Appar- 
ently, therefore,  there  is  a  flow  of  current  through  the  condenser 
the  same  as  if  it  were  a  conductor. 

Means  for  Assorting  Currents.  In  conclusion,  it  is  obvious 
that  the  telephone  engineer  has  within  his  reach  in  the  various 
coils — whether  non-inductive  or  inductive,  or  whether  having  one 
or  several  windings — and  in  the  condenser,  a  variety  of  tools  by 
which  he  may  achieve  a  great  many  useful  ends  in  his  circuit 
work.  Obviously,  the  condenser  affords  a  means  for  transmitting 
voice  currents  or  fluctuating  currents,  and  for  excluding  steady 
currents.  Likewise  the  impedance  coil  affords  a  means  for  readily 
transmitting  steady  currents  but  practically  excluding  voice  cur- 
rents or  fluctuating  currents.  By  the  use  of  these  very  simple 
devices  it  is  possible  to  sift  out  the  voice  currents  from  a  circuit 
containing  both  steady  and  fluctuating  currents,  or  it  is  possible 
in  the  same  manner  to  sift  out  the  steady  currents  and  to  leave  the 
voice  currents  alone  to  traverse  the  circuit. 

Great  use  is  made  in  the  design  of  telephone  circuits  of  the  fact 
that  the  electromagnets,  which  accomplish  the  useful  mechanical 
results  in  causing  the  movement  of  parts,  possess  the  quality  of 
impedance.  Thus,  the  magnets  which  operate  various  signaling 
relays  at  the  central  office  are  often  used  also  as  impedance  coils  in 
portions  of  the  circuit  through  which  it  is  desired  to  have  only  steady 
currents  pass.  If,  on  the  other  hand,  it  is  necessary  to  place  a  relay 
magnet,  having  considerable  impedance,  directly  in  a  talking  circuit, 
the  bad  effects  of  this  on  the  voice  currents  may  be  eliminated  by 
shunting  this  coil  with  a  condenser,  or  with  a  comparatively  high  non- 
inductive  resistance.  The  voice  currents  will  flow  around  the  high 
impedance  of  the  relay  coil  through  the  condenser  or  resistance, 
while  the  steady  currents,  which  are  the  ones  which  must  be 
depended  upon  to  operate  the  relay,  are  still  forced  in  whole  or  in 
part  to  pass  through  the  relay  coil  where  they  belong. 

In  a  similar  way  the  induction  coil  affords  a  means  for  keeping 
two  circuits  completely  isolated  so  far  as  the  direct  flow  of  current 
between  them  is  concerned,  and  yet  of  readily  transmitting,  by 
electromagnetic  induction,  currents  from  one  of  these  circuits  to 
the  other.  Here  is  a  means  of  isolation  so  far  as  direct  current  is 
concerned,  with  complete  communication  for  alternating  current. 


CHAPTER   XIII 
CURRENT  SUPPLY  TO  TRANSMITTERS 

The  methods  by  which  current  is  supplied  to  the  transmitter  of 
a  telephone  for  energizing  it,  may  be  classified  under  two  divisions : 
first,  those  where  the  battery  or  other  source  of  current  is  located 
at  the  station  with  the  transmitter  which  it  supplies;  and  second, 
those  where  the  battery  or  other  source  of  current  is  located  at  a  dis- 
tant point  from  the  transmitter,  the  battery  in  such  cases  serving  as 
a  common  source  of  current  for  the  supply  of  transmitters  at  a  num- 
ber of  stations. 

The  advantages  of  putting  the  transmitter  and  the  battery  which 
supplies  it  with  current  in  a  local  circuit  with  the  primary  of  an  in- 
duction coil,  and  placing  the  secondary  of  the  induction  coil  in  the 
line,  have  already  been  pointed  out  but  may  be  briefly  summarized 
as  follows:  When  the  transmitter  is  placed  directly  in  the  line  cir- 
cuit and  the  line  is  of  considerable  length,  the  current  which  passes 
through  the  transmitter  is  necessarily  rather  small  unless  a  battery 
of  high  potential  is  used;  and,  furthermore,  the  total  change  in  re- 
sistance which  the  transmitter  is  capable  of  producing  is  but  a  small 
proportion  of  the  total  resistance  of  the  line,  and,  therefore,  the  cur- 
rent changes  produced  by  the  transmitter  are  relatively  small.  On 
the  other  hand,  when  the  transmitter  is  placed  in  a  local  circuit  with 
the  battery,  this  circuit  may  be  of  small  resistance  and  the  current  rel- 
atively large,  even  though  supplied  by  a  low-voltage  battery;  so  that 
the  transmitter  is  capable  of  producing  relatively  large  changes  in  a 
relatively  large  current. 

To  draw  a  comparison  between  these  two  general  classes  of 
transmitter  current  supply,  a  number  of  cases  will  be  considered  in 
connection  with  the  following  figures,  in  each  of  which  two  sta- 
tions connected  by  a  telephone  line  are  shown.  Brief  reference  to 
the  local  battery  method  of  supplying  current  will  be  made  in  order 
to  make  this  chapter  contain,  as  far  as  possible,  all  of  the  com- 
monly used  methods  of  current  supply  to  transmitters. 


CURRENT  SUPPLY  TO  TRANSMITTERS 


177 


Local  Battery.  In  Fig.  125  two  stations  are  shown  connected 
by  a  grounded  line  wire.  The  transmitter  of  each  station  is  in- 
cluded in  a  low-resistance  primary  circuit  including  a  battery  and 
the  primary  winding  of  an  induction  coil,  the  relation  between  the 


II-. 


»S TA T/ON  -XI  -  STA T/O/Y  -B- 

Fig.  125.     Local-Battery  Stations  with  Grounded  Circuit 

primary  circuits  and  the  line  circuits  being  established  by  the  in- 
ductive action  between  the  primary  and  the  secondary  windings  of 
induction  coils,  the  secondary  in  each  case  being  in  the  line  circuits 
with  the  receivers.  • 

Fig.  126  shows  exactly  the  same  arrangement  but  with  a  metallic 
circuit  rather  than  a  grounded  circuit.  The  student  should  become 
accustomed  to  the  replacing  of  one  of  the  line  wires  of  a  metallic  cir- 


H'h 


STAT/OM-A-  STAT/ON-B- 

Fig.  126.     Local-Battery  Stations  with  Metallic  Circuit 

cuit  by  the  earth,  and  to  the  method,  employed  in  Figs.  125  and  126, 
of  indicating  a  grounded  circuit  as  distinguished  from  a  metallic 
circuit. 

In  Fig.  127  is  shown  a  slight  modification  of  the  circuit  shown 
in  Fig.  126,  which  consists  of  connecting  one  end  of  the  primary 
winding  to  one  end  of  the  secondary  winding  of  the  induction  coil, 
thus  linking  together  the  primary  circuit  and  the  line  circuit,  a  por- 
tion of  each  of  these  circuits  being  common  to  a  short  piece  of  the 
local  wiring.  There  is  no  difference  whatever  in  the  action  of  the 
circuits  shown  in  Figs.  126  and  127,  the  latter  being  shown  merely 


178 


TELEPHONY 


for  the  purpose  of  bringing  out  this  fact.  It  is  very  common,  par- 
ticularly in  local-battery  circuits,  to  connect  one  end  of  the  primary 
and  the  secondary  windings,  as  by  doing  so  it  is  often  possible  to  save 
a  contact  point  in  the  hook  switch  and  also  to  simplify  the  wiring. 

The  advantages  to  be  gained  by  employing  a  local  battery  at 
each  subscriber's  station  associated  with  the  transmitter  in  the  pri- 
mary circuit  of  an  induction  coil  are  attended  by  certain  disadvan- 


STKT/ON-A-  <STAT/ON-B- 

Fig.  127.     Local-Battery  Stations  with  Metallic  Circuit 

• 

tages  from  a  commercial  standpoint.  The  primary  battery  is  not  an 
economical  way  to  generate  electric  energy.  In  all  its  commercial 
forms  it  involves  the  consumption  of  zinc  and  zinc  is  an  expensive 
fuel.  The  actual  amount  of  current  in  watts  required  by  a  telephone 
is  small,  however,  and  this  disadvantage  due  to  the  inexpensive 
method  of  generating  current  would  not  in  itself  be  of  great  impor- 
tance. A  more  serious  objection  to  the  use  of  local  batteries  at 
subscribers'  stations  appears  when  the  subject  is  considered  from 
the  standpoint  of  maintenance.  Batteries,  whether  of  the  so-called 
"dry"  or  "wet"  type,  gradually  deteriorate,  even  when  not  used, 
and  in  cases  where  the  telephone  is  used  many  times  a  day  the  dete- 
rioration is  comparatively  rapid.  This  makes  necessary  the  occa- 
sional renewals  of  the  batteries  with  the  attendant  expense  for  new 
batteries  or  new  material,  and  of  labor  and  transportation  in  visiting 
the  station.  The  labor  item  becomes  more  serious  when  the  stations 
are  scattered  in  a  sparsely  settled  community,  in  which  case  the 
visiting  of  the  stations,  even  for  the  performance  of  a  task  that  would 
require  but  a  few  minutes'  time,  may  consume  some  hours  on  the 
part  of  the  employes  in  getting  there  and  back.  . 

Common  Battery.  Advantages.  It  would  be  more  economical 
if  all  of  the  current  for  the  subscribers'  transmitters  could  be  supplied 
from  a  single  comparatively  efficient  generating  source  instead  of 


CURRENT  SUPPLY  TO  TRANSMITTERS  179 

from  a  multitude  of  inefficient  small  sources  scattered  throughout 
the  community  served  by  the  exchange.  The  advantage  of  such 
centralization  lies  not  only  in  more  economic  generating  means,  but 
also  in  having  the  common  source  of  current  located  at  one  place, 
where  it  may  be  cared  for  with  a  minimum  amount  of  expense. 
Such  considerations  have  resulted  in  me  so-called  "common-battery 
system,"  wherein  the  current  for  all  the  subscribers'  transmitters  is 
furnished  from  a  source  located  at  the  central  office. 

Where  such  a  method  of  supplying  current  is  practiced,  the  re- 
sult has  also  been,  in  nearly  all  cases,  the  doing  away  with  the  sub- 
scriber's magneto  generators,  relying  on  the  central-office  source 
of  current  to  furnish  the  energy  for  enabling  the  subscriber  to  signal 
the  operator.  Such  systems,  therefore,  concentrate  all  of  the  sources 
of  energy  at  the  central  office  and  for  that  reason  they  are  frequently 
referred  to  as  central-energy  systems. 

NOTE.  In  this  chapter  the  central-energy  or  common-battery  system 
will  be  considered  only  in  so  far  as  the  supply  of  current  for  energizing  the 
subscribers'  transmitters  is  concerned,  the  discussion  of  the  action  of  signaling 
being  reserved  for  subsequent  chapters. 

Series  Battery.  If  but  a  single  pair  of  lines  had  to  be  consid- 
ered, the  arrangement  shown  in  Fig.  128  might  be  employed.  In 
this  the  battery  is  located  at  the  central  office  and  placed  in  series  with 
the  two  grounded  lines  leading  from  the  central  office  to  the  two  sub- 
scribers' stations.  The  voltage  of  this  battery  is  made  sufficient  to 
furnish  the  required  current  over  the  resistance  of  the  entire  line 


|l|l|l|l  ------- 


Fig.  128.     Battery  in  Series  with  Two  Lines 

circuit  with  its  included  instruments.  Obviously,  changes  in  re- 
sistance in  the  transmitter  at  Station  A  will  affect  the  flow  of  current 
in  the  entire  line  and  the  fluctuations  resulting  from  the  vibration 
of  the  transmitter  diaphragm  will,  therefore,  reproduce  these  sounds 
in  the  receiver  at  Station  B,  as  well  as  in  that  at  Station  A. 

An  exactly  similar  arrangement  applied  to  a  metallic  circuit  is 
shown  in  Fig.  129.     In  thus  placing  the  battery  in  series  in  the  circuit 


180  TELEPHONY 

between  the  two  stations,  as  shown  in  Figs.  128  and  129,  it  is  obvious 
that  the  transmitter  at  each  station  is  compelled  to  vary  the  resistance 
of  the  entire  circuit  comprising  the  two  lines  in  series,  in  order  to 
affect  the  receiver  at  distant  stations.  This  is  in  effect  making  the 
transmitter  circuit  twice  as  long  as  is  necessary,  as  will  be  shown  in 


I  I 


STAT/ON-A-  STAT/OH-B- 

Pig.  129.     Battery  in  Series  with  Two  Lines 

the  subsequent  systems  considered.  Furthermore,  the  placing  of 
the  battery  in  series  in  the  circuit  of  the  two  combined  lines  does  not 
lend  itself  readily  to  the  supply  of  current  from  a  common  source  to 
more  than  a  single  pair  of  lines. 

Series  Substation  Circuit.  The  arrangement  at  the  substations 
— consisting  in  placing  the  transmitter  and  the  receiver  in  series  in  the 
line  circuit,  as  shown  in  Figs.  128  and  129 — is  the  simplest  possible 
one,  and  has  been  used  to  a  considerable  extent,  but  it  has  been 
subject  to  the  serious  objection,  where  receivers  having  permanent 
magnets  were  used,  of  making  it  necessary  to  so  connect  the  receiver 
in  the  line  circuit  that  the  steady  current  from  the  battery  would  not 
set  up  a  magnetization  in  the  cores  of  the  receiver  in  such  a  direc- 
tion as  to  neutralize  or  oppose  the  magnetization  of  the  permanent 
magnets.  As  long  as  the  current  flowed  through  the  receiver  coils 
in  such  a  direction  as  to  supplement  the  magnetization  of  the  per- 
manent magnets,  no  harm  was  usually  done,  but  when  the  current 
flowed  through  the  receiver  coils  in  such  a  way  as  to  neutralize  or 
oppose  the  magnetizing  force  of  the  permanent  magnets,  the  action 
of  the  receiver  was  greatly  interfered  with.  As  a  result,  it  was 
necessary  to  always  connect  the  receivers  in  the  line  circuit  in  a  cer- 
tain way,  and  this  operation  was  called  poling. 

In  order  to  obviate  the  necessity  for  poling  and  also  to  bring 
about  other  desirable  features,  it  has  been,  until  recently,  almost 
universal  practice  to  so  arrange  the  receiver  that  it  would  be  in  the 


CURRENT  SUPPLY  TO  TRANSMITTERS 


181 


circuit  of  the  voice  currents  passing  over  the  line,  but  would  not  be 
traversed  by  direct  currents,  this  condition  being  brought  about  by 
various  arrangements  of  condensers,  impedance  coils,  or  induction 
coils,  as  will  be  shown  later.  During  the  year  1909,  however,  the 
adoption  by  several  concerns  of  the  so-called  "direct-current"  receiver 
has  made  it  necessary  for  the  direct  current  to  flow  through  the  re- 
ceiver coils  in  order  to  give  the  proper  magnetization  to  the  receiver 
cores,  and  this  has  brought  about  a  return  to  the  very  simple  form 
of  substation  circuit,  which  includes  the  receiver  and  the  transmitter 
directly  in  the  circuit  of  the  line.  This  illustrates  well  an  occur- 
rence that  is  frequently  observed  by  those  who  have  opportunity  to 
watch  closely  the  development  of  an  art.  At  one  time  the  condi- 
tions will  be  such  as  to  call  for  complicated  arrangements,  and  for 
years  the  aim  of  inventors  will  be  to  perfect  these  arrangements; 
then,  after  they  are  perfected,  adopted,  and  standardized,  a  new  idea, 
or  a  slight  alteration  in  the  practice  in  some  other  respect,  will  de- 
mand a  return  to  the  first  principles  and  wipe  out  the  necessity  for 
the  things  that  have  been  so  arduously  striven  for. 

Bridging  Battery  with  Repeating  Coil.  As  pointed  out,  the 
placing  of  the  battery  in  series  in  the  line  circuit  in  the  central 
office  is  not  desirable,  and,  so  far  as  we  are  aware,  has  never  been 
extensively  used.  The  universal  practice,  therefore,  is  to  place  it 
in  a  bridge  path  across  the  line  circuit,  and  a  number  of  arrange- 
ments employing  this  basic  idea  are  in  wide  use.  In  Fig.  130  is 
shown  the  standard  arrangement  of  the  Western  Electric  Com- 


STA  T/OM  -A- 


S7^\  T/O/V  -B- 


Fig.  130.     Bridging  Battery  with  Repeating  Coil 


pany,  employed  by  practically  all  the  Bell  operating  companies. 
In  this  the  battery  at  the  central  office  is  connected  in  the  middle  of 
the  two  sides  of  a  repeating  coil  so  that  the  current  from  the 
battery  is  fed  out  to  the  two  connected  lines  in  multiple. 


182  TELEPHONY 

Referring  to  the  middle  portion  of  this  figure  showing  the  cen- 
tral-office apparatus,  1  and  2  may  be  considered  as  the  two  halves  of 
one  side  of  a  repeating  coil  divided  so  that  the  battery  may  be  cut 
into  their  circuit.  Likewise,  3  and  4  m&y  be  considered  as  the  two 
halves  of  the  other  side  of  the  repeating  coil  similarly  divided  for  the 
same  purpose.  The  windings  of  this  repeating  coil  are  ordinarily 
alike;  that  is,  1  and  2  combined  have  the  same  resistance,  number  of 
turns,  and  impedance  as  3  and  4  combined.  The  two  sides  of  this 
coil  are  alternately  used  as  primary  and  secondary,  1  and  2  forming 
the  primary  when  Station  A  is  talking,  and  3  and  4>  the  secondary; 
and  vice  vcrsd  when  Station  B  is  talking. 

As  will  be  seen,  the  current  flowing  from  the  positive  pole  of  the 
battery  will  divide  and  flow  through  the  windings  2  and  4>  thence 
over  the  upper  limb  of  each  line,  through  the  transmitter  at  each 
station,  and  back  over  the  lower  limbs  of  the  line,  through  the  wind- 
ings 1  and  3,  where  the  two  paths  reunite  and  pass  to  the  negative 
pole  of  the  battery.  It  is  evident  that  when  neither  transmitter  is 
being  used  the  current  flowing  through  both  lines  will  be  a  steady 
current  and  that,  therefore,  neither  line  will  have  an  inductive  effect 
on  the  other.  When,  however,  the  transmitter  at  Station  A  is  used, 
the  variations  in  the  resistance  caused  by  it  will  cause  undulations 
in  the  current.  These  undulations,  passing  through  the  windings 
1  and  2  of  the  repeating  coil,  will  cause,  by  electromagnetic  induc- 
tion, alternating  currents  to  flow  in  the  windings  3  and  4>  and  these 
alternating  currents  will  be  superimposed  on  the  steady  currents 
flowing  in  that  line  and  will  affect  the  receiver  at  Station  B,  as  will 
be  pointed  out.  The  reverse  conditions  exist  when  Station  B  is 
talking. 

Bell  Substation  Arrangement.  The  substation  circuits  at  the 
stations  in  Fig.  130  are  illustrative  of  one  of  the  commonly  em- 
ployed methods  of  preventing  the  steady  current  from  the  battery 
from  flowing  through  the  receiver  coil.  This  particular  arrange- 
ment is  that  employed  by  the  common-battery  instruments  of 
the  various  Bell  companies.  Considering  the  action  at  Station 
B,  it  is  evident  that  the  steady  current  will  pass  through  the  trans- 
mitter and  through  the  secondary  winding  of  the  induction  coil, 
and  that  as  long  as  this  current  is  steady  no  current  will  flow  through 
the  telephone  receiver.  The  receiver,  transmitter,  and  primary 


CURRENT  SUPPLY  TO  TRANSMITTERS  183 

winding  of  the  induction  coil  are,  however,  included  in  a  local  cir- 
cuit with  the  condenser.  The  presence  of  the  condenser  precludes 
the  possibility  of  direct  current  flowing  in  this  path.  Considering 
Station  A  as  a  receiving  station,  it  is  evident  that  the  voice  currents 
coming  to  the  station  over  the  line  will  pass  through  the  secondary 
winding  and  will  induce  alternating  currents  in  the  primary  winding 
which  will  circulate  through  the  local  circuit  containing  the  receiver 
and  the  condenser,  and  thus  actuate  the  receiver.  The  considera- 
tions are  not  so  simple  when  the  station  is  being  treated  as  a  trans- 
mitting station.  Under  this  condition  the  steady  current  passes 
through  the  transmitter  in  an  obvious  manner.  It  is  clear  that  if 
the  local  circuit  containing  the  receiver  did  not  exist,  the  circuit 
would  be  operative  as  a  transmitting  circuit  because  the  transmitter 
would  produce  fluctuations  in  the  steady  current  flowing  in  the  line 
and  thus  be  able  to  affect  the  distant  station.  The  transmitter, 
therefore,  has  a  direct  action  on  the  currents  flowing  in  the  line  by 
the  variation  in  resistance  which  it  produces  in  the  line  circuit.  There 
is,  however,  a  subsidiary  action  in  this  circuit.  Obviously,  there 
is  a  drop  of  potential  across  the  transmitter  terminals  due  to  the  flow 
of  steady  current.  This  means  that  the  upper  terminal  of  the  con- 
denser will  be  charged  to  the  same  potential  as  the  upper  terminal 
of  the  transmitter,  while  the  lower  terminal  of  the  condenser  will 
be  of  the  same  potential  as  the  lower  terminal  of  the  transmitter. 
When,  now,  the  transmitter  varies  its  resistance,  a  variation  in  the 
potential  across  its  terminals  will  occur;  and  as  a  result,  a  variation 
in  potential  across  the  terminals  of  the  condenser  will  occur,  and 
this  means  that  alternating  currents  will  flow  through  the  primary 
winding  of  the  induction  coil.  The  transmitter,  therefore,  by  its 
action,  causes  alternating  currents  to  flow  through  the  primary  of 
this  induction  coil  and  it  causes,  by  direct  action  on  the  circuit  of  the 
line,  fluctuations  in  the  steady  current  flowing  in  the  line.  The 
alternating  currents  flowing  in  the  primary  of  the  coil  induce  currents 
in  the  secondary  of  the  coil  which  supplement  and  augment  the 
fluctuations  produced  by  the  direct  action  of  the  transmitter.  This 
circuit  may  be  looked  at,  therefore,  in  the  light  of  combining  the 
direct  action  which  the  transmitter  produces  in  the  current  in  the 
line  with  the  action  which  the  transmitter  produces  in  the  local  cir- 
cuit containing  the  primary  of  the  induction  coil,  this  action  being 


184 


TELEPHONY 


repeated  in  the  line  circuit  through  the  secondary  of  the  induction 
coil. 

The  receiver  in  this  circuit  is  placed  in  the  local  circuit,  and  is 
thus  not  traversed  by  the  steady  currents  flowing  in  the  line.  There 
is  thus  no  necessity  for  poling  it.  This  circuit  is  very  efficient,  but  is 
subject  to  the  objection  of  producing  a  heavy  side  tone  in  the  receiver 
of  the  transmitting  station.  By  "side  tone"  is  meant  the  noises  which 


STA  r/o/y  -xv- 


STAT/O/y  -B- 


Fig.  131.     Bridging  Battery  with  Impedance  Coils 

are  produced  in  the  receiver  at  a  station  by  virtue  of  the  action  of  the 
transmitter  at  that  station.  Side  tone  is  objectionable  for  several 
reasons:  first,  it  is  sometimes  annoying  to  the  subscriber;  second,  and 
of  more  importance,  the  subscriber  who  is  talking,  hearing  a  very 
loud  noise  in  his  own  receiver,  unconsciously  assumes  that  he  is 
talking  too  loud  and,  therefore,  lowers  his  voice,  sometimes  to  such 
an  extent  that  it  will  not  properly  reach  the  distant  station. 

Bridging  Battery  with  Impedance  Coils.  The  method  of  feeding 
current  to  the  line  from  the  common  battery,  shown  in  Fig.  130, 
is  called  the  "split  repeating-coil"  method.  As  distinguished  from 
this  is  the  impedance-coil  method  which  is  shown  in  Fig.  131.  In 
this  the  battery  is  bridged  across  the  circuit  of  the  combined  lines 
in  series  with  two  impedance  coils,  1  and  2,  one  on  each  side  of  the 
battery.  The  steady  currents  from  the  battery  find  ready  path  through 
these  impedance  coils  which  are  of  comparatively  low  ohmic  resis- 
tance, and  the  current  divides  and  passes  in  multiple  over  the  circuits 
of  the  two  lines.  Voice  currents,  however,  originating  at  either  one  of 
the  stations,  will  not  pass  through  the  shunt  across  the  line  at  the 
central  office  on  account  of  the  high  impedance  offered  by  these  coils, 
and  as  a  result  they  are  compelled  to  pass  on  to  the  distant  station 
and  affect  the  receiver  there,  as  desired. 

This    impedance-coil  method  seems  to  present  the  advantage 


CURRENT  SUPPLY  TO  TRANSMITTERS  185 

of  greater  simplicity  over  the  repeating-coil  method  shown  in  Fig. 
130,  and  so  far  as  talking  efficiency  is  concerned,  there  is  little  to 
choose  between  the  two.  The  repeating-coil  method,  however,  has 
the  advantage  over  this  impedance-coil  method,  because  by  it  the 
two  lines  are  practically  divided  except  by  the  inductive  con- 
nection between  the  two  windings,  and  as  a  result  an  unbalanced 
condition  of  one  of  the  connected  lines  is  not  as  likely  to  produce 
an  unbalanced  condition  in  the  other  as  where  the  two  lines  are 
connected  straight  through,  as  with  the  impedance-coil  method. 
The  substation  arrangement  of  Fig.  131  is  the  same  as  that  of 
Fig.  130. 

Double  Battery  with  Impedance  Coils.  A  modification  of  the 
impedance-coil  method  is  used  in  all  of  the  central-office  work  of 
the  Kellogg  Switchboard  and  Supply  Company.  This  employs 
a  combination  of  impedance  coils  and  condensers,  and  in  effect 
isolates  the  lines  conductively  from  each  other  as  completely  as 
the  repeating-coil  method.  It  is  characteristic  of  all  the  Kellogg 
common-battery  systems  that  they  employ  two  batteries  instead 
of  one,  one  of  these  being  connected  in  all  cases  with  the  calling 
line  of  a  pair  of  connected  lines  and  the  other  in  all  cases 
with  the  called  line.  As  shown  in  Fig.  132,  the  left-hand  battery 
is  connected  with  the  line  leading  to  Station  A  through  the  im- 
pedance coils  1  and  2.  Likewise,  the  right-hand  battery  is  con- 


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STAT/ON-A-  5  STAT/ON-B- 

Fig.  132.     Double-Battery  Kellogg  System 

nected  to  the  line  of  Station  B  through  the  impedance  coils  3  and 
4.  These  four  impedance  coils  are  wound  on  separate  cores  and  do 
not  have  any  inductive  relation  whatsoever  with  each  other.  Con- 
densers 5  and  6  are  employed  to  completely  isolate  the  lines  conduc- 
tively. Current  from  the  left-hand  battery,  therefore,  passes  only 
to  Station  A,  and  current  from  the  right-hand  battery  to  Station  B. 


186  TELEPHONY 

Whenever  the  transmitter  at  Station  A  is  actuated  the  undulations  of 
current  which  it  produces  in  the  line  cause  a  varying  difference  of 
potential  across  the  outside  terminals  of  the  two  impedance  coils  1 
and  2.  This  means  that  the  two  left-hand  terminals  of  condensers 
5  and  6  are  subjected  to  a  varying  difference  of  potential  and  these, 
of  course,  by  electrostatic  induction,  cause  the  right-hand  terminals 
of  these  condensers  to  be  subject  to  a  correspondingly  varying 
difference  of  potential.  From  this  it  follows  that  alternating  currents 
will  be  impressed  upon  the  right-hand  line  and  these  will  affect  the 
receiver  at  Station  B. 

A  rough  way  of  expressing  the  action  of  this  circuit  is  to  con- 
sider it  in  the  same  light  as  that  of  the  impedance-coil  circuit  shown 
in  Fig.  131,  and  to  consider  that  the  voice  currents  originating  in  one 
line  are  prevented  from  passing  through  the  bridge  paths  at  the  cen- 
tral office  on  account  of  the  impedance,  and  are,  therefore,  forced  to 
continue  on  the  line,  being  allowed  to  pass  readily  by  the  condensers 
in  series  between  the  two  lines. 

Kellogg  Substation  Arrangement.  An  interesting  form  of  sub- 
station circuit  which  is  employed  by  the  Kellogg  Company  in  all 
of  its  common-battery  telephones  is  shown  in  Fig.  132.  In  passing) 
it  may  be  well  to  state  that  almost  any  of  the  substation  circuits 
shown  in  this  chapter  are  capable  of  working  with  any  of  the  central- 
office  circuits.  The  different  ones  are  shown  for  the  purpose  of  giving 
a  knowledge  of  the  various  substation  circuits  that  are  employed, 
and,  as  far  as  possible,  to  associate  them  with  the  particular  central- 
office  arrangements  with  which  they  are  commonly  used. 

In  this  Kellogg  substation  arrangement  the  line  circuit  passes 
first  through  the  transmitter  and  then  divides,  one  branch  passing 
through  an  impedance  coil  7  and  the  other  through  the  receiver  and 
the  condenser  8,  in  series.  The  steady  current  from  the  central- 
office  battery  finds  ready  path  through  the  transmitter  and  the  im- 
pedance coil,  but  is  prevented  from  passing  through  the  receiver  by 
the  barrier  set  up  by  the  condenser  8.  Voice  currents,  however, 
coming  over  the  line  to  the  station,  find  ready  path  through  the  re- 
ceiver and  the  condenser  but  are  barred  from  passing  through  the 
impedance  coil  by  virtue  of  its  high  impedance. 

In  considering  the  action  of  the  station  as  a  transmitting  sta- 
tion, the  variations  set  up  by  the  transmitter  pass  through  the  con- 


CURRENT  SUPPLY  TO  TRANSMITTERS 


187 


denser  and  the  receiver  at  the  same  station,  while  the  steady  current 
which  supplies  the  transmitter  passes  through  the  impedance  coil. 
Impedance  coils  used  for  this  purpose  are  made  of  low  ohmic  resist- 
ance but  of  a  comparatively  great  number  of  turns,  and,  therefore, 
present  a  good  path  for  steady  currents  and  a  difficult  path  for  voice 
currents.  This  divided  circuit  arrangement  employed  by  the  Kel- 
logg Company  is  one  of  the  very  simple  ways  of  eliminating  direct 
currents  from  the  receiver  path,  at  the  same  time  allowing  the  free 
passage  of  voice  currents. 

Dean  Substation  Arrangement.  In  marked  contrast  to  the 
scheme  for  keeping  steady  current  out  of  the  receiver  circuit  em- 
ployed by  the  Kellogg  Company,  is  that  shown  in  Fig.  133,  which 
has  been  largely  used  by  the  Dean  Electric  Company,  of  Elyria, 
Ohio.  The  central-office  arrangement  in  this  case  is  that  using 


STAT/OH  -A- 


Fig.  133.     Dean  System 


the  split  repeating  coil,  which  needs  no  further  description.  The 
substation  arrangement,  however,  is  unique  and  is  a  beautiful 
example  of  what  can  be  done  in  the  way  of  preventing  a  flow  of 
current  through  a  path  without  in  any  way  insulating  that  path  or 
placing  any  barrier  in  the  way  of  the  current.  It  is  an  example  of 
the  prevention  of  the  direct  flow  of  current  through  the  receiver 
by  so  arranging  the  circuits  tha.t  there  will  always  be  an  equal 
potential  on  each  side  of  it,  and,  therefore,  no  tendency  for  current  to 
flow  through  it. 

In  this  substation  arrangement  four  coils  of  wire — 1,  2,  3,  and 
4 — are  so  arranged  as  to  be  connected  in  the  circuit  of  the  line,  two 
in  series  and  two  in  multiple.  The  current  flowing  from  the  battery 
at  the  central  office,  after  passing  through  the  transmitter,  divides 
between  the  two  paths  containing,  respectively,  the  coils  1  and  3 
and  the  coils  2  and  4-  The  receiver  is  connected  between  the  junc- 
tion of  the  coils  2  and  4  and  tnat  °f  ^  an^  3.  The  resistances  of 


188  TELEPHONY 

the  coils  are  so  chosen  that  the  drop  of  potential  through  the  coil 
2  will  be  equal  to  that  through  the  coil  1,  and  likewise  that  through 
the  coil  4  will  t*e  equal  to  that  through  the  coil  3.  As  a  result,  the 
receiver  will  be  connected  between  two  points  of  equal  potential, 
and  no  direct  current  will  flow  through  it.  How,  then,  do  voice 
currents  find  their  way  through  the  receiver,  as  they  evidently  must, 
if  the  circuit  is  to  fulfill  any  useful  function?  The  coils  2  and  3 
are  made  to  have  high  impedance,  while  1  and  4  are  so  wound  as  to 
be  non-inductive  and,  therefore,  offer  no  impedance  save  that  of  their 
ohmic  resistance.  What  is  true,  therefore,  of  direct  currents  does 
not  hold  for  voice  currents,  and  as  a  result,  the  voice  currents,  in- 
stead of  taking  the  divided  path  which  the  direct  currents  pursued, 
are  debarred  from  the  coils  2  and  3  by  their  high  impedance  and  thus 
pass  through  the  non-inductive  coil  1,  the  receiver,  and  the  non- 
inductive  coil  4- 

This  circuit  employs  a  Wheatstone-bridge  arrangement,  adjusted 
to  a  state  of  balance  with  respect  to  direct  currents,  such  currents 
being  excluded  from  the  receiver,  not  because  the  receiver  circuit 
is  in  any  sense  opaque  to  such  direct  currents,  but  because  there 
is  no  difference  of  potential  between  the  terminals  of  the  receiver 
circuit,  and,  therefore,  no  tendency  for  current  to  flow  through  the 
receiver.  In  order  that  fluctuating  currents  may  not,  for  the  same 
reason,  be  caused  to  pass  by,  rather  than  through,  the  receiver 
circuit,  the  diametrically-opposed  arms  of  the  Wheatstone  bridge 
are  made  to  possess,  in  large  degree,  self-induction,  thereby  giving 
these  two  arms  a  high  impedance  to  fluctuating  currents.  The 
conditions  which  exist  for  direct  currents  do  not,  therefore,  exist  for 
fluctuating  currents,  and  it  is  this  distinction  which  allows  alter- 
nating currents  to  pass  through  the  receiver  and  at  the  same  time 
excludes  direct  currents  therefrom. 

In  practice,  the  coils  1,  2,  3,  and  4  of  the  Dean  substation 
circuit  are  wound  on  the  same  core,  but  coils  1  and  4 — the  non- 
inductive  ones — are  wound  by  doubling  the  wire  back  on  itself  so  as 
to  neutralize  their  self-induction. 

Stromberg-Carlson.  Another  modification  of  the  central- 
office  arrangement  and  also  of  the  subscribers'  station  circuits,  is 
shown  in  Fig.  134,  this  being  a  simplified  representation  of  the  cir- 
cuits commonly  employed  by  the  Stromberg-Carlson  Telephone 


CURRENT  SUPPLY  TO  TRANSMITTERS 


189 


Manufacturing  Company.  The  battery  feed  at  the  central  office 
differs  only  from  that  shown  in  Fig.  132,  in  that  a  single  battery  rather 
than  two  batteries  is  used,  the  current  being  supplied  to  one  of  the 
lines  through  the  impedance  coils  1  and  2,  and  to  the  other  line 
through  the  impedance  coils  3  and  4>  condensers  5  and  6  serve 


STAT/ON -A- 


STAT/ON-&- 


Fig.  134.     Stromberg-Carlson  System 


conductively  to  isolate  the  two  lines.  At  the  subscriber's  station 
the  line  circuit  passes  through  the  secondary  of  an  induction  coil  and 
the  transmitter.  The  receiver  is  kept  entirely  in  a  local  circuit  so 
that  there  is  no  tendency  for  direct  current  to  flow  through  it,  but  it 
is  receptive  to  voice  currents  through  the  electromagnetic  induc- 
tion between  the  primary  and  the  secondary  of  the  induction  coil. 

North.  Another  arrangement  of  central-office  battery  feed  is 
employed  by  the  North  Electric  Company,  and  is  shown  in  Fig.  135. 
In  this  two  batteries  are  used  which  supply  current  respectively  to 
the  two  connected  lines,  condensers  being  employed  to  conductively 
isolate  the  lines.  This  differs  from  the  Kellogg  arrangement 


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Fig.  135.     North  Electric  Company  System 


shown  in  Fig.  132  in  that  the  two  coils  i  and  2  are  wound  on  the 
same  core,  while  the  coils  3  and  4  are  wound  together  upon  an- 
other core.  In  this  case,  in  order  that  the  inductive  action  of 
one  of  the  coils  may  not  neutralize  that  of  the  other  coil  on  the 
same  core,  the  two  coils  are  wound  in  such  relative  direction 


190 


TELEPHONY 


that  their  magnetizing  influence  will  always  be  cumulative  rather 
than  differential. 

The  central-office  arrangements  discussed  in  Figs.  130  to  135, 
inclusive,  are  those  which  are  in  principal  use  in  commercial  prac- 
tice in  common-battery  exchanges. 

Current  Supply  ever  Limbs  cf  Line  in  Parallel.  As  indicating 
further  interesting  possibilities  in  the  method  of  supplying  current 


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Fig.  136.     Current  Supply  over  Parallel  Limbs  of  Line 

from  a  common  source  to  a  number  of  substations,  several 
other  systems  will  be  briefly  referred  to  as  being  of  interest, 
although  these  have  not  gone  into  wide  commercial  use.  The 
system  shown  in  Fig.  136  is  one  proposed  by  Dean  in  the  early 
days  of  common-battery  working,  and  this  arrangement  was  put 
into  actual  service  and  gave  satisfactory  results,  but  was  after- 
wards supplanted  by  the  Bell  equipment  operating  under  the 
system  shown  in  Fig.  130,  which  became  standardized  by  that  com- 
pany. In  this  the  current  from  the  common  battery  at  the  central 
office  is  not  fed  over  the  two  line  wires  in  series,  but  in  multiple, 
using  a  ground  return  from  the  subscriber's  station  to  the  ^central 
office.  Across  the  metallic  circuit  formed  by  two  connected  lines 
there  is  bridged,  at  the  central  office,  an  impedance  coil  1,  and  be- 
tween the  center  point  of  this  impedance  coil  and  the  ground  is  con- 
nected the  common  battery.  At  the  subscriber's  station  is  placed 
an  impedance  coil  2,  also  bridged  across  the  two  limbs  of  the  line, 
and  between  the  center  point  of  this  impedance  coil  and  the  ground 
is  connected  the  transmitter,  which  is  shunted  by  the  primary  wind- 
ing of  an  induction  coil.  Connected  between  the  two  limbs  of  the 
line  at  the  substation  there  is  also  the  receiver  and  the  secondary  of 
an  induction  coil  in  series. 

The  action  of  this  circuit  at  first  seems  a  little  complex,  but  if 
taken  step  by  step  may  readily  be  understood.  The  transmitter  supply 


CURRENT  SUPPLY  TO  TRANSMITTERS 


191 


circuit  may  be  traced  from  the  central-office  battery  through  the  two 
halves  of  the  impedance  coil  1  in  multiple;  thence  over  the  two  limbs 
of  the  line  in  multiple  to  Station  A,  for  instance;  thence  in  multiple 
through  the  two  halves  of  impedance  coil  2,  to  the  center  point  of 
that  coil;  thence  through  the  two  paths  offered  respectively  by  the 
primary  of  the  induction  coil  and  by  the  transmitter;  then  to 
ground  and  back  to  the  other  pole  of  the  central-office  battery.  By 
this  circuit  the  transmitter  at  the  substation  is  supplied  with  cur- 
rent. 

Variations  in  the  resistance  of  the  transmitter  when  in  action, 
cause  complementary  variations  in  the  supply  current  flowing  through 
the  primary  of  the  induction  coil.  These  variations  induce  similar 
alternating  currents  in  the  secondary  of  this  coil,  which  is  in  series 
in  the  line  circuit.  The  currents,  so  induced  in  this  secondary,  flow 
in  series  through  one  side  of  the  line  to  the  distant  station;  thence 
through  the  secondary  and  the  receiver  at  that  station  to  the  other 
side  of  the  line  and  back  through  that  side  of  the  line  to  the  receiver. 
These  currents  are  not  permitted  to  pass  through  the  bridged  paths 
across  the  metallic  circuit  that  are  offered  by  the  impedance  coils 
1  and  2,  because  they  are  voice  currents  and  are,  therefore,  debarred 
from  these  paths  by  virtue  of  the  impedance. 

An  objection  to  this  form  of  current  supply  and  to  other  similar 
forms,  wherein  the  transmitter  current  is  fed  over  the  two  sides  of 


STAT/O/Y  -A- 

Fig.  137.     Current  Supply  over  Parallel  Limbs  of  Line 

the  line  in  multiple  with  a  ground  return,  is  that  the  ground-return 
circuit  formed  by  the  two  sides  of  the  line  in  multiple  is  subject  to 
inductive  disturbances  from  other  lines  in  the  same  way  as  an  or- 
dinary grounded  line  is  subject  to  inductive  disturbance.  The  cur- 
rent-supply circuit  is  thus  subject  to  external  disturbances  and  such 
disturbances  find  their  way  into  the  metallic  circuit  and,  therefore, 


192  TELEPHONY 

through  the  instruments  by  means  of  the  electromagnetic  induction 
between  the  primary  and  the  secondary  coils  at  the  substations. 

Another  interesting  method  of  current  supply  from  a  central- 
office  battery  is  shown  in  Fig.  137.  This,  like  the  circuit  just  con- 
sidered, feeds  the  energy  to  the  subscriber's  station  over  the  two 
sides  of  the  line  in  multiple  with  a  ground  return.  In  this  case, 
however,  a  local  circuit  is  provided  at  the  substation,  in  which  is 
placed  a  storage  battery  1  and  the  primary  2  of  an  induction  coil, 
together  with  the  transmitter.  The  idea  in  this  is  that  the  current 
supply  from  the  central  office  will  pass  through  the  storage  battery 
and  charge  it.  Upon  the  use  of  the  transmitter,  this  storage  battery 
acts  to  supply  current  to  the  local  circuit  containing  the  transmitter 
and  the  primary  coil  2  in  exactly  the  same  manner  as  in  a  local  bat- 
tery system.  The  fluctuating  current  so  produced  by  the  action  of  the 
transmitter  in  this  local  circuit  acts  on  the  secondary  winding  3  of 
the  induction  coil,  and  produces  therein  alternating  currents  which 
pass  to  the  central  office  and  are  in  turn  repeated  to  the  distant  sta- 
tion. 

Supply  Many  Lines  from  Common  Source.  We  come  now  to 
the  consideration  of  the  arrangement  by  which  a  single  battery  may 
be  made  to  supply  current  at  the  central  office  to  a  large  number 
of  pairs  of  connected  lines  simultaneously.  Up  to  this  point  in  this 
discussion  it  has  been  shown  only  how  each  battery  served  a  single 
pair  of  connected  lines  and  no  others. 

Repeating  Coil: — In  Fig.  138  is  shown  how  a  single  battery 
supplies  current  simultaneously  to  four  different  pairs  of  lines,  the 
lines  of  each  pair  being  connected  for  conversation.  It  is  seen  that 
the  pairs  of  lines  shown  in  this  figure  are  arranged  in  each  case  in 
accordance  with  the  system  shown  in  Fig.  130.  Let  us  inquire  why  it 
is  that,  although  all  of  these  four  pairs  of  lines  are  connected  with  a 
common  source  of  energy  and  are,  therefore,  all  conductively  joined, 
the  stations  will  be  able  to  communicate  in  pairs  without  interference 
between  the  pairs.  In  other  words,  why  is  it  that  voice  currents 
originating  at  Station  A  will  pass  only  to  the  receiver  at  Station  B 
and  not  to  the  receivers  at  Station  C  or  Station  H,  for  instance? 
The  reason  is  that  separate  supply  conductors  lead  from  the  points 
such  as  1  and  2  at  the  junctions  of  the  repeating-coil  windings  on 
each  pair  of  circuits  to  the  battery  terminals,  and  the  resistance 


CURRENT  SUPPLY  TO  TRANSMITTERS 


193 


STATION  If! 

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Fig.  138.    Common  Source  for  Many  Lines 


STAT/ON 


Fig.  139.    Common  Source  for  Many  Lines 


194  TELEPHONY 

and  impedance  of  the  battery  itself  and  of  the  common  leads  to  it 
are  so  small  that  although  the  feeble  voice  currents  originating  in 
the  pair  of  lines  connecting  Station  A  and  Station  B  pass  through 
the  battery,  they  are  not  able  to  alter  the  potential  of  the  battery  in 
any  appreciable  degree.  As  a  result,  therefore,  the  supply  wires 
leading  from  the  common-battery  terminals  to  the  points  7  and  8, 
for  instance,  cannot  be  subjected  to  any  variations  in  potential  by 
virtue  of  currents  flowing  through  the  battery  from  the  points  1  and 
2  of  the  lines  joining  Station  A  and  Station  B. 

Retardation  Coil — Single  Battery: — In  Fig.  139  is  shown  in 
similar  manner  the  current  supply  from  a  single  battery  to  four  dif- 
ferent pairs  of  lines,  the  battery  being  associated  with  the  lines  by 
the  combined  impedance  coil  and  condenser  method,  which  was 
specifically  dealt  with  in  connection  with  Fig.  133.  The  reasons 
why  there  will  be  no  interference  between  the  conversations  carried 
on  in  the  various  pairs  of  connected  lines  in  this  case  are  the  same  as 
those  just  considered  in  connection  with  the  system  shown  in  Fig. 
138.  The  impedance  coils  in  this  case  serve  to  keep  the  telephone 
currents  confined  to  their  respective  pairs  of  lines  in  which  they 
originate,  and  this  same  consideration  applies  to  the  system  of  Fig. 
138,  for  each  of  the  separate  repeating-coil  windings  of  Fig.  138  is 
in  itself  an  impedance  coil  with  respect  to  such  currents  as  might 
leak  away  from  one  pair  of  lines  on  to  another. 

Retardation  Coil — Double  Battery : — The  arrangement  of  feed- 
ing a  number  of  pairs  of  lines  according  to  the  Kellogg  two-battery 
system  is  indicated  in  Fig.  140,  which  needs  no  further  explanation 
in  view  of  the  description  of  the  preceding  figures.  It  is  interesting 
to  note  in  this  case  that  the  left-hand  battery  serves  only  the  left- 
hand  lines  and  the  right-hand  battery  only  the  right-hand  lines.  As 
this  is  worked  out  in  practice,  the  left-hand  battery  is  always  con- 
nected to  those  lines  which  originate  a  call  and  the  right-hand  bat- 
tery always  to  those  lines  that  are  called  for.  The  energy  supplied 
to  a  calling  line  is  always,  therefore,  from  a  different  source  than  that 
which  supplies  a  called  line. 

Current  Supply  from  Distant  Point.  Sometimes  it  is  con- 
venient to  supply  current  to  a  group  of  lines  centering  at  a  certain 
point  from  a  source  of  current  located  at  a  distant  point.  This 
is  often  the -case  in  the  so-called  private  branch  exchange,  where  a 


CURRENT  SUPPLY  TO  TRANSMITTERS  195 


STAT/O/y-C-  S7XT/0/V  -D- 

Fig.  140.     Two  Sources  for  Many  Lines 


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r 


STATJON-C-  STAT/ON-D- 

Pig.  141.     Current  Supply  from  Distant  Point 


196  TELEPHONY 

given  business  house  or  other  institution  is  provided  with  its  own 
switchboard  for  interconnecting  the  lines  leading  to  the  various  tele- 
phones of  that  concern  or  institution  among  themselves,  and  also 
for  connecting  them  with  lines  leading  to  the  city  exchange.  It 
is  not  always  easy  or  convenient  to  maintain  at  such  private  switch- 
boards a  separate  battery  for  supplying  the  current  needed  by  the 
local  exchange. 

In  such  cases  the  arrangement  shown  in  Fig.  141  is  sometimes 
employed.  This  shows  two  pairs  of  lines  connected  by  the  impe- 
dance-coil system  with  common  terminals  1  and  2,  between  which 
ordinarily  the  common  battery  would  be  connected.  Instead  of 
putting  a  battery  between  these  terminals,  however,  at  the  local 
exchange,  a  condenser  of  large  capacity  is  connected  between  them 
and  from  these  terminals  circuit  wires  3  and  4  are  led  to  a  battery 
of  suitable  voltage  at  a  distant  central  office.  The  condenser  in  this 
case  is  used  to  afford  a  short-circuit  path  for  the  voice  currents 
that  leak  from  one  side  of  one  pair  of  lines  to  the  other,  through 
the  impedance  coils  bridged  across  the  line.  In  this  way  the 
effect  of  the  necessarily  high  resistance  in  the  common  leads  3  and 
4,  leading  to  the  storage  battery,  is  overcome  and  the  tendency  to 
cross-talk  between  the  various  pairs  of  connected  lines  is  eliminated. 
Frequently,  instead  of  employing  this  arrangement,  a  storage  battery 
of  small  capacity  will  be  connected  between  the  terminals  1  and  2, 
instead  of  the  condenser,  and  these  will  be  charged  over  the  wires  3 
and  4  from  a  source  of  current  at  a  distant  point. 

A  consideration  of  the  various  methods  of  supplying  current 
from  a  common  source  to  a  number  of  lines  will  show  that  it  is  essen- 
tial that  the  resistance  of  the  battery  itself  be  very  low.  It  is  also 
necessary  that  the  resistance  and  the  impedance  of  the  common 
leads  from  the  battery  to  the  point  of  distribution  to  the  various 
pairs  of  lines  be  very  low,  in  order  that  the  voice  currents  which 
flow  through  them,  by  virtue  of  the  conversations  going  on  in  the 
different  pairs  of  lines,  shall  not  produce  any  appreciable  altera- 
tion in  the  difference  of  potential  between  the  battery  terminals. 


CHAPTER  XIV 
THE  TELEPHONE  SET 

We  have  considered  what  may  be  called  the  elemental  parts  of  a 
complete  telephone;  that  is,  the  receiver,  transmitter,  hook  switch, 
battery,  generator,  call  bell,  condenser,  and  the  various  kinds  of 
coils  which  go  to  make  up  the  apparatus  by  which  one  is  enabled  to 
transmit  and  receive  speech  and  signals.  We  will  now  consider  the 
grouping  of  these  various  elements  into  a  complete  working  organ- 
ization known  as  a  telephone. 

Before  considering  the  various  types  it  is  well  to  state  that 
the  term  telephone  is  often  rather  loosely  used.  We  sometimes 
hear  the  receiver  proper  called  a  telephone  or  a  hand  telephone. 
Since  this  was  the  original  speaking  telephone,  there  is  some  reason 
for  so  calling  the  receiver.  The  modern  custom  more  often  applies 
the  term  telephone  to  the  complete  organization  of  talking  and  sig- 
naling apparatus,  together  with  the  associated  wiring  and  cabinet  or 
standard  on  which  it  is  mounted.  The  name  telephone  set  is  per- 
haps to  be  preferred  to  the  word  telephone,  since  it  tends  to  avoid 
misunderstanding  as  to  exactly  what  is  meant.  Frequently,  also, 
the  telephone  or  telephone  set  is  referred  to  as  a  subscriber's  station 
equipment,  indicating  the  equipment  that  is  to  be  found  at  a  sub- 
scriber's station.  This,  as  applying  to  a  telephone  alone,  is  not 
proper,  since  the  subscriber's  station  equipment  includes  more  than 
a  telephone.  It  includes  the  local  wiring  within  the  premises  of  the 
subscriber  and  also  the  lightning  arrester  and  other  protective  de- 
vices, if  such  exist. 

To  avoid  confusion,  therefore,  the  collection  of  talking  and  sig- 
naling apparatus  with  its  wiring  and  containing  cabinet  or  standard 
will  be  referred  to  in  this  work  as  a  telephone  or  telephone  set.  The 
receiver  will,  as  a  rule,  be  designated  as  such,  rather  than  as  a 
telephone.  The  term  subscriber's  station  equipment  will  refer  to 
the  complete  equipment  at  a  subscriber's  station,  and  will  include 
the  telephone  set,  the  interior  wiring,  and  the  protective  devices, 


198  TELEPHONY 

together  with  any  other  apparatus  that  may  be  associated  with  the 
telephone  line  and  be  located  within  the  subscriber's  premises. 

Classification  of  Sets.  Telephones  may  be  classified  under 
two  general  headings,  magneto  telephones  and  common-battery 
telephones,  according  to  the  character  of  the  systems  in  which  they 
are  adapted  to  work. 

Magneto  Telephone.  The  term  magneto  telephone,  as  it  was 
originally  employed  in  telephony,  referred  to  the  type  of  instrument 
now  known  as  a  receiver,  particularly  when  this  was  used  also  as  a 
transmitter.  As  the  use  of  this  instrument  as  a  transmitter  has 
practically  ceased,  the  term  magneto  telephone  has  lost  its  signifi- 
cance as  applying  to  the  receiver,  and,  since  many  telephones  are 
equipped  with  magneto  generators  for  calling  purposes,  the  term 
magneto  telephone  has,  by  common  consent,  come  to  be  used  to 
designate  any  telephone  including,  as  a  part  of  its  equipment,  a  mag- 
neto generator.  Magneto  telephones  usually,  also,  include  local 
batteries  for  furnishing  the  transmitter  with  current,  and  this  has 
led  to  these  telephones  being  frequently  called  local  battery  telephones. 
However,  a  local  battery  telephone  is  not  necessarily  a  magneto 
telephone  and  vice  versa,  since  sometimes  magneto  telephones  have 
no  local  batteries  and  sometimes  local  battery  telephones  have  no 
magnetos.  Nearly  all  of  the  telephones  which  are  equipped  with 
magneto  generators  are,  however,  also  equipped  with  local  batteries 
for  talking  purposes,  and,  therefore,  the  terms  magneto  telephone 
and  local  battery  telephone  usually  refer  to  the  same  thing. 

Common- Battery  Telephone.  Common-battery  telephones,  on  the 
other  hand,  are  those  which  have  no  local  battery  and  no  magneto 
generator,  all  the  current  for  both  talking  and  signaling  being  fur- 
nished from  a  common  source  of  current  at  the  central  office. 

Wall  and  Desk  Telephones.  Again  we  may  classify  telephones 
or  telephone  sets  in  accordance  with  the  manner  in  which  their  var- 
ious parts  are  associated  with  each  other  for  use,  regardless  of  \vhat 
parts  are  contained  in  the  set.  We  may  refer  to  all  sets  adapted  to 
be  mounted  on  a  wall  or  partition  as  wall  telephones,  and  to  all  in 
which  the  receiver,  transmitter,  and  hook  are  provided  with  a  stan- 
dard of  their  owrn  to  enable  them  to  rest  on  any  flat  surface,  such  as 
a  desk  or  table,  as  desk  telephones.  These  latter  are  also  referred  to 
as  portable  telephones  and  as  portable  desk  telephones. 


THE  TELEPHONE  SET 


199 


In  general,  magneto  or  local  battery  telephones  differ  from  com- 
mon-battery telephones  in  their  component  parts,  the  difference  re- 
siding principally  in  the  fact  that  the  magneto  telephone  always  has  a 
magneto  generator  and  usually  a  local  battery,  while  the  common- 
battery  telephone  has  no  local  source  of  current  whatever.  On  the 
other  hand,  the  differences  between  wall  telephones  and  desk  tele- 
phones are  principally  structural,  and  obviously  either  of  these  types 
of  telephones  may  be  for  common-battery  or  magneto  work.  The 
same  component  parts  go  to  make  up  a  desk  telephone  as  a  wall 
telephone,  provided  the  two  instruments  are  adapted  for  the  same 


Fig.  142.   Magneto  Wall  Set 


Fig.  143.     Magneto  Wall  Set 


class  of  service,  but  the  difference  between  the  two  lies  in  the  struc- 
tural features  by  which  these  same  parts  are  associated  with  each 
other  and  protected  from  exposure. 

MagnetoTelephone  Sets.  Wall.  In  Fig.  142  is  shown  a  familiar 
type  of  wall  set.  The  containing  box  includes  within  it  all  of  the 
working  parts  of  the  apparatus  except  that  which  is  necessarily  left 
outside  in  order  to  be  within  the  reach  of  the  user.  Fig.  143  shows 
the  same  set  with  the  door  open.  This  gives  a  good  idea  of  the 
ordinary  arrangement  of  the  apparatus  within.  It  is  seen  that 
the  polarized  bell  or  ringer  has  its  working  parts  mounted  on  the 
inside  of  the  door  or  cover  of  the  box,  the  tapper  projecting  through 


200  TELEPHONY 

so  as  to  play  between  the  gongs  on  the  outside.  Likewise  the 
transmitter  arm,  which  supports  the  transmitter  and  allows  its 
adjustment  up  and  down  to  accommodate  itself  to  the  height 
of  the  user,  is  mounted  on  the  front  of  the  door,  and  the  con- 
ductors leading  to  it  may  be  seen  fastened  to  the  rear  of  the  door 
in  Fig.  143. 

In  some  wall  sets  the  wires  leading  to  the  bell  and  trans- 
mitter are  connected  to  the  wiring  of  the  rest  of  the  set  through  the 
hinges  of  the  door,  thus  allowing  the  door  to  be  opened  and  closed 
repeatedly  without  breaking  off  the  wires.  In  order  to  always  insure 
positive  electrical  contact  between  the  stationary  and  movable  parts 
of  the  hinge  a  small  wire  is  wound  around  the  hinge  pin,  one  end 
being  soldered  to  the  stationary  part  and  the  other  end  to  the  mov- 
able part  of  the  hinge.  In  other  forms  of  wall  set  the  wires  to  the 
bell  and  the  transmitter  lead  directly  from  the  stationary  portion  of 
the  cabinet  to  the  back  of  the  door,  the  wires  being  left  long  enough 
to  have  sufficient  flexibility  to  allow  the  door  to  be  opened  and  closed 
without  injuring  the  wires. 

At  the  upper  portion  of  the  box  there  is  mounted  the  hook 
switch,  this  being,  in  this  case,  of  the  short  lever  type.  The  lever  of 
the  hook  projects  through  the  side  of  the  box  so  as  to  make  the  hook 
available  as  a  support  for  the  receiver.  Immediately  at  the  right  of 
the  hook  switch  is  mounted  the  induction  coil,  and  immediately 
below  this  the  generator,  its  crank  handle  projecting  through  the 
right-hand  side  of  the  box  so  as  to  be  available  for  use  there.  The 
generator  is  usually  mounted  on  a  transverse  shelf  across  the  middle 
of  the  cabinet,  this  shelf  serving  to  form  a  compartment  below  it  in 
which  the  dry  battery  of  two  or  three  cells  is  placed. 

The  wall  telephone-set  cabinets  have  assumed  a  multitude  of 
forms.  When  wet  cells  rather  than  dry  cells  were  ordinarily  em- 
ployed, as  was  the  case  up  to  about  the  year  1895,  the  magneto 
generator,  polarized  bell,  and  hook  switch  were  usually  mounted 
in  a  rectangular  box  placed  at  the  top  of  a  long  backboard.  Im- 
mediately below  this  on  the  backboard  was  mounted  the  transmitter 
arm,  and  sometimes  the  base  of  this  included  the  induction  coil. 
Below  this  was  the  battery  box,  this  being  a  large  affair  usually 
adapted  to  accommodate  two  and  sometimes  three  ordinary  LeClanche* 
cells  side  by  side. 


THE  TELEPHONE  SET 


201 


The  dry  cell  has  almost  completely  replaced  the  wet  cell  in  this 
country,  and  as  a  result,  the  general  type  of  wall  set  as  shown  in 
Figs.  142  and  143,  has  gradually  replaced  the  old  wet-cell  type, 
which  was  more  cumbrous  and  unsightly.  It  is  usual  on  wall  sets 
to  provide  some  sort  of  a  shelf,  as  indicated  in  Fig.  142,  for  the  con- 
venience of  the  user  in  making  notes  and  memoranda. 

Desk.  In  the  magneto  desk-telephone  sets,  the  so-called  desk 
stand,  containing  the  transmitter,  the  receiver,  and  the  hook  switch, 
with  the  standard  upon  which  they  are  mounted,  is  shown  in  Fig. 

144.  This  desk  stand  evidently  does  not  comprise  the  complete 
equipment    for    a    magneto    desk- 
telephone  set,  since  the  generator, 

polarized  bell,  and  battery  are  lack- 
ing. The  generator  and  bell  are 
usually  mounted  together  in  a  box, 
either  on  the  under  side  of  the  desk 
of  the  user  or  on  the  wall  within 
easy  reach  of  his  chair.  Connec- 
tions are  made  between  the  appa- 
ratus in  the  desk  stand  proper  and 
the  battery,  generator,  and  bell  by 
means  of  flexible  conducting  cords, 
these  carrying  a  plurality  of  con- 
ductors, as  required  by  the  particu- 
lar circuit  of  the  telephone  in  ques- 
tion. Such  a  complete  magneto 
desk-telephone  set  is  shown  in  Fig. 

145,  this  being  one  of  the  types  manufactured  by  the  Stromberg- 
Carlson  Manufacturing  Company. 

A  great  varietv  of  arrangements  of  the  various  parts  of  magneto 
desk-telephone  apparatus  is  employed  in  practice.  Sometimes,  as 
shown  in  Fig.  1.45,  the  magneto  bell  box  is  equipped  with  binding 
posts  for  terminating  all  of  the  conductors  in  the  cord,  the  line  wires 
also  running  to  some  of  these  binding  posts. 

In  the  magneto-telephone  set  illustrated  the  box  is  made  large 
enough  to  accommodate  only  the  generator  and  call  bell,  and  the  bat- 
teries are  mounted  elsewhere,  as  in  a  drawer  of  the  desk,  while  in 
other  cases  there  is  no  other  equipment  but  that  shown  in  the  cut, 


Fig.   144.     Desk  Stand 


202 


TELEPHONY 


the  batteries  being  mounted  within  the  magneto  bell  box  itself.  In 
still  other  cases,  the  polarized  bell  is  contained  in  one  box,  the  gener- 
ator in  another,  the  batteries  in  the  drawer  of  the  desk,  the  induction 
coil  being  mounted  either  in  the  base  of  the  desk  stand,  in  the  bell 

box,  or  in  the  generator  box.  In  such 
cases  all  of  the  circuits  of  the  various 
scattered  parts  are  wired  to  a  terminal 
strip,  located  at  some  convenient  point, 
this  strip  containing  terminals  for  all 
the  wires  leading  from  the  various  parts 
and  for  the  line  wires  themselves.  By 
combining  the  various  wires  on  the 
terminals  of  this  terminal  strip,  the 
complete  t  circuits  of  the  telephone  are 
built  up.  In  still  other  cases  the  in- 
duction coil  is  mounted  on  the  terminal 
strip  and  separate  wires  or  sets  of 
wires  are  run  to  the  polarized  bell 
and  generator,  to  the  desk  stand  itself, 
and  to  the  batteries.  These  various 
arrangements  are  subject  largely  to 
the  desire  or  personal  ideas  of  the 
manufacturer  or  user.  All  of  them 
work  on  the  same  principle  so  far  as  the  operation  of  the  talking 
and  signaling  circuits  is  concerned. 

Circuits  of  Magneto=TeIephone  Sets.  Magneto  telephones, 
whether  of  the  wall  or  desk  type,  may  be  divided  into  two  general 
classes,  series  and  bridging,  according  to  whether  the  magnet  of  the 
bell  is  included  in  series  or  bridge  relation  with  the  telephone  line 
when  the  hook  is  down. 

Series.  In  the  so-called  series  telephone  line,  where  several 
telephones  are  placed  in  series  in  a  single  line  circuit,  the  employ- 
ment of  the  series  type  of  telephone  results  in  all  of  the  telephone 
bells  being  in  series  in  the  line  circuit.  This  means  that  the  voice 
currents  originating  in  the  telephones  that  are  in  use  at  a  given  time 
must  pass  in  series  through  the  magnets  of  the  bells  of  the  stations  that 
are  not  in  use.  In  order  that  these  magnets,  through  which  the 
voice  currents  must  pass,  may  interfere  to  as  small  a  degree  as  possi- 


Fig.   145.     Magneto  Desk  Set 


THE  TELEPHONE  SET 


203 


ble  with  the  voice  currents,  it  is  common  to  employ  low-resistance 
magnets  in  series  telephones,  these  magnets  being  wound  with  com- 
paratively few  turns  and  on  rather  short  cores  so  that  the  impedance 
will  be  as  small  as  possible.  Likewise,  since  the  generators  are 
required  to  ring  all  of  the  bells  in  series,  they  need  not  have  a  large 
current  output,  but  must  have  sufficient  voltage  to  ring  through  all 
of  the  bells  in  series  and  through  the  resistance  of  the  line.  For 
this  reason  the  generators  are  usually  of  the  three-bar  type  and 
sometimes  have  only  two  bars. 

In  Fig.  146  are  shown,  in  simplified  form,  the  circuits  of  an  ordi- 
nary series  telephone.  The  receiver  in  this  is  shown  as  being  re- 
moved from  the  hook  and  thus  the  talking  apparatus  is  brought  into 
play.  The  line  wires  1  and  2  connect  respectively  to  the  binding 
posts  3  and  4>  which  form  the  terminals  of  the  instrument.  When 
the  hook  is  up,  the  circuit  between  the  binding  posts  3  and  4  includes 
the  receiver  and  the  secondary  winding  of  the  induction  coil,  together 
with  one  of  the  upper  contacts  5 
of  the  switch  hook  and  the  hook 
lever  itself.  This  completes  the 
circuit  for  receiving  speech.  The 
hook  switch  is  provided  with 
another  upper  contact  6,  between 
which  and  the  contact  5  is  con- 
nected the  local  circuit  contain- 
ing the  transmitter,  the  battery, 
and  the  primary  of  the  induction 
coil  in  series.  The  primary  and  the  secondary  windings  are  con- 
nected together  at  one  end  and  connected  with  the  switch  contact 
5,  as  shown.  It  is  thus  seen  that  when  the  hook  is  up  the  circuit 
through  the  receiver  is  automatically  closed  and  also  the  local  cir- 
cuit containing  the  primary,  the  battery,  and  the  transmitter.  Thus, 
all  the  conditions  for  transmitting  and  receiving  speech  are  fulfilled. 

When  the  hook  is  down,  however,  the  receiving  and  transmit- 
ting circuits  are  broken,  but  another  circuit  is  completed  by  the 
engagement  of  the  hook-switch  lever  with  the  lower  hook  contact  7. 
Between  this  contact  and  one  side  of  the  line  is  connected  the  polar- 
ized ringer  and  the  generator.  With  the  hook  down,  therefore, 
the  circuit  may  be  traced  from  the  line  wire  1  to  binding  post  3, 


Fig.  146.     Circuit  of  Series  Magneto  Set 


204 


TELEPHONY 


thence  through  the  generator  shunt  to  the  call  bell,  and  thence  through 
the  lower  switching  contact  7  to  the  binding  post  4  and  line  wire  2. 
The  generator  shunt,  as  already  described  in  Chapter  VIII,  normally 
keeps  the  generator  shunted  out  of  circuit.  When,  however,  the 
generator  is  operated  the  shunt  is  broken,  which  allows  the  arma- 
ture of  the  generator  to  come  into  the  circuit  in  series  with  the  wind- 
ing of  the  polarized  bell.  The  normal  shunting  of  the  generator 
armature  from  the  circuit  of  the  line  is  advantageous  in  several  ways. 
In  the  first  place,  the  impedance  of  the  generator  winding  is  normally 

cut  out  of  the  circuit  so  that  in 
the  case  of  a  line  with  several 
stations  the  talking  or  voice  cur- 
rents do  not  have  to  flow  through 
the  generator  armatures  at  the 
stations  which  are  not  in  use. 
Again,  the  normal  shunting  of 
the  generator  tends  to  save  the 
generator  armature  from  injury 
by  lightning. 

The  more  complete  circuits  of 
a  series  magneto  telephone  are 
shown  in  Fig.  147.  In  this  the 
line  binding  posts  are  shown  as  1 
and  2.  At  the  bottom  of  the 
telephone  cabinet  are  four  other 
binding  posts  marked  3,  4>  5>  and 
Pig.  147.  circuit  of  series  Magneto  Set  6.  Of  these  3  and  4  serve  for  the 

receiver  terminals  and  5  and  6 

for  the  transmitter  and  battery  terminals.  The  circuits  of  this 
diagram  will  be  found  to  be  essentially  the  same  as  those  of  Fig. 
146,  except  that  they  are  shown  in  greater  detail.  This  particular 
type  of  circuit  is  one  commonly  employed  where  the  generator, 
ringer,  hook  switch,  and  induction  coil  are  all  mounted  in  a  so- 
called  magneto  bell  box  at  the  top  of  the  instrument,  and  where 
the  transmitter  is  mounted  on  an  arm  just  below  this  box,  and  the 
battery  in  a  separate  compartment  below  the  transmitter.  The 
only  wiring  that  has  to  be  done  between  the  bell  box  and  the  other 
parts  of  the  instrument  in  assembling  the  complete  telephone  is  to 


THE  TELEPHONE  SET 


205 


connect  the  receiver  to  the  binding  posts  3  and  4  and  to  connect  the 
battery  and  transmitter  circuit  to  the  binding  posts  5  and  6. 

Bridging.  In  other  cases,  where  several  telephones  are  placed 
on  a  single-line  circuit,  the  bells  are  arranged  in  multiple  across  the 
line.  For  this  reason  their  magnets  are  wound  with  a  very  great 
number  of  turns  and  consequently  to  a  high  resistance.  In  order  to 
further  increase  the  impedance,  the  cores  are  made  long  and  heavy. 
Since  the  generators  on  these  lines  must  be  capable  of  giving  out  a 
sufficient  volume  of  current  to  divide  up  between  all  of  the  bells  in 
multiple,  it  follows  that  these  generators  must  have  a  large  current 
output,  and  at  the  same  time  a  sufficient  voltage  to  ring  the  bells  at 
the  farthest  end  of  the  line.  Such  instruments  are  commonly  called 
bridging  instruments,  on  account  of  the  method  of  connecting  their 
bells  across  the  circuit  of  the  lire. 

The  fundamental  characU--.stic  of  the  bridging  telephone  is 
that  it  contains  three  possible  bridge  paths  across  the  line  wires. 
The  first  of  these  bridge  paths  is 
through  the  talking  apparatus, 
the  second  through  the  generator, 
and  the  third  through  the  ringer. 
This  is  shown  in  simplified  form 
in  Fig.  148.  The  talking  appa- 
ratus is  associated  with  the  two 
upper  contacts  of  the  hook  switch 
in  the  usual  manner  and  needs  no 

further  description.  The  generator  is  the  second  separate  bridge 
path,  normally  open,  but  adapted  to  be  closed  when  the  generator 
is  operated,  this  automatic  closure  being  performed  by  the  move- 
ment of  the  crank  shaft.  The  third  bridge  contains  the  polarized 
bell,  and  this,  as  a  rule,  is  permanently  closed.  Sometimes,  how- 
ever, the  arrangement  is  such  that  the  bell  path  is  normally  closed 
through  the  switch  which  is  operated  by  the  generator  crank  shaft, 
and  this  path  is  automatically  broken  when  the  generator  is  opera- 
ted, at  which  time,  also,  the  generator  path  is  automatically  closed. 
This  arrangement  brings  about  the  result  that  the  generator  never 
can  ring  its  own  bell,  because  its  switch  always  operates  to  cut 
out  the  bell  at  its  own  station  just  before  the  generator  itself  is  cut 
into  the  circuit 


Circuit  of  Bridging  Magneto  Set 


206 


TELEPHONY 


In  Fig.  149  is  shown  the  complete  circuit  of  a  bridging  telephone. 
The  circuit  given  in  this  figure  is  for  a  local-battery  wall  set  similar 
in  type  to  that  shown  in  Figs.  142  and  143.  A  simplified  diagram- 
matic arrangement  is  shown  in  the  lower  left-hand  corner  of  this  fig- 
ure, and  from  a  consideration  of  this  it  will  be  seen  that  the  bell 
circuit  across  the  line  is  normally  completed  through  the  two  right- 
hand  normally  closed  contacts  of  the  switch  on  the  generator.  When, 
however,  the  generator  is  operated  these  two  contacts  are  made  to 
disengage  each  other  while  the  long  spring  of  the  generator  switch 
engages  the  left-hand  spring  and  thus  brings  the  generator  itself  into 
the  circuit. 

Of  the  three  binding  posts,  1,  2,  and  .3,  at  the  top  of  Fig.  149,  1 
and  2  are  for  connecting  with  the  line  wires,  while  3  is  for  a  ground 

connection,  acting  in  conjunc- 
tion with  the  lightning  arrester 
mounted  at  the  top  of  the  tele- 
phone and  indicated  at  4  in  Fig. 
149.  This  has  no  function  in 
talking  or  ringing,  and  will  be 
referred  to  more  fully  in  Chapter 
XIX.  Suffice  it  to  say  at  this 
point  that  these  arresters  usually 
consist  of  two  conducting  bodies, 
one  connected  permanently  to 
each  of  the  line  binding  posts, 
and  a  third  conducting  body  con- 
nected to  the  ground  binding 
post.  These  three  conducting 

bodies  are  in  close  proximity  but  carefully  insulated  from  each  other; 
the  idea  being  that  when  the  line  wires  are  struck  by  lightning  or 
subjected  otherwise  to  a  dangerous  potential,  the  charge  on  the  line 
will  jump  across  the  space  between  the  conducting  bodies  and  pass 
harmlessly  to  ground. 

If  the  large  detailed  circuit  of  Fig.  149  be  compared  with  the 
small  theoretical  circuit  in  the  same  figure,  the  various  conducting 

NOTE.  The  student  should  practice  making  simplified  diagrams  from 
actual  wiring  diagrams.  The  difference  between  the  two  is  that  one  is  laid 
out  for  ease  in  understanding  it,  while  4he  other  is  laid  out  to  show  the  actual 
course  of  the  wires  as  installed. 


Fig.  149.     Circuit  of  Bridging  Magneto  Set 


THE  TELEPHONE  SET 


207 


paths  will  be  found  to  be  the  same.  Such  a  simplified  circuit  does 
more  to  enable  one  to  grasp  the  fundamental  scheme  of  a  complex 
circuit  than  much  description,  since  it  shows  at  a  glance  the  general 
arrangement.  The  more  detailed  circuits  are,  however,  necessary 
to  show  the  actual  paths  followed  by  the  wiring. 

The  circuits  of  desk  stands  do  not  differ  from  those  of  wall 
sets  in  any  material  degree,  except  as  may  be  necessitated  by  the  fact 
that  the  various  parts  of  the  telephone  set  are  not  all  mounted  in 
the  same  cabinet  or  on  the  same  standard.  To  provide  for  the 
necessarv  relative  movement  between  the  desk  stand  and  the  other 

tf 

portions  of  the  set,  flexible  conductors  are  run  from  the  desk  stand 

itself    to     the    stationary 

portions  of  the  equipment, 

such  as  the  battery  and 

the  parts  contained  in  the 

generator  and  bell  box. 

In  Fig.  150  is  shown 
the  circuit  of  the  Strom- 
berg-Carlson  magneto 
desk-telephone  set,  illus- 
trated in  Fig.  145.  This 
diagram  needs  no  expla- 
nation in  view  of  what 
has  already  been  said. 
The  conductors,  leading 
from  the  desk-stand  group  of  apparatus  to  the  bell-box  group  of 
apparatus,  are  grouped  together  in  a  flexible  cord,  as  shown  in  Fig. 
145,  and  are  connected  respectively  to  the  various  binding  posts 
or  contact  points  within  the  desk  stand  at  one  end  and  at  the  base 
of  the  bell  box  at  the  other  end.  These  flexible  conductors  are 
insulated  individually  and  covered  by  a  common  braided  covering. 
They  usually  are  individualized  by  having  a  colored  thread  woven 
into  their  insulating  braid,  so  that  it  is  an  easy  matter  to  identify 
the  two  ends  of  the  same  conductor  at  either  end  of  the  flexible 
cord  or  cable. 

Common=Battery  Telephone  Sets.  Owing  to  the  fact  that  com- 
mon-battery telephones  contain  no  sources  of  current,  they  are 
usually  somewhat  simpler  than  the  magneto  type.  The  compo- 


Fig.  150.     Circuit  of  Bridging  Magneto  Desk  Set 


208 


TELEPHONY 


nent  parts  of  a  common-battery  telephone,  whether  of  the  wall  or 
desk  type,  are  the  transmitter,  receiver,  hook  switch,  polarized 
bell,  condenser,  and  sometimes  an  induction  coil.  The  purpose 
of  the  condenser  is  to  prevent  direct  or  steady  currents  from  passing 
through  the  windings  of  the  ringer  while  the  ringer  is  connected  across 

the  circuit  of  the  line  during  the 
time  when  the  telephone  is  not  in 
use.  The  requirements  of  com- 
mon-battery signaling  demand 
that  the  ringer  shall  be  connected 


Fig.  151.     Common-Battery  Wall  Set 


Fig.  152.     Common-Battery  Wall  Set 


with  the  line  so  as  to  be  receptive  of  a  call  at  any  time  while  the 
telephone  is  not  in  use.  The  requirements  also  demand  that  no 
conducting  path  shall  normally  exist  between  the  two  sides  of  the 
line.  These  two  apparently  contradictory  requirements  are  met  by 
placing  a  condenser  in  series  with  the  ringer  so  that  the  ringer  will 
be  in  a  path  that  will  readily  transmit  the  alternating  ringing  cur- 
rents sent  out  from  the  central-office  generator,  while  at  the  same 
time  the  condenser  will  afford  a  complete  bar  to  the  passage  of  steady 


THE  TELEPHONE  SET 


209 


Fig.    153. 


currents.     Sometimes  the  condenser  is  also  used  as  a  portion  of  the 
talking  apparatus,  as  will  be  pointed  out. 

Wall.  In  Figs.  151  and  152  are  given  two  views  of  a  character- 
istic form  of  common-battery  wall-telephone  set,  made  by  the  Strom- 
berg-Carlson  Manufacturing  Company.  The  common-battery  wall 
set  has  usually  taken  this  general 
form.  In  it  the  transmitter  is 
mounted  on  an  adjustable  arm  at 
the  top  of  the  backboard,  while 
the  box  containing  the  bell  and  all 
working  parts  of  the  instrument 
is  placed  below  the  transmitter, 
the  top  of  the  box  affording  a 
shelf  for  writing  purposes.  In 
Fig.  151  are  shown  the  hook 
switch  and  the  receiver;  just  below 
these  may  be  seen  the  magnets  of 
the  polarized  bell,  back  of  which 
is  shown  a  rectangular  box  con- 
taining the  condenser.  Immedi- 
ately in  front  of  the  ringer  magnets  is  the  induction  coil. 

In  Fig.  153  are  shown  the  details  of  the  circuit  of  this  instrument. 
This  figure  also  includes  a  simplified  circuit  arrangement  from 
which  the  principles  involved  may  be  more  readily  understood.  It 
is  seen  that  the  primary  of  the  induction  coil  and  the  transmitter 
are  included  in  series  across  the  line.  The  secondary  of  the  induc- 
tion coil,  in  series  with  the  receiver,  is  connected  also  across  the  line 
in  series  with  a  condenser  and  the  transmitter. 

Hotel.  Sometimes,  in  order  to  economize  space,  the  shelf  of 
common-battery  wall  sets  is  omitted  and  the  entire  apparatus 
mounted  in  a  small  rectangular  box,  the  front  of  which  carries  the 
transmitter  mounted  on  the  short  arm  or  on  no  arm  at  all,  Such 
instruments  are  commonly  termed  hotel  sets,  because  of  the  fact 
that  their  use  was  first  confined  largely  to  the  rooms  in  hotels.  Later, 
however,  these  instruments  have  become  very  popular  in  general 
use,  particularly  in  residences.  Sometimes  the  boxes  or  cabinets  of 
these  sets  are  made  of  wood,  but  of  recent  years  the  tendency  has 
been  growing  to  make  them  of  pressed  steel.  The  steel  box  is  usually 


Stromberg-Carlson  Common- 
Battery  Wall  Set 


210 


TELEPHONY 


finished  in  black  enamel,  baked  on,  the  color  being  sometimes  varied 
to  match  the  color  of  the  surrounding  woodwork.  In  Figs.  154  and 
1 55  are  shown  two  views  of  a  common-batteiy  hotel  set  manufactured 
by  the  Dean  Electric  Company. 

Such  sets  are  extremely  neat  in  appearance  and  have  the  ad- 
vantage of  taking  up  little  room  on  the  wall  and  the  commercial 
advantage  of  being  light  and  compact  for  shipping  purposes.  A 
possible  disadvantage  of  this  type  of  instrument  is  the  somewhat 
crowded  condition  which  necessarily  follows  from  the  placing  of  all 


Pig.  154.     Steel  Box  Hotel  Set 

the  parts  in  so  confined  a  space.  This  interferes  somewhat  with 
the  accessibility  of  the  various  parts,  but  great  ingenuity  has  been 
manifested  in  making  the  parts  readily  get-at-able  in  case  of  necessity 
for  repairs  or  alterations. 

Desk.  The  common-battery  desk  telephone  presents  a  some- 
what simpler  problem  than  the  magneto  desk  telephone  for  the 
reason  that  the  generator  and  local  battery,  the  two  most  bulky 
parts  of  a  magneto  telephone,  do  not  have  to  be  provided  for.  Some 
companies,  in  manufacturing  desk  stands  for  common-battery  pur- 


,    THE  TELEPHONE  SET 


211 


poses,  mount  the  condenser  and  the  induction  coil  or  impedance 
coil,  or  whatever  device  is  used  in  connection  with  the  talking  cir- 


Fig.  155.     Steel  Box  Hotel  Set 


Fig.  156.     Common-Battery  Desk  Set 

cuit,  in  the  base  of  the  desk  stand  itself,  and  mount  the  polarized 
ringer  and  the  condenser  used  for  ringing  purposes  in  a  separate 


212 


TELEPHONY 


bell  box  adapted  to  be  mounted  on  the  wall  or  some  portion  of  the 
desk.     Other  companies  mount  only  the  transmitter,  receiver,  and 

hook  switch  on  the  desk  stand  proper  and 
put  the  condenser  or  induction  coil,  or  other 
device  associated  with  the  talking  circuit, 
in  the  bell  box.  There  is  little  to  choose 
between  the  two  general  practices.  The 
number  of  conducting  strands  in  the  flexi- 
ble cord  is  somewhat  dependent  on  the 
arrangement  of  the  circuit  employed. 

The  Kellogg  Switchboard  and  Supply 
Company  is  one  which  places  all  the  parts, 
except  the  polarized  ringer  and  the  associ- 
ated condenser,  in  the  desk  stand  itself.  In 
Fig.  156  is  shown  a  bottom  view  of  the 
desk  stand  with  the  bottom  plate  removed. 
In  the  upper  portion  of  the  circle  of  the  base 
is  shown  a  small  condenser  which  is  placed  in  the  talking  circuit  in 
series  with  the  receiver.  In  the  right-hand  portion  of  the  circle  of  the 
base  is  shown  a  small  impedance  coil,  which  is  placed  in  series  with  the 
transmitter  but  in  shunt  relation  with  the  condenser  and  the  receiver. 


Fig.  157.     Bell  for  Common- 
Battery  Desk  Set 


Fig.  158.     Bell  for  Common-Battery  Desk  Set 


In  Figs.  157  and  158  are  shown  two  views  of  the  type  of  bell 
box  employed  by  the  Kellogg  Company  in  connection  with  the  com- 


THE  TELEPHONE  SET 


213 


men-battery  desk  sets,  this  box  being  of  pressed-steel  construction  and 
having  a  removable  lid,  as  shown  in  Fig.  158,  by  which  the  working 
parts  of  the  ringer  are  made  readily  accessible,  as  are  also  the  ter- 
minals for  the  cord  leading  from  the  desk  stand  and  for  the  wires 
of  the  line  circuit.  The  condenser  that  is  placed  in  series  with  the 
ringer  is  also  mounted  in  this  same  box.  By  employing  two  conden- 
sers, one  in  the  bell  box  large  enough  to  transmit  ringing  currents  and 
the  other  in  the  base  of  the  desk  stand  large  enough  only  to  trans- 


Pig.  159.    Microtelephone  Set 

mit  voice  currents,  a  duplication  of  condensers  is  involved,  but  it 
has  the  corresponding  advantages  of  requiring  only  two  strands  to 
the  flexible  cord  leading  from  the  bell  box  to  the  desk  stand  proper. 
•  A  form  of  desk-telephone  set  that  is  used  largely  abroad,  but 
that  has  found  very  little  use  in  this  country,  is  shown  in  Fig.  159. 
In  this  the  transmitter  and  the  receiver  are  permanently  attached 
together,  the  receiver  being  of  the  watch-case  variety  and  so  posi- 
tioned relatively  to  the  transmitter  that  when  the  receiver  is  held  at 
the  ear,  the  mouthpiece  of  the  transmitter  will  be  just  in  front  of  the 


214 


TELEPHONY 


lips  of  the  user.  In  order  to  maintain  the  transmitter  in  a  vertical 
position  during  use,  this  necessitates  the  use  of  a  curved  mouthpiece 
as  shown.  This  transmitter  and  receiver  so  combined  is  commonly 

called,  in  this  country,  the  micro- 
telephone  set,  although  there 
seems  to  be  no  logical  reason  for 
this  name.  The  combined  trans- 
mitter and  receiver,  instead  of 
being  supported  on  an  ordinary 
form  of  hook  switch,  are  sup- 
ported on  a  forked  bracket  as 
shown,  this  bracket  serving  to 
operate  the  switch  springs  which 
are  held  in  one  position  when 
the  bracket  is  subjected  to  the 
Fig.  160.  Kellogg  Common-Battery  Desk  set  weight  of  the  microtelephone, 

and    in    the   alternate   position 

when  relieved  therefrom.  This  particular  microtelephone  set  is  the 
product  of  the  L.  M.  Ericsson  Telephone  Manufacturing  Company, 
of  Buffalo,  New  York.  The  circuits  of  such  sets  do  not  differ  ma- 
terially from  those  of  the  ordinary  desk  telephone  set. 

Circuits  of  Common=Battery  Telephone  Sets.     The  complete 


Fig.  161.     Dean  Common-Battery  Set 


circuits  of  the  Kellogg  desk-stand  arrangement  are  shown  in  Fig. 
160,  the  desk-stand  parts  being  shown  at  the  left  and  the  bell-box 


THE  TELEPHONE  SET 


215 


parts  at  the  right.  As  is  seen,  but  two  conductors  extend  from  the 
former  to  the  latter.  A  simplified  theoretical  sketch  is  also  shown 
in  the  upper  right-hand  corner  of  this  figure. 

The  details  of  the  common-battery  telephone  circuits  of  the 
Dean  Electric  Company  are  shown  in  Fig.  161.  This  involves  the 
use  of  the  balanced  Wheatstone  bridge.  The  only  other  thing  about 
this  circuit  that  needs  description,  in  view  of  what  has  previously 
been  said  about  it,  is  that  the  polarized  bell  is  placed  in  series  with 
a  condenser  so  that  the  two  sides  of  the  circuit  may  be  insulated  from 
each  other  while  the  "telephone  is  not  in  use,  and  yet  permit  the  pas- 
sage of  ringing  current  through  the  bell. 

The  use  of  the  so-called  direct-current  receiver  has  brought 
about  a  great  simplification  in  the  common-battery  telephone  cir- 


Fig.  162.    Monarch  Common-Battery  Wall  Set 

cuits  of  several  of  the  manufacturing  companies.  By  this  use  the 
transmitter  and  the  receiver  are  placed  in  series  across  the  line,  this 
path  being  normally  opened  by  the  hook-switch  contacts.  The 
polarized  bell  and  condenser  are  placed  in  another  bridge  path  across 
the  line,  this  path  not  being  affected  by  the  hook-switch  contacts. 
All  that  there  is  to  such  a  complete  common-battery  telephone  set, 
therefore,  is  a  receiver,  transmitter,  hook  switch,  bell,  condenser, 
and  cabinet,  or  other  support. 

The  extreme  simplicity  of  the  circuits  of  such  a  set  is  illustrated 
in  Fig.  162,  which  shows  how  the  Monarch  Telephone  Manufacturing 
Company  connect  up  the  various  parts  of  their  telephone  set,  using  the 
direct-current  receiver  already  described  in  connection  with  Fig.  54. 


CHAPTER  XV 
NON=SELECTIVE  PARTY=LINE  SYSTEMS 

A  party  line  is  a  line  that  is  for  the  joint  use  of  several  stations. 
It  is,  therefore,  a  line  that  connects  a  central  office  with  two  or  more 
subscribers'  stations,  or  where  no  central  office  is  involved,  a  line 
that  connects  three  or  more  isolated  stations  with  each  other.  The 
distinguishing  feature  of  a  party  line,  therefore,  is  that  it  serves  more 
than  two  stations,  counting  the  central  office,  if  there  is  one,  as  a  sta- 
tion. 

Strictly  speaking,  the  term  party  line  should  be  used  in  contra- 
distinction to  the  term  private  line.  Companies  operating  tele- 
phone exchanges,  however,  frequently  lease  their  wires  to  individuals 
for  private  use,  with  no  central-office  switchboard  connections,  and 
such  lines  are,  by  common  usage,  referred  to  as  "private  lines." 
Such  lines  may  be  used  to  connect  two  or  more  isolated  stations.  A 
private  line,  in  the  parlance  of  telephone  exchange  working,  may, 
therefore,  be  a  party  line,  as  inconsistent  as  this  may  seem. 

A  telephone  line  that  is  connected  with  an  exchange  is  an  ex- 
change line,  and  it  is  a  party  line  if  it  has  more  than  one  station  on 
it.  It  is  an  individual  line  or  a  single  party  line  if  it  has  but  a  single 
station  on  it.  A  line  which  has  no  central-office  connection  is  called 
an  "isolated  line,"  and  it  is  a  party  line  if  it  has  more  than  two 
stations  on  it. 

The  problem  of  mere  speech  transmission  on  party  lines  is 
comparatively  easy,  being  scarcely  more  complex  than  that  in- 
volved in  private  or  single  party  lines.  This  is  not  true,  however, 
of  the  problem  of  signaling  the  various  stations.  This  is  because 
the  line  is  for  the  common  use  of  all  its  patrons  or  subscribers,  as 
they  are  termed,  and  the  necessity  therefore  exists  that  the  person 
sending  a  signal,  whether  operator  or  subscriber,  shall  be  able  in 
some  way  to  inform  a  person  at  the  desired  station  that  the  call  is 


218 


TELEPHONY 


intended  for  that  station.  There  are  two  general  ways  of  accom- 
plishing this  purpose. 

(/)  The  first  and  simplest  of  these  ways  is  to  make  no  pro- 
vision for  ringing  any  one  bell  on  the  line  to  the  exclusion  of  the  others, 
and  thus  allow  all  bells  to  ring  at  once  whenever  any  station  on  the 
line  is  wanted.  Where  this  is  done,  in  order  to  prevent  all  stations 
from  answering,  it  is  necessary,  in  some  way,  to  convey  to  the  de- 
sired station  the  information  that  the  call  is  intended  for  that  sta- 
tion, and  to  all  of  the  other  stations  the  information  that  the  call  is 
not  intended  for  them.  This  is  done  on  such  lines  by  what  is  called 
"code  ringing,"  the  code  consisting  of  various  combinations  of  long 
and  short  rings. 

(#)  The  other  and  more  complex  way  is  to  arrange  for  selec- 
tive ringing,  so  that  the  person  sending  the  call  may  ring  the  bell  at 
the  station  desired,  allowing  the  bells  at  all  the  other  stations  to  re- 
main quiet. 

These  two  general  classes  of  party-line  systems  may,  therefore, 
be  termed  "non-selective"  and  "selective"  systems.  Non-selective 


Pig.  163.     Grounded-Circuit  Series  Line 

party  lines  are  largely  used  both  on  lines  having  connection  with  a 
central  office,  and  through  the  central  office  the  privilege  of  connec- 
tion with  other  lines,  and  on  isolated  lines  having  no  central-office 
connection.  The  greatest  field  of  usefulness  of  non-selective  lines 
is  in  rural  districts  and  in  connection  with  exchanges  in  serving 
rather  sparsely  settled  districts  where  the  cost  of  individual  lines 
or  even  lines  serving  but  a  few  subscribers,  is  prohibitive. 

Non-selective  telephone  party  lines  most  often  employ  mag- 
neto telephones.  The  early  forms  of  party  lines  employed  the  or- 
dinary series  magneto  telephone,  the  bells  being  of  low  resistance  and 
comparatively  low  impedance,  while  the  generators  were  provided 
with  automatic  shunting  devices,  so  that  their  resistance  would 
normally  be  removed  from  the  circuit  of  the  line. 


NON-SELECTIVE  PARTY-LINE  SYSTEMS 


219 


Series  Systems.  The  general  arrangement  of  a  series  party  line 
employing  a  ground  return  is  shown  in  Fig.  163.  In  this  three 
ordinary  series  instruments  are  connected  together  in  series,  the  end 
stations  being  grounded,  in  order  to  afford  a  return  path  for  the  ring- 
ing and  voice  currents. 

In  Fig.  164  there  is  shown  a  metallic-circuit  series  line  on  which 


Fig.  164.     Metallic-Circuit  Series  Line 

five  ordinary  series  telephones  are  placed  in  series.  In  this  no  ground 
is  employed,  the  return  being  through  a  line  wire,  thus  making  the 
circuit  entirely  metallic. 

The  limitations  of  the  ordinary  series  party  line  may.be  best 
understood  by  reference  to  Fig.  165,  in  which  the  circuits  of  three 
series  telephones  are  shown  connected  with  a  single  line.  The  re- 
ceiver of  Station  A  is  represented  as  being  on  its  hook,  while  the  re- 
ceivers of  Stations  B  and  C  are  removed  from  their  hooks,  as  when 
the  subscribers  at  those  two  stations  are  carrying  on  a  conversation. 


Pig.  165.     Series  Party  Line 


The  hook  switches  of  Stations  B  and  C  being  in  raised  positions,  the 
generators  and  ringers  of  those  stations  are  cut  out  of  the  circuit, 
and  only  the  telephone  apparatus  proper  is  included,  but  the  hook 


220  TELEPHONY 

switch  of  Station  A  being  depressed  by  the  weight  of  its  receiver, 
includes  the  ringer  of  that  station  in  circuit,  and  through  this  ringer, 
therefore,  the  voice  currents  of  Stations  B  and  C  must  pass. 

The  generator  of  Station  A  is  not  in  the  circuit  of  voice  cur- 
rents, however,  because  of  the  automatic  shunt  with  which  the  gen- 
erator is  provided,  as  described  in  Chapter  VIII. 

A  slight  consideration  of  the  series  system  as  shown  in  this  fig- 
ure, indicates  that  the  voice  currents  of  any  two  stations  that  are  in 
use,  must  pass  (as  indicated  by  the  heavy  lines)  through  the  ringers 
of  all  the  stations  that  are  not  in  use;  and  when  a  great  number  of 
stations  are  placed  upon  a  single  line,  as  has  been  frequently  the 
case,  the  impedance  offered  by  these  ringers  becomes  a  serious  bar- 
rier to  the  passage  of  the  voice  currents.  This  defect  in  the  series 
party  line  is  fundamental,  as  it  is  obvious  that  the  ringers  must  be 
left  in  the  circuit  of  the  stations  which  are  not  in  use,  in  order  that 
those  stations  may  always  be  in  such  condition  as  to  be  able  to  re- 
ceive a  call. 

This  defect  may  in  some  measure  be  reduced  by  making  the 
ringers  of  low  impedance.  This  is  the  general  practice  with  series 
telephones,  the  ringers  ordinarily  having  short  cores  and  a  compara- 
tively small  number  of  turns,  the  resistance  being  as  a  rule  about 
80  ohms. 

Bridging  Systems.  Very  much  better  than  the  series  plan  of 
party-line  connections,  is  the  arrangement  by  which  the  instruments 
are  placed  in  bridges  across  the  line,  such  lines  being  commonly 
known  as  bridged  or  bridging  lines.  This  was  first  strongly  advo- 
cated and  put  into  wide  practical  use  by  J.  J.  Carty,  now  the  Chief 
Engineer  of  the  American  Telephone  and  Telegraph  Company. 

A  simple  illustration  of  a  bridging  telephone  line  is  shown  in 
Fig.  166,  where  the  three  telephones  shown  are  each  connected  in 
a  bridge  path  from  the  line  wire  to  ground,  a  type  known  as  a 
"grounded  bridging  line."  Its  use  is  very  common  in  rural  dis- 
tricts. 

A  better  arrangement  is  shown  in  Fig.  167,  which  represents  a 
metallic-circuit  bridging  line,  three  telephone  instruments  being  shown 
in  parallel  or  bridge  paths  across  the  two  line  wires. 

The  actual  circuit  arrangements  of  a  bridging  party  line  are  bet- 
ter shown  in  Fig.  168.  There  are  three  stations  and  it  will  be  seen 


NON-SELECTIVE  PARTY-LINE  SYSTEMS 


221 


that  at  each  station  there  are  three  possible  bridges,  or  bridge  paths, 
across  the  two  limbs  of  the  line.     The  first  of  these  bridges  is  con- 


Fig.  166.    Grounded  Bridging  Line 


trolled  by.  the  hook  switch  and  is  normally  open.  When  the  hook 
is  raised,  however,  this  path  is  closed  through  the  receiver  and  sec- 
ondary of  the  induction  coil,  the  primary  circuit  being  also  closed  so 


Fig.  167.     Metallic  Bridging  Line 


as  to  include  the  battery  and  transmitter.     This  constitutes  an  ordi- 
nary local-battery  talking  set. 

A  second  bridge  at  each  station  is  led  through  the  ringer  or  call- 
bell,  and  this,  in  most  bridging  telephones,  is  permanently  closed, 


STAT/OW  -A-  STATION -B-  STAT/ON-C- 

Fig.   168.    Metallic  Bridging  Line 

the  continuity  of  this  path  between  the  two  limbs  of  the  line  not  being 
affected  either  by  the  hook  switch  or  by  the  automatic  switch  in 
connection  with  the  generator. 


222  TELEPHONY 

A  third  bridge  path  at  each  station  is  led  through  the  generator. 
This,  as  indicated,  is  normally  open,  but  the  automatic  cut-in  switch 
of  the  generator  serves,  when  the  generator  is  operated,  to  close  its 
path  across  the  line,  so  that  it  may  send  its  currents  to  the  line  and 
ring  the  bells  of  all  the  stations. 

When  any  generator  is  operated,  its  current  divides  and  passes 
over  the  line  wires  and  through  all  of  the  ringers  in  multiple.  It 
is  seen,  therefore,  that  the  requirements  for  a  bridging  generator 
are  that  it  shall  be  capable  of  generating  a  large  current,  sufficient 
when  divided  up  amongst  all  the  bells  to  ring  each  of  them;  and 
that  it  shall  be  capable  of  producing  a  sufficient  voltage  to  send  the 
required  current  not  only  to  the  near-by  stations,  but  to  the  stations 
at  the  distant  end  of  the  line. 

It  might  seem  at  first  that  the  bridging  system  avoided  one 
difficulty  only  to  encounter  another.  It  clearly  avoids  the  difficulty 
of  the  series  system  fn  that  the  voice  currents,  in  order  to  reach  dis- 
tant stations,  do  not  have  to  pass  through  all  of  the  bells  of  the  idle 
stations  in  series.  There  is,  however,  presented  at  each  station  a 
leakage  path  through  the  bell  bridged  across  the  line,  through  which 
it  would  appear  the  voice  currents  might  leak  uselessly  from  one 
side  of  the  line  to  the  other  and  not  pass  on  in  sufficient  volume 
to  the  distant  station.  This  difficulty  is,  however,  more  apparent 
than  real.  It  is  found  that,  by  making  the  ringers  of  high  impedance, 
the  leakage  of  voice  currents  through  them  from  one  side  of  the  line 
to  the  other  is  practically  negligible. 

It  is  obvious  that  in  a  heavily  loaded  bridged  line,  the  bell  at 
the  home  station,  that  is  at  the  station  from  which  the  call  is  being 
sent,  will  take  slightly  more  than  its  share  of  the  current,  and  it  is 
also  obvious  that  the  ringing  of  the  home  bell  performs  no  useful 
function.  The  plan  is  frequently  adopted,  therefore,  of  having 
the  operation  of  the  generator  serve  to  cut  its  own  bell  out  of  the 
circuit.  The  arrangement  by  which  this  is  done  is  clearly  shown 
in  Fig.  169.  The  circuit  of  the  bell  is  normally  complete  across 
the  line,  while  the  circuit  of  the  generator  is  normally  open.  When, 
however,  the  generator  crank  is  turned  these  conditions  are  reversed, 
the  bell  circuit  being  broken  and  the  generator  circuit  closed,  so  as 
to  allow  its  current  all  to  pass  the  line.  This  feature  of  having  the 
local  bell  remain  silent  upon  the  operation  of  its  own  generator  is 


NON-SELECTIVE  PARTY-LINE  SYSTEMS 


223 


Fig.  169.     Circuits  of  Bridging  Station 


also  of  advantage  because  other  parties  at  the  same  station  are  not 
disturbed  by  the  ringing  of  the  bell  when  a  call  is  being  made  by 
that  station. 

A  difficulty  encountered  on  non-selective  bridging  party  lines, 
which  at  first  seems  amusing  rather  than  serious,  but  which  never- 
theless is  often  a  vexatious  trouble,  is  that  due  to  the  propensity  of 
some  people  to  "listen  in"  on  the 
line  on  hearing  calls  intended 
for  other  than  their  own  stations. 
People  whose  ethical  standards 
would  not  permit  them  to  listen 
at,  or  peep  through,  a  keyhole, 
often  engage  in  this  telephonic 
eavesdropping. 

Frequently,  not  only  one  but 
many  subscribers  will  respond 
to  a  call  intended  for  others  and  will  listen  to  the  ensuing  conver- 
sation. This  is  disadvantageous  in  several  respects:  It  destroys 
the  privacy  of  conversation  between  any  two  parties;  it  subjects 
the  local  batteries  to  an  unnecessary  and  useless  drain;  and  it  greatly 
impairs  the  ringing  efficiency  of  the  line.  The  reason  for  this  inter- 
ference with  ringing  is  that  the  presence  of  the  low-resistance  re- 
ceivers across  the  line  allows  the  current  sent  out  by  any  of  the 
generators  to  pass  in  large  measure  through  the  receivers,  thus  de- 
priving the  ringers,  which  are  of  comparatively  high  resistance  and 
impedance,  of  the  energy  necessary  to  operate  them.  As  a  result 
of  this  it  is  frequently  impossible  for  one  party  to  repeat  the  call 
for  another  because,  during  the  interval  between  the  first  and  second 
call,  a  number  of  parties  remove  their  receivers  from  their  hooks  in 
order  to  listen.  Ring-off  or  clearing-out  signals  are  likewise  inter- 
fered with. 

A  partial  remedy  for  this  interference  with  ringing,  due  to 
eavesdropping,  is  to  introduce  a  low-capacity  condenser  into  the 
receiver  circuit  at  each  station,  as  shown  in  Fig.  169.  This  does 
not  seriously  interfere  with  the  speech  transmission  since  the  con- 
densers will  readily  transmit  the  high-frequency  voice  currents. 
Such  condensers,  however,  have  not  sufficient  capacity  to  enable 
them  readily  to  transmit  the  low-frequency  ringing  currents  and 


224  TELEPHONY 

hence  these  are  forced,  in  large  measure,  to  pass  through  the  bells 
for  which  they  are  intended  rather  than  leaking  through  the  low- 
resistance  receiver  paths. 

The  best  condenser  for  this  use  is  of  about  ^-microfarad  capacity, 
which  is  ample  for  voice-transmitting  purposes,  while  it  serves  to 
effectively  bar  the  major  portion  of  the  generator  currents.  A 
higher  capacity  condenser  would  carry  the  generator  currents  much 
more  readily  and  thus  defeat  the  purpose  for  which  it  was  intended. 

In  order  that  the  requisite  impedance  may  be  given  to  the 
ringers  employed  for  bridging  party  lines,  it  is  customary  to  make 
the  cores  rather  long  and  of  somewhat  larger  diameter  than  in  series 
ringers  and  at  the  same  time  to  wind  the  coils  with  rather  fine  wire 
so  as  to  secure  the  requisite  number  of  turns.  Bridging  bells  are 
ordinarily  wound  to  a  resistance  of  1,000  or  1,600  ohms,  these  two 
figures  having  become  standard  practice.  It  is  not,  however,  the 
high  resistance  so  much  as  the  high  impedance  that  is  striven  for 
in  bridging  bells;  it  is  the  number  of  turns  that  is  of  principal  im- 
portance. 

As  has  already  been  stated,  the  generators  used  for  bridging 
lines  are  made  capable  of  giving  a  greater  current  output  than  is 
necessary  in  series  instruments,  and  for  this  purpose  they  are  usually 
provided  with  at  least  four,  and  usually  five,  bar  magnets.  The 
armature  is  made  correspondingly  long  and  is  wound,  as  a  rule,  with 
about  No.  33  wire. 

Sometimes  where  a  bridged  party  line  terminates  in  a  central- 
office  switchboard  it  is  desired  to  so  operate  the  line  that  the  sub- 
scribers shall  not  be  able  to  call  up  each  other,  but  shall,  instead, 
be  able  to  signal  only  the  central-office  operator,  who,  in  turn,  will 
be  enabled  to  call  the  party  desired,  designating  his  station  by  a 
suitable  code  ring.  One  common  way  to  do  this  is  to  use  biased  bells 
instead  of  the  ordinary  polarized  bells.  In  order  that  the  bells 
may  not  be  rung  by  the  subscribers'  generators,  these  generators 
are  made  of  the  direct-current  type  and  these  are  so  associated  with 
the  line  that  the  currents  which  they  send  out  will  be  in  the  wrong 
direction  to  actuate  the  bells.  On  the  other  hand,  the  central-office 
generator  is  of  direct-current  type  and  is  associated  with  the  line 
in  the  right  direction  to  energize  the  bells.  Thus  any  subscriber 
on  the  line  may  call  the  central  office  by  merely  turning  his  generator 


NON-SELECTIVE  PARTY-LINE  SYSTEMS 


225 


crank,  which  action  will  not  ring  the  bells  of  the  subscribers  on  the 
line.  The  operator  will  then  be  able  to  receive  the  call  and  in  turn 
send  out  currents  of  the  proper  direction  to  ring  all  the  bells  and, 
by  code,  call  the  desired  party  to  the  telephone. 

Signal  Code.  The  code  by  which  stations  are  designated  on 
non-selective  party  lines  usually  consists  in  combinations  of  long 
and  short  rings  similar  to  the  dots  and  dashes  in  the  Morse  code. 
Thus,  one  short  ring  may  indicate  Station  No.  1;  two  short  rings 
Station  No.  2;  and  so  on  up  to,  say,  five  short  rings,  indicating  Station 
No.  5.  It  is  not  good  practice  to  employ  more  than  five  successive 
short  rings  because  of  the  confusion  which  often  arises  in  people's 
minds  as  to  the  number  of  rings  that  they  hear.  When,  therefore, 
the  number  of  stations  to  be  rung  by  code  exceeds  five,  it  is  better 
to  employ  combinations  of  long  and  short  rings,  and  a  good  way 
is  to  adopt  a  partial  decimal  system,  omitting  the  numbers  higher 
than  five  in  each  ten,  and  employing  long  rings  to  indicate  the  tens 
digits  and  short  rings  to  indicate  the  units  digit,  Table  X. 

TABLE  X 
Signal  Code 


STATION  NUMBER 

RlNO 

STATION  NUMBER 

RlNO 

1 

1   short 

12 

1  long,  2   short 

2 

2  short 

13 

1  long,  3  short 

3 

3  short 

14 

1  long,  4  short 

4 

4  short 

15 

1  long,  5  short 

5 

5  short 

21 

2  long,  1  short 

11 

1  long,  1  short 

22 

2  long,  2  short 

Other  arrangements  are  often  employed  and  by  almost  any  of 
them  a  great  variety  of  readily  distinguishable  signals  may  be  secured. 
The  patrons  of  such  lines  learn  to  distinguish,  with  comparatively 
few  errors,  between  the  calls  intended  for  them  and  those  intended 
for  others,  but  frequently  they  do  not  observe  the  distinction,  as  has 
already  been  pointed  out. 

Limitations.  With  good  telephones  the  limit  as  to  the  number 
of  stations  that  it  is  possible  to  operate  upon  a  single  line  is  usually 
due  more  to  limitations  in  ringing  than  in  talking.  As  the  number 
of  stations  is  increased  indefinitely  a  condition  will  be  reached  a» 


226  TELEPHONY 

which  the  generators  will  not  be  able  to  generate  sufficient  current 
to  ring  all  of  the  bells,  and  this  condition  is  likely  to  occur  before 
the  talking  efficiency  is  seriously  impaired  by  the  number  of  bridges 
across  the  line. 

Neither  of  these  considerations,  however,  should  determine 
the  maximum  number  of  stations  to  be  placed  on  a  line.  The 
proper  limit  as  to  the  number  of  stations  is  not  the  number  that 
can  be  rung  by  a  single  generator,  or  the  number  with  which  it  is 
possible  to  transmit  speech  properly,  but  rather  the  number  of  sta- 
tions that  may  be  employed  without  causing  undue  interference 
between  the  various  parties  who  may  desire  to  use  the  line.  Over- 
loaded party  lines  cause  much  annoyance,  not  only  for  the  reason 
that  the  subscribers  are  often  not  able  to  use  the  line  when  they 
want  it,  but  also,  in  non-selective  lines,  because  of  the  incessant 
ringing  of  the  bells,  and  tl  e  liability  of  confusion  in  the  interpretation 
of  the  signaling  code,  which  of  course  becomes  more  complex  as  the 
number  of  stations  increases. 

The  amount  of  business  that  is  done  over  a  telephone  line  is 
usually  referred  to  as  the  "traffic."  It  will  be  understood,  however, 
in  considering  party-line  working  that  the  number  of  calls  per  day 
or  per  hour,  or  per  shorter  unit,  is  not  the  true  measure  of  the  traffic 
and,  therefore,  not  the  true  measure  of  the  amount  of  possible  inter- 
ference between  the  various  subscribers  on  the  line. 

An  almost  equally  great  factor  is  the  average  length  of  the 
conversation.  In  city  lines,  that  is,  in  lines  in  city  exchanges,  the 
conversation  is  usually  short  and  averages  perhaps  two  minutes 
in  duration.  In  country  lines,  however,  serving  people  in  rural 
districts,  who  have  poor  facilities  for  seeing  each  other,  particularly 
during  the  winter  time,  the  conversations  will  average  very  much 
longer.  In  rural  communities  the  people  often  do  much  of  their 
visiting  by  telephone,  and  conversations  of  half  an  hour  in  length 
are  not  unusual.  It  is  obvious  that  under  such  conditions  a  party 
line  having  a  great  many  stations  will  be  subject  to  very  grave 
interference  between  the  parties,  people  desiring  to  use  the  line  for 
business  purposes  often  being  compelled  to  wait  an  undue  time  be- 
fore they  may  secure  the  use  of  the  line. 

It  is  obvious,  therefore,  that  the  amount  of  traffic  on  the  line, 
whether  due  to  many  short  conversations  or  to  a  comparatively  few 


NON-SELECTIVE  PARTY-LINE  SYSTEMS  227 

long  ones,  is  the  main  factor  that  should  determine  the  number  of 
stations  that,  economically,  may  be  placed  on  a  line.  The  facilities 
also  for  building  lines  enter  as  a  factor  in  this  respect,  since  it  is 
obvious  that  in  comparatively  poor  communities  the  money  may 
not  be  forthcoming  to  build  as  many  lines  as  are  needed  to  properly 
take  care  of  the  traffic.  A  compromise  is,  therefore,  often  neces- 
sary, and  the  only  rule  that  may  be  safely  laid  down  is  to  place  as 
few  parties  on  a  given  line  as  conditions  will  admit. 

No  definite  limit  may  be  set  to  apply  to  all  conditions  but  it 
may  be  safely  stated  that  under  ordinary  circumstances  no  more  than 
ten  stations  should  be  placed  on  a  non-selective  line.  Twenty 
stations  are,  however,  common,  and  sometimes  forty  and  even  fifty 
have  been  connected  to  a  single  line.  In  such  cases  the  confusion 
which  results,  even  if  the  talking  and  the  ringing  efficiency  are  toler- 
able, makes  the  service  over  such  overloaded  lines  unsatisfactory  to 
all  concerned. 


CHAPTER  XVI 
SELECTIVE  PARTY=LINE  SYSTEMS 

The  problem  which  confronts  one  in  the  production  of  a  system 
of  selective  ringing  on  party  lines  is  that  of  causing  the  bell  of  any 
chosen  one  of  the  several  parties  on  a  circuit  to  respond  to  a  signal 
sent  out  from  the  central  office  without  sounding  any  of  the  other 
bells.  This,  of  course,  must  be  accomplished  without  interfering 
with  the  regular  functions  of  the  telephone  line  and  apparatus. 
By  this  is  meant  that  the  subscribers  must  be  able  to  call  the  cen- 
tral office  and  to  signal  for  disconnection  when  desired,  and  also  that 
the  association  of  the  selective-signaling  devices  with  the  line  shall 
not  interfere  with  the  transmission  of  speech  over  the  line.  A  great 
many  ways  of  accomplishing  selective  ringing  on  party  lines  have 
been  proposed,  and  a  large  number  of  them  have  been  used.  All 
of  these  ways  may  be  classified  under  four  different  classes  accord- 
ing to  the  underlying  principle  involved. 

Classification.  (1)  Polarity  systems  are  so  called  because  they 
depend  for  their  operation  on  the  use  of  bells  or  other  responsive 
devices  so  polarized  that  they  will  respond  to  one  direction  of  cur- 
rent only.  These  bells  or  other  devices  are  so  arranged  in  connec- 
tion with  the  line  that  the  one  to  be  rung  will  be  traversed  by  current 
in  the  proper  direction  to  actuate  it,  while  all  of  the  others  will  either 
not  be  traversed  by  any  current  at  all,  or  by  current  in  the  wrong 
direction  to  cause  their  operation. 

(#)  The  harmonic  systems  have  for  their  underlying  principle 
the  fact  that  a  pendulum  or  elastic  reed,  so  supported  as  to  be 
capable  of  vibrating  freely,  will  have  one  particular  rate  of  vibration 
which  it  may  easily  be  made  to  assume.  This  pendulum  or  reed 
is  placed  under  the  influence  of  an  electromagnet  associated  with  the 
line,  and  owing  to  the  fact  that  it  will  vibrate  easily  at  one  particular 
rate  of  vibration  and  with  extreme  difficulty  at  any  other  rate,  it  is 
clear  that  for  current  impulses  of  a  frequency  corresponding  to  its 


SELECTIVE  PARTY-LINE  SYSTEMS  229 

natural  rate  the  reed  will  take  up  the  vibration,  while  for  other  fre- 
quencies it  will  fail  to  respond. 

Selection  on  party  lines  by  means  of  this  system  is  provided  for 
by  tuning  all  of  the  reeds  on  the  line  at  different  rates  of  vibration 
and  is  accomplished  by  sending  out  on  the  line  ringing  currents  of 
proper  frequency  to  ring  the  desired  bell.  The  current-generating 
devices  for  ringing  these  bells  are  capable  of  sending  out  different 
frequencies  corresponding  respectively  to  the  rates  of  vibration  of 
each  of  the  vibrating  reed  tongues.  To  select  any  one  station, 
therefore,  the  current  frequency  corresponding  to  the  rate  of  vibra- 
tion of  the  reed  tongue  at  that  station  is  sent  and  this,  being  out  of 
tune  with  the  reed  tongues  at  all  of  the  other  stations,  operates  the 
tongue  of  the  desired  station,  but  fails  to  operate  those  at  all  of  the 
other  stations. 

(3)  In  the  step-by-step  system  the  bells  on  the  line  are  normally 
not  in  operative  relation  with  the  line  and  the  bell  of  the  desired  party 
on  the  line  is  made  responsive  by  sending  over  the  line  a  certain 
number  of  impulses  preliminary  to  ringing  it.  These  impulses 
move  step-by-step  mechanisms  at  each  of  the  stations  in  unison,  the 
arrangement  being  such  that  the  bells  at  the  several  stations  are 
each  made  operative  after  the  sending  of  a  certain  number  of  pre- 
liminary impulses,  this  number  being  different  for  all  the  stations. 

(4}  The  broken-line  systems  are  new  in  telephony  and  for 
certain  fields  of  work  look  promising.  In  these  the  line  circuit  is 
normally  broken  up  into  sections,  the  first  section  terminating  at  the 
first  station  out  from  the  central  office,  the  second  section  at  the  second 
station,  and  so  on.  When  the  line  is  in  its  normal  or  inactive  condi- 
tion only  the  bell  at  the  first  station  is  so  connected  with  the  line 
circuit  as  to  enable  it  to  be  rung,  the  line  being  open  beyond.  Send- 
ing a  single  preliminary  impulse  will,  however,  operate  a  switching 
device  so  as  to  disconnect  the  bell  at  the  first  station  and  to  connect 
the  line  through  to  the  second  station.  This  may  be  carried  out, 
by  sending  the  proper  number  of  preliminary  impulses,  so  as  to 
build  up  the  line  circuit  to  the  desired  station,  after  which  the  send- 
ing of  the  ringing  current  will  cause  the  bell  to  ring  at  that  station 
only. 

Polarity  Method.  The  polarity  method  of  selective  signaling 
on  party  lines  is  probably  the  most  extensively  used.  The  standard 


230 


TELEPHONY 


selective  system  of  the  American  Telephone  and  Telegraph  Com- 
pany operates  on  this  principle. 

Two-Party  Line.  It  is  obvious  that  selection  may  be  had  be- 
tween two  parties  on  a  single  metallic-circuit  line  without  the  use  of 
biased  bells  or  current  of  different  polarities.  Thus,  one  limb  of  a 
metallic  circuit  may  be  used  as  one  grounded  line  to  ring  the  bell  at 
one  of  the  stations,  and  the  other  limb  of  the  metallic  circuit  may 
be  used  as  another  grounded  line  to  ring  the  bell  of  the  other  station ; 
and  the  two  limbs  may  be  used  together  as  a  metallic  circuit  for 
talking  purposes  as  usual. 

This  is  shown  in  Fig.  170,  where  the  ringing  keys  at  the  cen- 
tral office  are  diagrammatically  shown  in  the  left-hand  portion  of 
the  figure  as  K1  and  K2.  The  operation  of  these  keys  will  be  more 


G/ 


""•U^"-  H 
±       Gz     * 

5 

•  ..v  TV 

fJ 

/—  * 

.  r 

* 

, 

j 

.                       . 

^  ^T\_rv*^_                                   ^ 

Fig.  170.     Simple  Two-Party  Line  Selection 


fully  pointed  out  in  a  subsequent  chapter,  but  a  correct  understand- 
ing will  be  had  if  it  be  remembered  that  the  circuits  are  normally 
maintained  by  these  keys  in  the  position  showrn.  When,  however, 
either  one  of  the  keys  is  operated,  the  two  long  springs  may  be  con- 
sidered as  pressed  apart  so  as  to  disengage  the  normal  contacts  be- 
tween the  springs  and  to  engage  the  two  outer  contacts,  with  which 
they  are  shown  in  the  cut  to  be  disengaged.  The  two  outer  con- 
tacts are  connected  respectively  to  an  ordinary  alternating-current 
ringing  generator  and  to  ground,  but  the  connection  is  reversed  on 
the  two  keys. 

At  Station  A  the  ordinary  talking  set  is  shown  in  simplified 
form,  consisting  merely  of  a  receiver,  transmitter,  and  hook  switch 
in  a  single  bridge  circuit  across  the  line.  An  ordinary  polarized  bell 
is  shown  connected  in  series  with  a  condenser  between  the  lower 
limb  <jl  «,bc  Mne  and  ground.  At  Station  B  the  same  talking  circuit 


SELECTIVE  PARTY-LINE  SYSTEMS  231 

is  shown,  but  the  polarized  bell  and  condenser  are  bridged  between 
the  upper  limb  of  the  line  and  ground. 

If  the  operator  desires  to  call  Station  A,  she  will  press  key  K1 
which  will  ground  the  upper  side  of  the  line  and  connect  the  lower 
side  of  the  line  with  the  generator  Gl,  and  this,  obviously,  will  cause  the 
bell  at  Station  A  to  ring.  The  bell  at  Station  B  will  not  ring  because 
it  is  not  in  the  circuit.  If,  on  the  other  hand,  the  operator  desires 
to  ring  the  bell  at  Station  B,  she  will  depress  key  K2,  which  will  al- 
low the  current  from  generator  G2  to  pass  over  the  upper  side  of  the 
line  through  the  bell  and  condenser  at  Station  B  and  return  by  the 
path  through  the  ground.  The  object  of  grounding  the  opposite 
sides  of  the  keys  at  the  central  office  is  to  prevent  cross-ringing,  that 
is,  ringing  the  wrong  bell.  Were  the  keys  not  grounded  this  might 
occur  when  a  ringing  current  was  being  sent  out  while  the  receiver 
at  one  of  the  stations  was  off  its  hook;  the  ringing  current  from,  say, 
generator  Gl  then  passing  not  only  through  the  bell  at  Station  A  as 
intended,  but  also  through  the  bell  at  Station  B  by  way  of  the  bridge 
path  through  the  receiver  that  happened  to  be  connected  across  the 
line.  With  the  ringing  keys  grounded  as  shown,  it  is  obvious  that 
this  will  not  occur,  since  the  path  for  the  ringing  current  through  the 
wrong  bell  will  always  be  shunted  by  a  direct  path  to  ground  on  the 
same  side  of  the  line. 

In  such  a  two-party-line  selective  system  the  two  generators 
G1  and  G2  may  be  the  same  generator  and  may  be  of  the  ordinary 
alternating-current  type.  The  bells  likewise  may  be  of  the  ordi- 
nary alternating-current  type. 

The  two-party  selective  line  just  described  virtually  employs 
two  separate  circuits  for  ringing.  Now  each  of  these  circuits  alone 
may  be  employed  to  accomplish  selective  ringing  between  two  sta- 
tions by  using  two  biased  bells  oppositely  polarized,  and  employing 
pulsating  ringing  currents  of  one  direction  or  the  other  according 
to  which  bell  it  is  desired  to  ring.  One  side  of  a  circuit  so  equipped 
is  shown  in  Fig.  171.  In  this  the  two  biased  bells  are  at  Station  A 
and  Station  B,  these  being  bridged  to  ground  in  each  case  and  adapted 
to  respond  only  to  positive  and  negative  impulses  respectively.  At 
the  central  office  the  two  keys  K1  and  K2  are  shown.  A  single  alter- 
nating-current generator  G  is  shown,  having  its  brush  1  grounded 
arid  brush  2  connected  to  a  commutator  disk  3  mounted  on  the 


232  TELEPHONY 

generator  shaft  so  as  to  revolve  therewith.  One-half  of  the  periph- 
ery of  this  disk  is  of  insulating  material  so  that  the  brushes  4  and  5, 
which  bear  against  the  disk,  will  be  alternately  connected  with  the 
disk  and,  therefore,  with  the  brush  2  of  the  generator.  Now  the 
brush  2,  being  one  terminal  of  an  alternating-current  machine,  is 
alternately  positive  and  negative,  and  the  arrangement  of  the  com- 
mutator is  such  that  the  disk,  which  is  always  at  the  potential  of  the 
brush  2,  will  be  connected  to  the  brush  5  only  while  it  is  positively 
charged  and  with  the  brush  4  onty  while  it  is  negatively  charged. 
As  a  result,  brush  5  has  a  succession  of  positive  impulses  and  brush 
4  a  succession  of  negative  ones.  Obviously,  therefore,  when  key 


Fig.  171.     Principle  of  Selection  by  Polarity 

K1  is  depressed  only  the  bell  at  Station  A  will  be  rung,  and  likewise 
the  depression  of  key  K2  will  result  only  in  the  ringing  of  the  bell 
at  Station  B. 

Four-Party  Line.  From  the  two  foregoing  two-party  line  sys- 
tems it  is  evident  that  a  four-party  line  system  may  be  readily  ob- 
tained, that  is,  by  employing  two  oppositely  polarized  biased  bells 
on  each  side  of  the  metallic  circuit.  The  selection  of  any  of  the  four 
bells  may  be  obtained,  choosing  between  the  pairs  connected,  re- 
spectively, with  the  two  limbs  of  the  line,  by  choosing  the  limb  on 
which  the  current  is  to  be  sent,  and  choosing  between  the  two  bells 
of  the  pair  on  that  side  of  the  line  by  choosing  which  polarity  of 
current  to  send. 

Such  a  four-party  line  system  is  shown  in  Fig.  172.  In  this 
the  generators  are  not  shown,  but  the  wires  leading  from  the  four 
keys  are  shown  marked  plus  or  minus,  according  to  the  terminal  of 
the  generator  to  which  they  are  supposed  to  be  connected.  Like- 
wise the  two  bells  connected  with  the  lower  side  of  the  line  are  marked 
positive  and  negative,  as  are  the  two  bells  connected  with  the  upper 


SELECTIVE  PARTY-LINE  SYSTEMS 


233 


side  of  the  line.  From  the  foregoing  description  of  Figs.  170  and 
171,  it  is  clear  that  if  key  K1  is  pressed  the  bell  at  Station  A  will  be 
rung,  and  that  bell  only,  since  the  bells  at  Station  C  and  Station  D 
are  not  in  the  circuit  and  the  positive  current  sent  over  the  lower 
side  of  the  line  is  not  of  the  proper  polarity  to  ring  the  bell  at  Sta- 
tion B. 

The  system  shown  in  Fig.  172  is  subject  to  one  rather  grave 
defect.  In  subsequent  chapters  it  will  be  pointed  out  that  in  com- 
mon-battery systems  the  display  of  the  line  signal  at  the  central  office 
is  affected  by  any  one  of  the  subscribers  merely  taking  his  receiver 
off  its  hook  and  thus  establishing  a  connection  between  the  two 


fL  fL  *J 


Fig.  172.     Four-Party  Polarity  Selection 

limbs  of  the  metallic  circuit.  Such  common-battery  systems  should 
have  the  two  limbs  of  the  line,  normally,  entirely  insulated  from 
each  other.  It  is  seen  that  this  is  not  the  case  in  the  system  just  de- 
scribed, since  there  is  a  conducting  path  from  one  limb  t>f  the  line 
through  the  two  bells  on  that  side  to  ground,  and  thence  through 
the  other  pair  of  bells  to  the  other  limb  of  the  line.  This  means 
that  unless  the  resistance  of  the  bell  windings  is  made  very  high, 
the  path  of  the  signaling  circuit  will  be  of  sufficiently  low  re- 
sistance to  actuate  the  line  signal  at  the  central  office. 

It  is  not  feasible  to  overcome  this  objection  by  the  use  of  con- 
densers in  series  with  the  bells,  as  was  done  in  the  system  shown  in 
Fig.  170,  since  the  bells  are  necessarily  biased  and  such  bells,  as 
may  readily  be  seen,  will  not  work  properly  through  condensers, 
since  the  placing  of  a  condenser  in  their  circuit  means  that  the 
current  which  passes  through  the  bell  is  alternating  rather  than 
pulsating,  although  the  original  source  may  have  been  of  pulsating 
nature  only. 


234 


TELEPHONY 


The  remedy  for  this  difficulty,  therefore,  has  been  to  place  in 
series  with  each  bell  a  very  high  non-inductive  resistance  of  about 
15,000  or  20,000  ohms,  and  also  to  make  the  windings  of  the  bells 
of  comparatively  high  resistance,  usually  about  2,500  ohms.  Even 
with  this  precaution  there  is  a  considerable  leakage  of  the  central- 
office  battery  current  from  one  side  of  the  line  to  the  other  through 
the  two  paths  to  ground  in  series.  This  method  of  selective  signal- 


Fig    173.     Standard  Polarity  System 

ing  has,  therefore,  been  more  frequently  used  with  magneto  systems. 
An  endeavor  to  apply  this  principle  to  common-battery  systems 
without  the  objections  noted  above  has  led  to  the  adoption  of  a 
modification,  wherein  a  relay  at  each  station  normally  holds  the 
ground  connection  open.  This  is  shown  in  Fig.  173  and  is  the 
standard  four-party  line  ringing  circuit  employed  by  the  American 
Telephone  and  Telegraph  Company  and  their  licensees 

In  this  system  the  biased  bells  are  normally  disconnected  from 
the  line,  and,  therefore,  the  leakage  path  through  them  from  one 
side  of  the*  line  to  the  other  does  not  exist.  At  each  station  there  is 
a  relay  winding  adapted  to  be  operated  by  the  ringing  current  bridged 
across  the  line  in  series  with  a  condenser.  As  a  result,  when  ringing 
current  is  sent  out  on  the  line  all  of  the  relays,  i.  e.,  one  at  each  sta- 
tion, are  energized  and  attract  their  armatures.  This  establishes  the 
connection  of  all  the  bells  to  line  and  really  brings  about  temporarily 
a  condition  equivalent  to  that  of  Fig.  172.  As  a  result,  the  sending 
of  a  positive  current  on  the  lower  line  with  a  ground  return  will  cause 
the  operation  of  the  bell  at  Station  A.  It  will  not  ring  the  bell  at 
Station  B  because  of  the  wrong  polarity.  It  will  not  ring  the  bells 
of  Station  C  and  Station  D  because  they  are  in  the  circuit  between 
the  other  side  of  the  line  and  ground.  As  soon  as  the  ringing  cur- 
rent ceases  all  of  the  relays  release  their  armatures  and  disconnect 
all  the  bells  from  the  line. 


SELECTIVE  PARTY-LINE  SYSTEMS 


235 


By  this  very  simple  device  the  trouble,  due  to  marginal  work- 
ing of  the  line  signal,  is  done  away  with,  since  normally  there  is  no 
leakage  from  one  side  of  the  line  to  the  other  on  account  of  the  pres- 
ence of  the  condensers  in  the  bridge  at  each  station. 

In  Fig.  174,  the  more  complete  connections  of  the  central-office 
ringing  keys  are  shown,  by  means  of  which  the  proper  positive  or 
negative  ringing  currents  are  sent  to  line  in  the  proper  way  to  cause 
the  ringing  of  any  one  of  the  four  bells  on  a  party  line  of  either  of 
the  types  shown  in  Figs.  172  and  173. 

In  this  the  generator  G  and  its  commutator  disk  3,  with  the 
various  brushes,  1,  2,  4,  and  5,  are  arranged  in  the  same  manner 
as  is  shown  in  Fig.  171.  It  is  evident  from  what  has  been  said 
that  wire  6  leading  from  generator  brush  2  and  commutator  disk 
3  will  carry  alternating  potential;  that  wire  7  will  carry  positive 
pulsations  of  potential;  and  that  wire  8  will  carry  negative  pulsa- 
tions of  potential.  There  are  five  keys  in  the  set  illustrated  in 
Fig.  174,  of  which  four,  viz,  K1,  K2,  Ks,  and  K4,  are  connected  in 
the  same  manner  as  diagrammatically  indicated  in  Figs.  172  and 
173,  and  will,  obviously,  serve  to  send  the  proper  current  over  the 
proper  limb  of  the  line  to  ring  one  of  the  bells.  Key  K5,  the  fifth 
one  in  the  set,  is  added 
so  as  to  enable  the  op- 
erator to  ring  an  ordi- 
nary unbiased  bell  on 
a  single  party  line  when 
connection  is  made  with 
such  line.  As  the  two 
outside  contacts  of  this 
key  are  connected  re- 
spectively to  the  two 
brushes  of  the  alterna- 
ting-current dynamo  G,  it  is  clear  that  it  will  impress  an  alternating 
current  on  the  line  when  its  contacts  are  closed. 

Circuits  of  Two-Party  Line  Telephones.  In  Fig.  175  is  shown 
in  detail  the  wiring  of  the  telephone  set  usually  employed  in  con- 
nection with  the  party-line  selective-ringing  system  illustrated  in 
Fig.  170.  In  the  wiring  of  this  set  and  the  two  following,  it  must 
be  borne  in  mind  that  the  portion  of  the  circuit  used  during  con- 


To  Line 


Fig.  174.     Ringing-Key  Arrangement 


236 


TELEPHONY 


versation  might  be  wired  in  a  number  of  ways  without  affecting  the 
principle  of  selective  ringing  employed;  however,  the  circuits  shown 
are  those  most  commonly  employed  with  the  respective  selective 
ringing  systems  which  they  are  intended  to  illustrate.  In  con- 
necting the  circuits  of  this  telephone  instrument  to  the  line,  the  two 
line  conductors  are  connected  to  binding  posts  1  and  2  and  a  ground 
connection  is  made  to  binding  post  3.  In  practice,  in  order  to 
avoid  the  necessity  of  changing  the  permanent  wiring  of  the  tele- 
phone set  in  connecting  it  as  an  A  or  B  Station  (Fig.  170),  the  line 
conductors  are  connected  to  the  binding  posts  in  reverse  order  at 
the  two  stations;  that  is,  for  Station  A  the  upper  conductor,  Fig. 
170,  is  connected  to  binding  post  1  and  the  lower  conductor  to 

binding  post  2,  while  at  Station 
B  the  upper  conductor  is  con- 
nected to  binding  post  2  and  the 
lower  conductor  to  binding  post 
1.  The  permanent  wiring  of 
this  telephone  set  is  the  same 
as  that  frequently  used  for  a 
set  connected  to  a  line  having 
only  one  station,  the  proper 
ringing  circuit  being  made  by 
the  method  of  connecting  up  the 
binding  posts.  For  example,  if 
this  telephone  set  were  to  be 
used  on  a  single  station  line, 
the  binding  posts  1  and  2  would 
be  connected  to  the  two  conduc- 
tors of  the  line  as  before,  while  binding  post  3  would  be  connected 
to  post  1  instead  of  being  grounded. 

Circuits  of  Four-P arty-Line  Telephones.  The  wiring  of  the 
telephone  set  used  with  the  system  illustrated  in  Fig  172  is  shown 
in  detail  in  Fig.  176.  The  wiring  of  this  set  is  arranged  for  local 
battery  or  magneto  working,  as  this  method  of  selective  ringing  is 
more  frequently  employed  with  magneto  systems,  on  account  of 
the  objectionable  features  which  arise  when  applied  to  common- 
battery  systems.  In  this  figure  the  line  conductors  are  connected 
to  binding  posts  1  and  2,  and  a  ground  connection  is  made  to  bind- 


Fig.  175.     Circuit  of  Two-Party  Station 


SELECTIVE  PARTY-LINE  SYSTEMS 


237 


Fig.  176. 


ing  post  3.  In  order  that  all  sets  may  be  wired  alike  and  yet  permit 
the  instrument  to  be  connected  for  any  one  of  the  various  stations, 
the  bell  is  not  permanently  wired 
to  any  portion  of  the  circuit  but 
has  flexible  connections  which 
will  allow  of  the  set  being  prop- 
erly connected  for  any  desired 
station.  The  terminals  of  the 
bell  are  connected  to  binding 
posts  9  and  10,  to  which  are  con- 
nected flexible  conductors  ter- 
minating in  terminals  7  and  8. 
These  terminals  may  be  con- 
nected to  the  binding  posts  4,  5, 
and  6  in  the  proper  manner  to 
connect  the  set  as  an  A,  B,  C,  or 
D  station,  as  required.  For  ex- 
ample, in  connecting  the  set  for 
Station  A,  Fig.  172,  terminal  7 
is  connected  to  binding  post  6 
and  8  to  5.  For  connecting  the 
set  for  Station  B  terminal  7  is 
connected  to  binding  post  5  and 
8  to  6.  For  connecting  the  set 
for  Station  C  terminal  7  is  con- 
nected to  binding  post  6  and  8  to 
4-  For  connecting  the  set  for 
Station  D  terminal  7  is  connected 
to  binding  post  4  and  8  to  6. 

The  detailed  wiring  of  the 
telephone  set  employed  in  con- 
nection with  the  system  illustrated 
in  Fig  173  is  shown  in  Fig.  177. 
The  wiring  of  this  set  is  ar- 
ranged for  a  common-battery  Fig.  177.  Circuit  of  Four-Party  Station 
,  .,  .  with  Relay 

system,    inasmuch    as     this    ar- 
rangement of  signaling  circuit  is  more  especially  adapted  for  com- 
mon-battery working.      However,    this    arrangement   is   frequently 


Circuit  of  Four-Party  Station 
without  Relay 


238  TELEPHONY 

adapted  to  magneto  systems  as  even  with  magneto  systems  a  per- 
manent ground  connection  at  a  subscriber's  station  is  objection- 
able inasmuch  as  it  increases  the  difficulty  of  determining  the  ex- 
istence or  location  of  an  accidental  ground  on  one  of  the  line  con- 
ductors. The  wiring  of  this  set  is  also  arranged  so  that  one  stand- 
ard type  of  wiring  may  be  employed  and  yet  allow  any  telephone 
set  to  be  connected  as  an  A,  B,  C,  or  D  station. 

Harmonic  Method.  Principles.  To  best  understand  the  prin- 
ciple of  operation  of  the  harmonic  party-line  signaling  systems,  it 
is  to  be  remembered  that  a  flexible  reed,  mounted  rigidly  at  one  end 
and  having  its  other  end  free  to  vibrate,  will,  like  a  violin  string, 
have  a  certain  natural  period  of  vibration;  that  is,  if  it  be  started  in 
vibration,  as  by  snapping  it  with  the  fingers,  it  will  take  up  a  cer- 
tain rate  of  vibration  which  will  continue  at  a  uniform  rate  until 
the  vibration  ceases  altogether.  Such  a  reed  will  be  most  easily 
thrown  into  vibration  by  a  series  of  impulses  having  a  frequency 
corresponding  exactly  to  the  natural  rate  of  vibration  of  the  reed 
itself;  it  may  be  thrown  into  vibration  by  very  slight  impulses  if 
they  occur  at  exactly  the  proper  times. 

It  is  familiar  to  all  that  a  person  pushing  another  in  a  swing 
may  cause  a  considerable  amplitude  of  vibration  with  the  exertion 
of  but  a  small  amount  of  force,  if  he  will  so  time  his  pushes  as 
to  conform  exactly  to  the  natural  rate  of  vibration  of  the  swing. 
It  is  of  course  possible,  however,  to  make  the  swing  take  up  other 
rates  of  vibrations  by  the  application  of  sufficient  force.  As  another 
example,  consider  a  clock  pendulum  beating  seconds.  By  gentle 
blows  furnished  by  the  escapement  at  exactly  the  proper  times,  the 
heavy  pendulum  is  kept  in  motion.  However,  if  a  person  grasps  the 
pendulum  weight  and  shakes  it,  it  may  be  made  to  vibrate  at  almost 
any  desired  rate,  dependent  on  the  strength  and  agility  of  the  in- 
dividual. 

The  conclusion  is,  therefore,  that  a  reed  or  pendulum  may  be 
made  to  start  and  vibrate  easily  by  the  application  of  impulses  at 
proper  intervals,  and  only  with  great  difficulty  by  the  application 
of  impulses  at  other  than  the  proper  intervals;  and  these  facts  form 
the  basis  on  which  harmonic-ringing  systems  rest. 

The  father  of  harmonic  ringing  in  telephony  was  Jacob  B. 
Currier,  an  undertaker  of  Lowell,  Mass.  His  harmonic  bells  were 


SELECTIVE  PARTY-LINE  SYSTEMS  239 

placed  in  series  in  the  telephone  line,  and  were  considerably  used  in 
New  England  in  commercial  practice  in  the  early  eighties.  Some- 
what later  James  A.  Lighthipe  of  San  Francisco  independently  in- 
vented a  harmonic-ringing  system,  which  was  put  in  successful 
commercial  use  at  Sacramento  and  a  few  other  smaller  California 
towns.  Lighthipe  polarized  his  bells  and  bridged  them  across  the 
line  in  series  with  condensers,  as  in  modern  practice,  and  save  for 
some  crudities  in  design,  his  apparatus  closely  resembled,  both  in 
principle  and  construction,  some  of  that  in  successful  use  today. 

Lighthipe's  system  went  out  of  use  and  was  almost  forgotten, 
when  about  1903,  "Win.  W.  Dean  again  independently  redeveloped 
the  harmonic  system,  and  produced  a  bell  astonishingly  like  that 
of  Lighthipe,  but  of  more  refined  design,  thus  starting  the  de- 
velopment which  has  resulted  in  the  present  wide  use  of  this 
system. 

The  signal-receiving  device  in  harmonic-ringing  systems  takes 
the  form  of  a  ringer,  having  its  armature  and  striker  mountec(  on  a 
rather  stiff  spring  rather  than  on  trunnions.  By  this  means  the 
moving  parts  of  the  bell  constitute  in  effect  a  reed  tongue,  which 
has  a  natural  rate  of  vibration  at  which  it  may  easily  be  made  to 
vibrate  with  sufficient  amplitude  to  strike  the  gongs.  The  harmonic 
ringer  differs  from  the  ordinary  polarized  bell  or  ringer,  therefore, 
in  that  its  armature  will  vibrate  most  easily  at  one  particular  rate, 
while  the  armature  of  the  ordinary  ringer  is  almost  indifferent,  be- 
tween rather  wide  limits,  as  to  the  rate  at  which  it  vibrates. 

As  a  rule  harmonic  party-line  systems  are  limited  to  four  stations 
on  a  line.  The  frequencies  employed  are  usually  16f ,  33£,  50,  and 
66f  cycles  per  second,  this  corresponding  to  1,000,  2,000,  3,000,  and 
4,000  cycles  per  minute.  The  reason  why  this  particular  set  of  fre- 
quencies was  chosen  is  that  they  represent  approximately  the  range 
of  desirable  frequencies,  and  that  the  first  ringing-current  machines 
in  such  systems  were  made  by  mounting  the  armatures  of  four  dif- 
ferent generators  on  a  single  shaft,  these  having,  respectively,  two 
poles,  four  poles,  six  poles,  and  eight  poles  each.  The  two-pole 
generator  gave  one  cycle  per  revolution,  the  four-pole  two,  the  six- 
pole  three,  and  the  eight-pole  four,  so  that  by  running  the  shaft 
of  the  machine  at  exactly  1,000  revolutions  per  minute  the  frequencies 
before  mentioned  were  attained.  This  range  of  frequencies  having 


240  TELEPHONY 

proved  about  right  for  general  practice  and  the  early  ringers  all 
having  been  attuned  so  as  to  operate  on  this  basis,  the  practice  of 
adhering  to  these  numbers  of  vibrations  has  been  kept  up  with 
one  exception  by  all  the  manufacturers  who  make  this  type  of 
ringer. 

Tuning.  The  process  of  adjusting  the  armature  of  a  ringer 
to  a  certain  rate  of  vibration  is  called  tuning,  and  it  is  customary 
to  refer  to  a  ringer  as  being  tuned  to  a  certain  rate  of  vibration,  just 
as  it  is  customary  to  refer  to  a  violin  string  as  being  tuned  to  a  cer- 
tain pitch  or  rate  of  vibration. 

The  physical  difference  between  the  ringers  of  the  various  fre- 
quencies consists  mainly  in  the  size  of  the  weights  at  the  end  of  the 
vibrating  reed,  that  is,  of  the  weights  which  form  the  tapper  for  the 
bell.  The  low-frequency  ringers  have  the  largest  weights  and  the 
high-frequency  the  smallest,  of  course.  The  ringers  are  roughly 
tuned  to  the  desired  frequencies  by  merely  placing  on  the  tapper 
rod  the  desired  weight  and  then  a  more  refined  tuning  is  given 
them  by  slightly  altering  the  positions  of  the  weights  on  the  tapper 
rod.  To  make  the  reed  have  a  slightly  lower  natural  rate  of  vibra- 
tion, the  weight  is  moved  further  from  the  stationary  end  of  the 
reed,  while  to  give  it  a  slightly  higher  natural  rate  of  vibration  the 
weight  is  moved  toward  the  stationary.  In  this  way  very  nice  adjust- 
ments may  be  made,  and  the  aim  of  the  various  factories  manufac- 
turing these  bells  is  to  make  the  adjustment  permanent  so  that  it 
will  never  have  to  be  altered  by  the  operating  companies.  Sev- 
eral years  of  experience  with  these  bells  has  shown  that  when  once 
properly  assembled  they  maintain  the  same  rate  of  vibration  with 
great  constancy. 

There  are  two  general  methods  of  operating  harmonic  bells. 
One  of  these  may  be  called  the  in-time  system  and  the  other  the 
under-tune  system.  The  under-tune  system  was  the  first  em- 
ployed. 

Under-Tune  System.  The  early  workers  in  the  field  of  har- 
monic-selective signaling  discovered  that  when  the  tapper  of  the 
reed  struck  against  gongs  the  natural  rate  of  vibration  of  the  reed 
was  changed,  or  more  properly,  the  reed  was  made  to  have  a  dif- 
ferent rate  of  vibration  from  its  natural  rate.  This  was  caused  by 
the  fact  that  the  elasticity  of  the  gongs  proved  another  factor  in  the 


SELECTIVE  PARTY-LINE  SYSTEMS  241 

set  of  conditions  causing  the  reeds  to  take  up  a  certain  rate  of  vibra- 
tion, and  the  effect  of  this  added  factor  was  always  to  accelerate 
the  rate  of  vibration  which  the  reed  had  when  it  was  not  striking 
the  gongs.  The  rebound  of  the  hammer  from  the  gongs  tended, 
in  other  words,  to  accelerate  the  rate  of  vibration,  which,  as  might 
be  expected,  caused  a  serious  difficulty  in  the  practical  operation  of 
the  bells.  To  illustrate:  If  a  reed  were  to  have  a  natural  rate  of 
vibration,  when  not  striking  the  gongs,  of  50  per  second  and  a  cur- 
rent of  50  cycles  per  second  were  impressed  on  the  line,  the  reed 
would  take  up  this  rate  of  vibration  easily,  but  when  a  sufficient 
amplitude  of  vibration  was  attained  to  cause  the  tapper  to  strike 
the  gongs,  the  reed  would  be  thrown  out  of  tune,  on  account  of  the 
tendency  of  the  gongs  to  make  the  reed  vibrate  at  a  higher  rate. 
This  caused  irregular  ringing  and  was  frequently  sufficient  to  make 
the  bells  cease  ringing  altogether  or  to  ring  in  an  entirely  unsatis- 
factory manner. 

In  order  to  provide  for  this  difficulty  the  early  bells  of  Currier 
and  Lighthipe  were  made  on  what  has  since  been  called  the  "under- 
tuned"  principle.  The  first  bells  of  the  Kellogg  Switchboard  and 
Supply  Company,  developed  by  Dean,  were  based  on  this  idea  as 
their  cardinal  principle.  The  reeds  were  all  given  a  natural  rate  of 
vibration,  when  not  striking  the  gongs,  somewhat  below  that  of  the 
current  frequencies  to  be  employed;  and  yet  not  sufficiently  below 
the  corresponding  current  frequency  to  make  the  bell  so  far  out  of 
tune  that  the  current  frequency  would  not  be  able  to  start  it.  This 
was  done  so  that  when  the  tapper  began  to  strike  the  gongs  the  tap- 
per would  be  accelerated  and  brought  practically  into  tune  with  the 
current  frequency,  and  the  ringing  would  continue  regularly  as  long 
as  the  current  flowed.  It  will  be  seen  that  the  under-tuned  sys- 
tem was,  therefore,  one  involving  some  difficulty  in  starting  in  order 
to  provide  for  proper  regularity  while  actually  ringing. 

Ringers  of  this  kind  were  always  made  with  but  a  single  gong, 
it  being  found  difficult  to  secure  uniformity  of  ringing  and  uniform- 
ity of  adjustment  when  two  gongs  were  employed.  Although  no 
ringers  of  this  type  are  being  made  at  present,  yet  a  large  number 
of  them  are  in  use  and  they  will  consequently  be  described.  Their 
action  is  interesting  in  throwing  better  light  on  the  more  improved 
types,  if  for  no  other  reason. 


242 


TELEPHONY 


Fig.  178.     Under-Tuned  Ringer 


Figs.  178  and  179  show,  respectively,  side  and  front  views  of 
the  original  Kellogg  bell.  The  entire  mechanism  is  self-contained, 
all  parts  being  mounted  on  the  base  plate  1.  The  electromagnet  is 
of  the  two-coil  type,  and  is  supported  on  the  brackets  2  and  3.  The 
bracket  2  is  of  iron  so  as  to  afford  a  magnetic  yoke  for  the  field  of 
the  electromagnet,  while  the  bracket  3  is  of  brass  so  as  not  to  short- 
circuit  the  magnetic  lines  across  the  air-gap.  The  reed  tongue — 

consisting  of  the  steel  spring  5,  the 
soft-iron  armature  pieces  6,  the  auxil- 
iary spring  7,  and  the  tapper  ball  8, 
all  of  which  are  riveted  together,  as 
shown  in  Fig.  178 — constitutes  the  only 
moving  part  of  the  bell.  The  steel 
spring  5  is  rigidly  mounted  in  the 
clamping  piece  9  at  the  upper  part  of 
the  bracket  3,  and  the  reed  tongue  is 
permitted  to  vibrate  only  by  the  flexi- 
bility of  this  spring.  The  auxiliary 
spring  7  is  much  lighter  than  the  spring 

5  and  has  for  its  purpose  the  provision  of  a  certain  small  amount 
of  flexibility  between  the  tapper  ball  and  the  more  rigid  portion  of 
the  armature  formed  by  the  iron  strips  6-6.  The  front  ends  of 
the  magnet  pole  pieces  extend  through  the  bracket  3  and  are  there 
provided  with  square  soft-iron  pole  pieces  10  set  at  right  angles  to 
the  magnet  cores  so  as  to  form  a  rather  narrow  air-gap  in  which  the 
armature  may  vibrate. 

The  cores  of  the  magnet  and  also  the  reed  tongue  are  polarized 
by  means  of  the  L-shaped  bar  magnet  4,  mounted  on  the  iron  yoke 
2  at  one  end  in  such  manner  that  its  other  end  will  lie  quite  close 
to  the  end  of  the  spring  5,  which,  being  of  steel,  will  afford  a  path 
for  the  lines  of  force  to  the  armature  proper.  We  see,  therefore, 
that  the  two  magnet  cores  are,  by  this  permanent  magnet,  given 
one  polarity,  while  the  reed  tongue  itself  is  given  the  other  polarity, 
this  being  exactly  the  condition  that  has  already  been  described  in 
connection  with  the  regular  polarized  bell  or  ringer. 

The  electromagnetic  action  by  which  this  reed  tongue  is  made 
to  vibrate  is,  therefore,  exactly  the  same  as  that  of  an  ordinary  po- 
larized ringer,  but  the  difference  between  the  two  is  that,  in  this 


SELECTIVE  PARTY-LINE  SYSTEMS 


243 


ringer,  the  reed  tongue  will  respond  only  to  one  particu- 
lar rate  of  vibrations,  while  the  regular  polarized  ringer  will  respond 
to  almost  any. 

As  shown  in  Fig.  178,  the  tapper  ball  strikes  on  the  inside  sur- 
/ace  of  the  single  gong.  The  function  of  the  auxiliary  spring  7  be- 
tween the  ball  and  the  main  portion  of  the  armature  is  to  allow  some 
resilience  between  the  ball  and  the  balance  of  the  armature  so  as  to 
counteract  in  some  measure  the  accelerating  influence  of  the  gong 
on  the  armature.  In  these  bells,  as  already  stated,  the  natural 
rate  of  vibration  of  the  reed  tongue  was  made  somewhat  lower  than 
the  rate  at  which  the  bell  was  to  be  operated,  so  that  the  reed  tongue 
had  to  be  started  by  a  current  slightly  out  of  tune  with  it,  and  then, 
as  the  tapper  struck  the  gong,  the  acceleration  due  to  the  gong  would 
bring  the  vibration  of  the  reed  tongue,  as  modified  by  the  gong, 
into  tune  with  the  current  that  was  operating  it.  In  other  words, 
in  this  system  the  ringing  currents  that  were  applied  to  the  line 
had  frequencies  corresponding  to  what  may  be  called  the  opera- 
tive rates  of  vibration  of  the  reed  tongues,  which  operative  rates  of 
vibration  were  in  each  case  the 
resultant  of  the  natural  pitch  of 
the  reed  as  modified  by  the 
action  of  the  bell  gong  when 
struck. 

In-Tune  System.  The  more 
modern  method  of  tuning  is  to 
make  the  natural  rate  of  vibration 
of  the  reed  tongue,  that  is,  the 
rate  at  which  it  naturally  vibrates 
when  not  striking  the  gongs, 
such  as  to  accurately  correspond 
to  the  rate  of  vibration  at  which  the  bells  are  to  be  operated — 
that  is,  the  natural  rate  of  vibration  of  the  reed  tongues  is  made 
the  same  as  the  operative  rate.  Thus  the  bells  are  attuned  for 
easy  starting,  a  great  advantage  over  the  under-tuned  system.  In 
the  under-tuned  system,  the  reeds  being  out  of  tune  in  starting 
require  heavier  starting  current,  and  this  is  obviously  conducive 
to  cross-ringing,  that  is,  to  the  response  of  bells  to  other  than  the 
intended  frequency. 


Fig.  179.     Under-Tuned  Ringer 


244  TELEPHONY 

Again,  easy  starting  is  desirable  because  when  the  armature 
is  at  rest,  or  in  very  slight  vibration,  it  is  at  a  maximum  distance 
from  the  poles  of  the  electromagnet,  and,  therefore,  subject  to  the 
weakest  influence  of  the  poles.  A  current,  therefore,  which  is 
strong  enough  to  start  the  vibration,  will  be  strong  enough  to  keep 
the  bell  ringing  properly. 

When  with  this  "in-tune"  mode  of  operation,  the  armature  is 
thrown  into  sufficiently  wide  vibration  to  cause  the  tapper  to  strike 
the  gong,  the  gong  may  tend  to  accelerate  the  vibration  of  the  reed 
tongue,  but  the  current  impulses  through  the  electromagnet  coils 


Fig.  180.     Dean  In-Tune  Ringer 

continue  at  precisely  the  same  rates  as  before.  Under  this  condi- 
tion of  vibration,  when  the  reed  tongue  has  an  amplitude  of  vibra- 
tion wide  enough  to  cause  the  tapper  to  strike  the  gongs,  the  ends 
of  the  armature  come  closest  to  the  pole  pieces,  so  that  the  pole  pieces 
have  their  maximum  magnetic  effect  on  the  armature,  with  the  result 
that  even  if  the  accelerating  tendency  of  the  gongs  were  considerable, 
the  comparatively  large  magnetic  attractive  impulses  occurring  at 
the  same  rate  as  the  natural  rate  of  vibration  of  the  reed  tongue, 
serve  wholly  to  prevent  any  actual  acceleration  of  the  reed  tongue. 
The  magnetic  attractions  upon  the  ends  of  the  armature,  continuing 
at  the  initial  rate,  serve,  therefore,  as  a  check  to  offset  any  acceler- 


SELECTIVE  PARTY-LINE  SYSTEMS 


245 


Fig.  181.     Tappers  for  Dean  Ringers 


ating  tendency  which  the  striking  of  the  gong  may  have  upon  the 
vibrating  reed  tongue. 

It  is  obvious,  therefore,  that  in  the  "in-tune"  system  the  elec- 
tromagnetic effect  on  the  armature  should,  when  the  armature  is 
closest  to  the  pole  pieces,  be  of  such  an  overpowering  nature  as  to 
prevent  whatever  acceler- 
ating tendency  the  gongs 
may  have  from  throwing 
the  armature  out  of  its 
"stride"  in  step  with  the 
current.  For  this  reason 
it  is  usual  in  this  type  to 
so  adjust  the  armature 
that  its  ends  will  actually 
strike  against  the  pole 
pieces  of  the  electromag- 
net when  thrown  into  vibration.  Sufficient  flexibility  is  given  to 
the  tapper  rod  to  allow  it  to  continue  slightly  beyond  the  point  at 
which  it  would  be  brought  to  rest  by  the  striking  of  the  armature 
ends  against  the  pole  pieces  and  thus  exert  a  whipping  action  so  as 
to  allow  the  ball  to  continue  in  its  movement  far  enough  to  strike 
against  the  gongs.  The  rebound  of  the  gong  is  then  taken  up  by 
the  elasticity  of  the  tapper  rod,  which  returns  to  an  unflexed  posi- 
tion, and  at  about  this  time  the  pole  piece  releases  the  armature  so 
that  it  may  swing  over  in  the  other  direction  to  cause  the  tapper 
to  strike  the  other  gong. 

The  construction  of  the  "in-tune"  harmonic  ringer  employed 
by  the  Dean  Electric  Company,  of  Elyria,  Ohio,  is  illustrated  in 
Figs.  180,  181,  and  182.  It  will  be  seen  from  Fig.  180  that  the  gen- 
eral arrangement  of  the  magnet  and  armature  is  the  same  as  that 
of  the  ordinary  polarized  ringer;  the  essential  difference  is  that  the 
armature  is  spring-mounted  instead  of  pivoted.  The  armature  and 
the  tapper  rod  normally  stand  in  the  normal  central  position  with 
reference  to  the  pole  pieces  of  the  magnet  and  the  gongs.  Fig.  181 
shows  the  complete  vibrating  parts  of  four  ringers,  adapted,  respec- 
tively, to  the  four  different  frequencies  of  the  system.  The  assem- 
bled armature,  tapper  rod,  and  tapper  are  all  riveted  together  and 
are  non-adjustable.  All  of  the  adjustment  that  is  done  upon  them 


246 


TELEPHONY 


is  done  in  the  factory  and  is  accomplished,  first,  by  choosing  the 
proper  size  of  weight,  and  second,  by  forcing  this  weight  into  the 


Pig.  182.     Dean  In-Tune  Ringer 

proper  position  on  the  tapper  rod  to  give  exactly  the  rate  of  vibra- 
tion that  is  desired. 

An  interesting  feature  of  this  Dean  harmonic  ringer  is  the  gong 
adjustment.  As  will  be  seen,  the  gongs  are  mounted  on  posts  which 
are  carried  on  levers  pivoted  to  the  ringer  frame.  These  levers 
have  at  their  outer  end  a  curved  rack  provided  with  gear  teeth 
adapted  to  engage  a  worm  or  screw  thread  mounted  on  the  ringer 

frame.  Obviously,  by  turning 
this  worm  screw  in  one  direc- 
tion or  the  other,  the  gongs 
are  moved  slightly  toward  or 
from  the  armature  or  tapper. 
This  affords  a  very  delicate 
means  of  adjusting  the  gongs, 
and  at  the  same  time  one 
which  has  no  tendency  to  work 
loose  or  to  get  out  of  adjust- 
ment. 

In  Fig.  183  is  shown  a  draw- 
Fig.  183.    Kellogg  in-Tune  Ringer         inS  of   the  "in-tune"  harmonic 

ringer  manufactured  by  the  Kel- 
logg Switchboard  and  Supply  Company.  This  differs  in  no  essen- 
tial respect  from  that  of  the  Dean  Company,  except  in  the  gong 


SELECTIVE  PARTY-LINE  SYSTEMS 


247 


adjustment,  this  latter  being  affected  by  a  screw  passing  through  a 
nut  in  the  gong  post,  as  clearly  indicated. 

In  both  the  Kellogg  and  the  Dean  in-tune  ringers,  on  account 
of  the  comparative  stiffness  of  the  armature  springs  and  on  account 
of  the  normal  position  of  the  armature  with  maximum  air  gaps  and 
consequent  minimum  magnetic  pull,  the  armature  will  practically 
not  be  affected  unless  the  energizing  current  is  accurately  attuned 
to  its  own  natural  rate.  When  the  proper  current  is  thrown  on  to 
the  line,  the  ball  will  be  thrown  into  violent  vibration,  and  the  ends 
of  the  armature  brought  into  actual  contact  with  the  pole  pieces, 
which  are  of  bare  iron  and  shielded  in  no  way.  The  armature  in 
this  position  is  very  strongly  attracted  and  comes  to  a  sudden  stop 
on  the  pole  pieces.  The  gongs  are  so  adjusted  that  the  tapper 
ball  will  have  to  spring  about  one  thirty-second  of  an  inch  in  order 


CENTRAL  OFF/CE 

n 


VTl.  wy"H     vr-H     wi-H 


<0 


Fig.  184.     Circuits  of  Dean  Harmonic  System 

to  hit  them.  The  armature  is  held  against  the  pole  piece  while  the 
tapper  ball  is  engaged  in  striking  the  gong  and  in  partially  return- 
ing therefrom,  and  so  strong  is  the  pull  of  the  pole  piece  on  the 
armature  in  this  position  that  the  accelerating  influence  of  the  gong 
has  no  effect  in  accelerating  the  rate  of  vibration  of  the  reed. 

Circuits.  In  Fig.  184  are  shown  in  simplified  form  the  circuits 
of  a  four-station  harmonic  party  line.  It  is  seen  that  at  the  central 
office  there  are  four  ringing  keys,  adapted,  respectively,  to  impress 
on  the  line  ringing  currents  of  four  different  frequencies.  At  the 
four  stations  on  the  line,  lettered  A,  B,  C,  and  D,  there  are  four  har- 
monic bells  tuned  accordingly.  At  Station  A  there  is  shown  the  talking 
apparatus  employing  the  Wheatstone  bridge  arrangement.  The  talking 
apparatus  at  all  of  the  other  stations  is  exactly  the  same,  but  is 
omitted  for  the  sake  of  simplicity.  A  condenser  is  placed  in  series 


248 


TELEPHONY 


with  each  of  the  bells  in  order  that  there  may  be  no  direct-current 
path  from  one  side  of  the  line  to  the  other  when  all  of  the  receivers 
are  on  their  hooks  at  the  several  stations. 

In  Fig.  185  is  shown  exactly  the  same  arrangement,  with  the 
exception  that  the  talking  apparatus  illustrated  in  detail  at  Station. 
A  is  that  of  the  Kellogg  Switchboard  and  Supply  Company.  Other- 
wise the  circuits  of  the  Dean  and  the  Kellogg  Company,  and  in  fact 
of  all  the  other  companies  manufacturing  harmonic  ringing  systems, 
are  the  same. 

Advantages.  A  great  advantage  of  the  harmonic  party-line 
system  is  the  simplicity  of  the  apparatus  at  the  subscriber's  station. 
The  harmonic  bell  is  scarcely  more  complex  than  the  ordinary  pol- 
arized ringer,  and  the  only  difference  between  the  harmonic-ringing 
telephone  and  the  ordinary  telephone  is  in  the  ringer  itself.  The 


CENTRAL  OFF/CE 


n 


Fig.  185.     Circuits  of  Kellogg  Harmonic  System 

absence  of  all  relays  and  other  mechanism  and  also  the  absence  of 
the  necessity  for  ground  connections  at  the  telephone  are  all  points  in 
favor  of  the  harmonic  system. 

Limitations.  As  already  stated,  the  harmonic  systems  of  the 
various  companies,  with  one  exception,  are  limited  to  four  frequencies. 
The  exception  is  in  the  case  of  the  North  Electric  Company,  which 
sometimes  employs  four  and  sometimes  five  frequencies  and  thus 
gets  a  selection  between  five  stations.  In  the  four-party  North  sys- 
tem, the  frequencies,  unlike  those  in  the  Dean  and  Kellogg  systems, 
wherein  the  higher  frequencies  are  multiples  of  the  lower,  are  ar- 
ranged so  as  to  be  proportional  to  the  whole  numbers  5,  7,  9,  and 
11,  which,  of  course,  have  no  common  denominator.  The  frequen- 
cies thus  employed  in  the  North  system  are,  in  cycles  per  second, 
30.3,  42.4,  54.5,  and  66.7.  In  the  five-party  system,  the  frequency 
of  16.7  is  arbitrarily  added. 


SELECTIVE  PARTY-LINE  SYSTEMS  249 

While  all  of  the  commercial  harmonic  systems  on  the  market 
are  limited  to  four  or  five  frequencies,  it  does  not  follow  that  a  greater 
number  than  four  or  five  stations  may  not  be  selectively  rung.  Double 
these  numbers  may  be  placed  on  a  party  line  and  selectively  actuated, 
if  the  first  set  of  four  or  five  is  bridged  across  the  line  and  the  second 
set  of  four  or  five  is  connected  between  one  limb  of  the  line  and 
ground.  The  first  set  of  these  is  selectively  rung,  as  already  de- 
scribed, by  sending  the  ringing  currents  over  the  metallic  circuit, 
while  the  second  set  may  be  likewise  selectively  rung  by  sending 
the  ringing  currents  over  one  limb  of  the  line  with  a  ground 
return.  This  method  is  frequently  employed  with  success  on  coun- 
try lines,  where  it  is  desired  to  place  a  greater  number  of  instruments 
on  a  line  than  four  or  five. 

Step=by=Step  Method.  A  very  large  number  of  step-by-step  sys- 
tems have  been  proposed  and  reduced  to  practice,  but  as  yet  they 
have  not  met  with  great  success  in  commercial  telephone  work,  and 
are  nowhere  near  as  commonly  used  as  are  the  polarity  and  har- 
monic systems. 

Principles.  An  idea  of  the  general  features  of  the  step-by- 
step  systems  may  be  had  by  conceiving  at  each  station  on  the  line 
a  ratchet  wheel,  having  a  pawl  adapted  to  drive  it  one  step  at  a 
time,  this  pawl  being  associated  with  the  armature  of  an  electro- 
magnet which  receives  current  impulses  from  the  line  circuit.  There 
is  thus  one  of  these  driving  magnets  at  each  station,  each  bridged 
across  the  line  so  that  when  a  single  impulse  of  current  is  sent  out 
from  the  central  office  all  of  the  ratchet  wheels  will  be  moved  one 
step.  Another  impulse  will  move  all  of  the  ratchet  wheels  another 
step,  and  so  on  throughout  any  desired  number  of  impulses.  The 
ratchet  wheels,  therefore,  are  all  stepped  in  unison. 

Let  us  further  conceive  that  all  of  these  ratchet  wheels  are 
provided  with  a  notch  or  a  hole  or  a  projection,  alike  in  all  respects 
at  all  stations  save  in  the  position  which  this  notch  or  hole  or  pro- 
jection occupies  on  the  wheel.  The  thing  to  get  clear  in  this  part 
of  the  conception  is  that  all  of  these  notches, 'holes,  or  projections 
are  alike  on  all  of  the  wheels,  but  they  occupy  a  different  position 
on  the  wheel  for  each  one  of  the  stations. 

Consider  further  that  the  bell  circuit  at  each  of  the  stations  is 
normally  open,  but  that  in  each  case  it  is  adapted  to  be  closed  when 


250  TELEPHONY 

the  notch,  hole,  or  projection  is  brought  to  a  certain  point  by  the 
revolution  of  the  wheel. 

Let  us  conceive  further  that  this  distinguishing  notch,  hole, 
or  projection  is  so  arranged  on  the  wheel  of  the  first  station  as  to 
close  the  bell  circuit  when  one  impulse  has  been  sent,  that  that  on 
the  second  station  will  close  the  bell  circuit  after  the  second  impulse 
has  been  sent,  and  so  on  throughout  the  entire  number  of  stations. 
It  will,  therefore,  be  apparent  that  the  bell  circuits  at  the  various 
stations  will,  as  the  wheels  are  rotated  in  unison,  be  closed  one  after 
the  other.  In  order  to  call  a  given  station,  therefore,  it  is  only  neces- 
sary to  rotate  all  of  the  wheels  in  unison,  by  sending  out  the  proper 
stepping  impulses  until  they  all  occupy  such  a  position  that  the  one 
at  the  desired  station  is  in  such  position  as  to  close  the  bell  circuit 
at  that  station.  Since  all  of  the  notches,  holes,  or  projections  are 
arranged  to  close  the  bell  circuits  at  their  respective  stations  at 
different  times,  it  follows  that  when  the  bell  circuit  at  the  desired 
station  is  closed  those  at  all  of  the  other  stations  will  be  open.  If, 
therefore,  after  the  proper  number  of  stepping  impulses  has  been 
sent  to  the  line  to  close  the  bell  circuit  of  the  desired  station,  ring- 
ing current  be  applied  to  the  line,  it  is  obvious  that  the  bell  of  that 
one  station  will  be  rung  to  the  exclusion  of  all  others.  It  is,  of  course, 
necessary  that  provision  be  made  whereby  the  magnets  which  fur- 
nish the  energy  for  stepping  the  wheels  will  not  be  energized  by  the 
ringing  current.  This  is  accomplished  in  one  of  several  ways,  the 
most  common  of  which  is  to  have  the  stepping  magnets  polarized 
or  biased  in  one  direction  and  the  bells  at  the  various  stations  op- 
positely biased,  so  that  the  ringing  current  will  not  affect  the  step- 
ping magnet  and  the  stepping  current  will  not  affect  the  ringer 
magnets. 

After  a  conversation  is  finished,  the  line  may  be  restored  to  its 
normal  position  in  one  of  several  ways.  Usually  so-called  release 
magnets  are  employed,  for  operating  on  the  releasing  device  at  each 
station.  These,  when  energized,  will  withdraw  the  holding  pawls 
from  the  ratchets  arid  allow  them  all  to  return  to  their  normal  posi- 
tions. Sometimes  these  release  magnets  are  -operated  by  a  long 
impulse  of  current,  being  made  too  sluggish  in  their  action  to  respond 
to  the  quick-stepping  impulses;  sometimes  the  release  magnets  are 
tapped  from  one  limb  of  the  line  to  ground,  so  as  not  to  be  affected 


SELECTIVE  PARTY-LINE  SYSTEMS  251 

by  the  stepping  or  ringing  currents  sent  over  the  metallic  circuit; 
and  sometimes  other  expedients  are  used  for  obtaining  the  release 
of  the  ratchets  at  the  proper  time,  a  large  amount  of  ingenuity  hav- 
ing been  spent  to  this  end. 

As  practically  all  step-by-step  party-line  systems  in  commercial 
use  have  also  certain  other  features  intended  to  assure  privacy  of 
conversation  to  the  users,  and,  therefore,  come  under  the  general 
heading  of  lock-out  party-line  systems,  the  discussion  of  commercial 
examples  of  these  systems  will  be  left  for  the  next  chapter,  which  is 
devoted  to  such  lock-out  systems. 

Broken=Line  Method.  The  broken-line  system,  like  the  step- 
by-step  system,  is  also  essentially  a  lock-out  system  and  for  that 
reason  only  its  general  features,  by  which  the  selective  ringing  is 
accomplished,  will  be  dealt  with  here. 

Principles.  In  this  system  there  are  no  tuned  bells,  no  posi- 
tively and  negatively  polarized  bells  bridged  to  ground  on  each 
side  of  the  line,  and  no  step-by-step  devices  in  the  ordinary  sense, 
by  which  selective  signaling  has  ordinarily  been  accomplished  on 
party  lines.  Instead  of  this,  each  instrument  on  the  line  is  exclu- 
sively brought  into  operative  relation  with  the  line,  and  then  re- 
moved from  such  operative  relation  until  the  subscriber  wanted  is 
connected,  at  which  time  all  of  the  other  instruments  are  locked 
out  and  the  line  is  not  encumbered  by  any  bridge  circuits  at  any  of 
the  instruments  that  are  not  engaged  in  the  conversation.  Further- 
more, in  the  selecting  of  a  subscriber  or  the  ringing  of  his  bell  there 
is  no  splitting  up  of  current  among  the  magnets  at  the  various  sta- 
tions as  in  ordinary  practice,  but  the  operating  current  goes  straight 
to  the  station  desired  and  to  that  station  alone  where  its  entire 
strength  is  available  for  performing  its  proper  work. 

In  order  to  make  the  system  clear  it  may  be  stated  at  the  out- 
set that  one  side  of  the  metallic  circuit  line  is  continued  as  in  ordi- 
nary practice,  passing  through  all  of  the  stations  as  a  continuous 
conductor.  The  other  side  of  the  line,  however,  is  divided  into 
sections,  its  continuity  being  broken  at  each  of  the  subscriber's 
stations.  Fig.  186  is  intended  to  show  in  the  simplest  possible  way 
how  the  circuit  of  the  line  may  be  extended  from  station  to  station 
in  such  manner  that  only  the  ringer  of  one  station  is  in  circuit  at  ?, 
time.  The  two  sides  of  the  line  are  shown  in  this  figure,  and  it  will 


252 


TELEPHONY 


be  seen  that  limb  L  extends  from  the  central  office  on  the  left  to  the 
last  station  on  the  right  without  a  break.  The  limb  R,  however, 
extends  to  the  first  station,  at  which  point  it  is  cut  off  from  the  ex- 


S  TAT/OH -B- 


3  TA  T/OM-C-         STAT/ON  -O- 


Fig.  186.     Principle  of  Broken-Line  System 

tension  Rx  by  the  open  contacts  of  a  switch.  For  the  purpose  of 
simplicity  this  switch  is  shown  as  an  ordinary  hand  switch,  but  as 
a  matter  of  fact  it  is  a  part  of  a  relay,  the  operating  coil  of  which  is 
shown  at  6,  just  above  it,  in  series  with  the  ringer. 

Obviously,  if  a  proper  ringing  current  is  sent  over  the  metallic 
circuit  from  the  central  office,  only  the  bell  at  Station  A  will  operate, 
since  the  bells  at  the  other  stations  are  not  in  the  circuit.  If  by 
any  means  the  switch  lever  2  at  Station  A  were  moved  out  of  en- 
gagement with  contact  1  and  into  engagement  with  contact  3,  it  is 
obvious  that  the  bell  of  Station  A  would  no  longer  be  in  circuit, 
but  the  limb  R  of  the  line  would  be  continued  to  the  extension 


•STAT/Ofi-A- 


STAT/O/y-B- 


STAT/O/y-C-         STATION -D- 


Fig.  187.     Principle  of  Broken-Line  System 

Rx  and  the  bell  of  Station  B  would  be  in  circuit.  Any  current  then 
sent  over  the  circuit  of  the  line  from  the  central  office  would  ring 
the  bell  of  this  station.  In  Fig.  187  the  switches  of  both  Station  A 
and  Station  B  have  been  thus  operated,  and  Station  C  is  thus  placed 
in  circuit.  Inspection  of  this  figure  will  show  that  the  bells  of  Sta- 
tion A,  Station  B,  and  Station  D  are  all  cut  out  of  circuit,  and  that, 
therefore,  no  current  from  the  central  office  can  affect  them.  This 
general  scheme  of  selection  is  a  new-comer  in  the  field,  and  for 
certain  classes  of  work  it  is  of  undoubted  promise. 


CHAPTER  XVII 
LOCK=OUT  PARTY=LINE  SYSTEMS 

The  party-line  problem  in  rural  districts  is  somewhat  different 
from  that  within  urban  limits.  In  the  latter  cases,  owing  to  the 
closer  grouping  of  the  subscribers,  it  is  not  now  generally  con- 
sidered desirable,  even  from  the  standpoint  of  economy,  to  place 
more  than  four  subscribers  on  a  single  line.  For  such  a  line  selec- 
tive ringing  is  simple,  both  from  the  standpoint  of  apparatus  and 
operation;  and  moreover  owing  to  the  small  number  of  stations  on 
a  line,  and  the  small  amount  of  traffic  to  and  from  such  subscribers 
as  usually  take  party-line  service,  the  interference  between  parties 
on  the  same  line  is  not  a  very  serious  matter. 

For  rural  districts,  particularly  those  tributary  to  small  towns, 
these  conditions  do  not  exist.  Owing  to  the  remoteness  of  the 
stations  from  each  other  it  is  not  feasible  from  the  standpoint  of 
line  cost  to  limit  'the  number  of  stations  to  four.  A  much  greater 
number  of  stations  is  employed  and  the  confusion  resulting  is  dis- 
tressing not  only  to  the  subscribers  themselves  but  also  to  the  man- 
agement of  the  company.  There  exists  then  the  need  of  a  party-line 
system  which  will  give  the  limited  user  in  rural  districts  a  service, 
at  least  approaching  that  which  he  would  get  if  served  by  an  indi- 
vidual line. 

The  principal  investment  necessary  to  provide  facilities  for 
telephone  service  is  that  required  to  produce  the  telephone  line. 
In  many  cases  the  cost  of  instruments  and  apparatus  is  small  in 
comparison  with  the  cost  of  the  line.  By  far  the  greater  number 
of  subscribers  in  rural  districts  are  those  who  use  their  instruments 
a  comparatively  small  number  of  times  a  day,  and  to  maintain  an 
expensive  telephone  line  for  the  exclusive  use  of  one  such  subscriber 
who  will  use  it  but  a  few  minutes  each  day  is  on  its  face  an  economic 
waste.  As  a  result,  where  individual  line  service  is  practiced  ex- 


254  TELEPHONY 

clusively  one  of  two  things  must  be  true :  either  the  average  subscriber 
pays  more  for  his  service  than  he  should,  or  else  the  operating  com- 
pany sells  the  service  for  less  than  it  costs,  or  at  best  for  an  insufficient 
profit.  Both  of  these  conditions  are  unnatural  and  cannot  be  per- 
manent. 

The  party-line  method  of  giving  service,  by  which  a  single  line 
is  made  to  serve  a  number  of  subscribers,  offers  a  solution  to  this 
difficulty,  but  the  ordinary  non-selective  or  even  selective  party 
line  has  many  undesirable  features  if  the  attempt  is  made  to  place 
on  it  such  a  large  number  of  stations  as  is  considered  economically 
necessary  in  rural  work.  These  undesirable  features  work  to  the 
detriment  of  both  the  user  of  the  telephone  and  the  operating  com- 
pany. 

Many  attempts  have  been  made  to  overcome  these  disadvan- 
tages of  the  party  line  in  sparsely  settled  communities,  by  producing 
what  are  commonly  called  lock-out  systems.  These,  as  their  name 
implies,  employ  such  an  arrangement  of  parts  that  when  the  line  is 
in  use  by  any  two  parties,  all  other  parties  are  locked  out  from  the 
circuit  and  cannot  gain  access  to  it  until  the  parties  who  are  using 
it  are  through.  System  after  system  for  accomplishing  this  purpose 
has  been  announced  but  for  the  most  part  these  have  involved  such 
a  degree  of  complexity  and  have  introduced  so  many  undesirable 
features  as  to  seriously  affect  the  smooth  operation  of  the  system 
and  the  reliability  of  the  service. 

We  believe,  however,  in  spite  of  numerous  failures,  that  the 
lock-out  selective-signaling  party  line  has  a  real  field  of  usefulness 
and  that  operating  companies  as  well  as  manufacturing  companies 
are  beginning  to  appreciate  this  need,  and  as  a  result  that  the  relief 
of  the  rural  subscriber  from  the  almost  intolerable  service  he  has 
often  had  to  endure  is  at  hand.  A  few  of  the  most  promising  lock- 
out party-line  systems  now  before  the  public  will,  therefore,  be  de- 
scribed in  some  detail. 

Poole  System.  The  Poole  system  is  a  lock-out  system  pure  and 
simple,  its  devices  being  in  the  nature  of  a  lock-out  attachment  for 
selective-signaling  lines,  either  of  the  polarity  or  of  the  harmonic 
type  wherein  common-battery  transmission  is  employed.  It  will 
be  here  described  as  employed  in  connection  with  an  ordinary 
harmonic-ringing  system. 


LOCK-OUT  PARTY-LINE  SYSTEMS 


255 


In  Fig.  188  there  is  shown  a  four-station  party  line  equipped 
with  Poole  lock-out  devices,  it  being  assumed  that  the  ringers  at 
each  station  are  harmonic  and  that  the  keys  at  the  central  office  are 
the  ordinary  keys  adapted  to  impress  the  proper  frequency  on  the 
line  for  ringing  any  one  of  the  stations.  In  addition  to  the  ordinary 
talking  and  ringing  apparatus  at  each  subscriber's  station,  there 
is  a  relay  of  special  form  and  also  a  push-button  key. 

Each  of  the  relays  has  two  windings,  one  of  high  resistance 
and  the  other  of  low  resistance.  Remembering  that  the  system  to 
which  this  device  is  applied  is  always  a  common-battery  system,  and 
that,  therefore,  the  normal  condition  of  the  line  will  be  one  in  which 
there  is  a  difference  of  potential  between  the  two  limbs,  it  will  be 
evident  that  whenever  any  subscriber  on  a  line  that  is  not  in  use 


HARMONIC    fltHGJHG  KEYS 


STAT/O/V-A-  STAT/ON-B-  STAT/OM-C-       &TAT/O/V -D- 

Fig.  188.     Poole  Lock-Out  System 

raises  his  receiver  from  its  hook,  a  circuit  will  be  established  from 
the  upper  contact  of  the  hook  through  the  lever  of  the  hook  to  the 
high-resistance  winding  1  of  the  relay  and  thence  to  .the  other  side 
of  the  line  by  way  of  wire  6.  This  will  result  in  current  passing 
through  the  high-resistance  winding  of  the  relay  and  the  relay  will 
pull  up  its  armature.  As  soon  as  it  does  so  it  establishes  two  other 
circuits  by  the  closure  of  the  relay  armature  against  the  contacts  4 
and  5.  i 

The  closing  of  the  contact  4  establishes  a  circuit  from  the  upper 
side  of  the  line  through  the  upper  contact  of  the  switch  hook,  thence 
through  the  contacts  of  the  push  button  3,  thence  through  the  low- 
resistance  winding  2  of  the  relay  to  the  terminal  4>  thence  through 
the  relay  armature  and  the  transmitter  to  the  lower  side  of  the  line. 
This  low-resistance  path  across  the  line  serves  to  hold  the  relay 
armature  attracted  and  also  to  furnish  current  to  the  transmitter 
for  talking.  The  establishment  of  this  low-resistance  path  across 


256  TELEPHONY 

the  line  does  another  important  thing,  however;  it  practically  short- 
circuits  the  line  with  respect  to  all  the  high-resistance  relay  windings, 
and  thus  prevents  any  of  the  other  high-resistance  relay  windings 
from  receiving  enough  current  to  actuate  them,  should  the  subscriber 
at  any  other  station  remove  his  receiver  from  the  hook  in  an  attempt 
to  listen  in  or  to  make  a  call  while  the  line  is  in  use.  As  a  subscriber 
can  only  establish  the  proper  conditions  for  talking  and  listening 
by  the  attraction  of  this  relay  armature  at  his  station,  it  is  obvious 
that  unless  he  can  cause  the  pulling  up  of  his  relay  armature  he  can 
not  place  himself  in  communication  with  the  line. 

The  second  thing  that  is  accomplished  by  the  pulling  up  of 
the  relay  armature  is  the  closure  of  the  contacts  5,  and  that  com- 
pletes the  talking  circuit  through  the  condenser  and  receiver  across 
the  line  in  an  obvious  fashion.  The  result  of  this  arrangement  is 
that  it  is  the  first  party  who  raises  his  receiver  from  its  hook  who 
is  enabled  to  successfully  establish  a  connection  with  the  line,  all 
subsequent  efforts,  by  other  subscribers,  failing  to  do  so  because  of 
the  fact  that  the  line  is  short-circuited  by  the  path  through  the 
low-resistance  winding  and  the  transmitter  of  the  station  that  is 
already  connected  with  the  line. 

A  little  target  is  moved  by  the  action  of  the  relay  so  that  a  visual 
indication  is  given  to  the  subscriber  in  making  a  call  to  show  whether 
or  not  he  is  successful  in  getting  the  use  of  the  line.  If  the  relay 
operates  and  he  secures  control  of  the  line,  the  target  indicates  the 
fact  by  its  movement,  while  if  someone  else  is  using  the  line  and 
the  relay  does  not  operate,  the  target,  by  its  failure  to  move,  indicates 
that  fact. 

When  one  party  desires  to  converse  with  another  on  the  same 
line,  he  depresses  the  button  3  at  his  station  until  after  the  called 
party  has  been  rung  and  has  responded.  This  holds  the  circuit 
of  his  low-resistance  winding  open,  and  thus  prevents  the  lock-out 
from  becoming  effective  until  the  called  party  is  connected  with  the 
line.  The  relay  armature  of  the  calling  party  does  not  fall  back 
with  the  establishment  of  the  low-resistance  path  at  the  called  sta- 
tion, because,  even  though  shunted,  it  still  receives  sufficient  cur- 
rent to  hold  its  armature  in  its  attracted  position.  After  the  called 
party  has  responded,  the  button  at  the  calling  station  is  released 
and  both  low-resistance  holding  coils  act  in  multiple. 


LOCK-OUT  PARTY-LINE  SYSTEMS  257 

No  induction  coil  is  used  in  this  system  and  the  impedance  of 
the  holding  coil  is  such  that  incoming  voice  currents  flow  through 
the  condenser  and  the  receiver,  which,  by  reference  to  the  figure,  will 
be  seen  to  be  in  shunt  with  the  holding  coil.  The  holding  coil  is 
in  series  with  the  local  transmitter,  thus  making  a  circuit  similar 
to  that  of  the  Kellogg  common-battery  talking  circuit  already 
discussed. 

A  possible  defect  in  the  use  of  this  system  is  one  that  has  been 
common  to  a  great  many  other  lock-out  systems,  depending  for  their 
operation  on  the  same  general  plan  of  action.  This  appears  when 
the  instruments  are  used  on  a  comparatively  long  line.  Since  the 
locking-out  of  all  the  instruments  that  are  not  in  use  by  the  one  that 
is  in  use  depends  on  the  low-resistance  shunt  that  is  placed  across 
the  line  by  the  instrument  that  is  in  use,  it  is  obvious  that,  in  the 
case  of  a  long  line,  the  resistance  of  the  line  wire  will  enter  into  the 
problem  in  such  a  way  as  to  tend  to  defeat  the  locking-out  function 
in  some  cases.  Thus,  where  the  first  instrument  to  use  the  line  is 
at  the  remote  end  of  the  line,  the  shunting  effect  that  this  instrument 
can  exert  with  respect  to  another  instrument  near  the  central  office 
is  that  due  to  the  resistance  of  the  line  plus  the  resistance  of  the 
holding  coil  at  the  end  instrument.  The  resistance  of  the  line  wire 
may  be  so  high  as  to  still  allow  a  sufficient  current  to  flow  through 
the  high-resistance  coil  at  the  nearer  station  to  allow  its  operation, 
even  though  the  more  remote  instrument  is  already  in  use. 

Coming  now  to  a  consideration  of  the  complete  selective-signal- 
ing lock-out  systems,  wherein  the  selection  of  the  party  and  the 
locking  out  of  the  others  are  both  inherent  features,  a  single  example 
of  the  step-by-step,  and  of  the  broken-line  selective  lock-out  systems 
will  be  discussed. 

Step=by=Step  System.  The  so-called  K.  B.  system,  manufac- 
tured by  the  Dayton  Telephone  Lock-out  Manufacturing  Com- 
pany of  Dayton,  Ohio,  operates  on  the  step-by-step  principle.  The 
essential  feature  of  the  subscriber's  telephone  equipment  in  this 
system  is  the  step-by-step  actuating  mechanism  which  performs 
also  the  functions  of  a  relay.  This  device  consists  of  an  electro- 
magnet having  two  cores,  with  a  permanent  polarizing  magnet 
therebetween,  the  arrangement  in  this  respect  being  the  same  as  in 
an  ordinary  polarized  bell.  The  armature  of  this  magnet  works 


258 


TELEPHONY 


a  rocker  arm,  which,  besides  stepping  the  selector  segment  around, 
also,  under  certain  conditions,  closes  the  bell  circuit  and  the 
talking  circuit,  as  will  be  described. 

Referring  first  to  Fig.  189,  which  shows  in  simplified  form  a 
four-station  K.  B.  lock-out  line,  the  electromagnet  is  shown  at  1 
and  the  rocker  arm  at  2.  The  ratchet  3  in  this  case  is  not  a  complete 
wheel  but  rather  a  segment  thereof,  and  it  is  provided  with  a  series 
of  notches  of  different  depths.  It  is  obvious  that  the  depth  of  the 
notches  will  determine  the  degree  of  movement  which  the  upper 
end  of  the  rocker  arm  may  have  toward  the  left,  this  being  depend- 
ent on  the  extent  to  which  the  pawl  6  is  permitted  to  enter  into  the 
segment.  The  first  or  normal  notch,  i.  e.,  the  top  notch,  is  always 
of  such  a  depth  that  it  will  allow  the  rocker-arm  lever  2  to  engage 


STAT/ON -B-  STAT/ON-C-  STAT/ON -D- 


Fig.  189.     K.  B.  Lock-Out  System 

the  contact  lever  4,  but  will  not  permit  the  rocker  arm  to  swing  far 
enough  to  the  left  to  cause  that  contact  to  engage  the  bell  contact 
5.  As  will  be  shown  later,  the  condition  for  the  talking  circuit 
to  be  closed  is  that  the  rocker  arm  2  shall  rest  against  the 
contact  4t  and  from  this  we  see  that  the  normal  notch  of  each 
of  the  segments  3  is  of  such  a  depth  as  to  allow  the  talking  circuit 
at  each  station  to  be  closed.  The  next  notch,  i.  e.,  the  second  one 
in  each  disk,  is  always  shallow,  as  are  all  of  the  other  notches  except 
one.  A  deep  notch  is  placed  on  each  disk  anywhere  from  the  third 
to  the  next  to  the  last  on  the  segment.  This  deep  notch  is  called 
the  selective  notch,  and  it  is  the  one  that  allows  of  contact  being 
made  with  the  ringer  circuit  of  that  station  when  the  pawl  6  drops 
into  it.  The  position  of  this  notch  differs  on  all  of  the  segments 
on  a  line,  and  obviously,  therefore,  the  ringer  circuit  at  any  station 
may  be  closed  to  the  exclusion  of  all  the  others  by  stepping  all  of 


LOCK-OUT  PARTY-LINE  SYSTEMS  259 

the  segments  in  unison  until  the  deep  notch  on  the  segment  of  the 
desired  station  lies  opposite  to  the  pawl  6,  which  will  permit  the 
rocker  arm  2  to  swing  so  far  to  the  left  as  to  close  not  only  the  cir- 
cuit between  2  and  4>  but  also  between  2,  4,  and  5.  In  this  posi- 
tion the  talking  and  the  ringing  circuits  are  both  closed. 

The  position  of  the  deepest  notch,  i.  e.,  the  selective  notch,  on 
the  circumference  of  the  segment  at  any  station  depends  upon  the 
number  of  that  station;  thus,  the  segment  of  Station  4  will  have  a 
deep  notch  in  the  sixth  position;  the  segment  for  Station  9  will  have 
a  deep  notch  in  the  eleventh  position;  the  segment  for  any  station 
will  have  a  deep  notch  in  the  position  corresponding  to  the  number 
of  that  station  plus  two. 

From  what  has  been  said,  therefore,  it  is  evident  that  the  first, 
or  normal,  notch  on  each  segment  is  of  such  a  depth  as  to  allow 
the  moving  pawl  6  to  fall  to  such  a  depth  in  the  segment  as  to  permit 
the  rocker  arm  2  to  close  the  talking  circuit  only.  All  of  the  other 
notches,  except  one,  are  comparatively  shallow,  and  while  they 
permit  the  moving  pawl  6  under  the  influence  of  the  rocker  arm  2 
to  move  the  segment  3,  yet  they  do  not  permit  the  rocker  arm  2  to 
move  so  far  to  the  left  as  to  close  even  the  talking  circuit.  The 
exception  is  the  deep  notch,  or  selective  notch,  which  is  of  such 
depth  as  to  permit  the  pawl  6  to  fall  so  far  into  the  segment  as  to 
allow  the  rocker  arm  2  to  close  both  the  talking  and  the  ringing 
circuits.  Besides  the  moving  pawl  6  there  is  a  detent  pawl  7.  This 
always  holds  the  segment  3  in  the  position  to  which  it  has  been  last 
moved  by  the  moving  pawl  6. 

The  actuating  magnet  1,  as  has  been  stated,  is  polarized  and 
when  energized  by  currents  in  one  direction,  the  rocker  arm  moves 
the  pawl  6  so  as  to  step  the  segment  one  notch.  When  this  relay 
is  energized  by  current  in  the  opposite  direction,  the  operation  is 
such  that  both  the  moving  pawl  6  and  the  detent  pawl  7  will  be 
pulled  away  from  the  segment,  thus  allowing  the  segment  to  return 
to  its  normal  position  by  gravity.  This  is  accomplished  by  the 
following  mechanism:  An  armature  stop  is  pivoted  upon  the  face 
of  the  rocker  arm  so  as  to  swing  in  a  plane  parallel  to  the  pole  faces 
of  the  relay,  and  is  adapted,  when  the  relay  is  actuated  by  selective 
impulses  of  one  polarity,  to  be  pulled  towards  one  of  the  pole  faces 
where  it  acts,  through  impact  with  a  plate  attached  to  the  pole  face 


260  TELEPHONY 

of  the  relay,  as  a  limiting  means  for  the  motion  of  the  rocker  arm 
when  the  rocker  arm  is  actuated  by  the  magnet.  When,  however, 
the  relay  is  energized  by  current  in  the  opposite  direction,  as  on  a 
releasing  impulse,  the  armature  stop  swings  upon  its  pivot  towards 
the  opposite  pole  face,  in  which  position  the  lug  on  the  end  of  the 
armature  stop  registers  with  a  hole  in  the  plate  on  the  relay,  thus 
allowing  the  full  motion  of  the  rocker  arm  when  it  is  attracted  by 
the  magnet.  This  motion  of  the  rocker  arm  withdraws  the  detent 
pawl  from  engagement  with  the  segment  as  well  as  the  moving  pawl, 
and  thereby  permits  the  segment  to  return  to  its  normal  position. 
As  will  be  seen  from  Fig.  189,  each  of  the  relay  magnets  1  is  per- 
manently bridged  across  the  two  limbs  of  the  line. 

Each  station  is  provided  with  a  push  button,  not  saown,  by 
means  of  which  the  subscriber  who  makes  a  call  may  prevent  the 
rocker  arm  of  his  instrument  from  being  actuated  while  selective 
impulses  are  being  sent  over  the  line.  The  purpose  of  this  is  to 
enable  one  party  to  make  a  call  for  another  on  the  same  line,  de- 
pressing his  push  button  while  the  operator  is  selecting  and  ringing 
the  called  party.  The  segment  at  his  own  station,  therefore,  re- 
mains in  its  normal  position,  in  which  position,  as  we  have  already 
seen,  his  talking  circuit  is  closed;  all  of  the  other  segments  are, 
however,  stepped  up  until  the  ringing  and  talking  circuits  of  the 
desired  station  are  in  proper  position,  at  which  time  ringing  current 
is  sent  over  the  line.  The  segments  in  Fig.  189,  except  at  Station 

C,  are  shown  as  having  been  stepped  up  to  the  sixth  position,  which 
corresponds  to  the  ringing  position  of  the  fourth  station,  or  Station 

D.  The    condition  shown   in    this  figure  corresponds   to   that  in 
which    the     subscriber    at    Station    C    originated    the    call    and 
pressed  his  button,  thus  retaining  his  own  segment  in  its  normal 
position    so    that   the  talking   circuits   would    be  established   with 
Station  D. 

When  the  line  is  in  normal  position  any  subscriber  may  call 
central  by  his  magneto  generator,  not  shown  in  Fig.  189,  which 
will  operate  the  drop  at  central,  but  will  not  operate  any  of  the  sub- 
scribers' bells,  because  all  bell  circuits  are  normally  open.  WTien 
a  subscriber  desires  connection  with,  another  line,  the  operator  sends 
an  impulse  back  on  the  line  which  steps  up  and  locks  out  all  instru- 
ments except  that  of  the  calling  subscriber. 


LOCK-OUT  PARTY-LINE  SYSTEMS 


261 


A  complete  K.  B.  lock-out  telephone  is  shown  in  Fig.  190. 
This  is  the  type  of  instrument  that  is  usually  furnished  when  new 
equipment  is  ordered.  If,  however,  it  is  desired  to  use  the  K.  B. 
system  in  connection  with  telephones  of  the  ordinary  bridging  type 
that  are  already  in  service,  the  lock-out  and  selective  mechanism, 


Fig.  190.     K.  B.  Lock-Out  Station 


which  is  shown  on  the  upper  inner  face  of  the  door  in  Fig.  190,  is 
furnished  separately  in  a  box  that  may  be  mounted  close  to  the  regular 
telephone  and  connected  thereto  by  suitable  wires,  as  shown  in  Fig. 
191.  It  is  seen  that  this  instrument  employs  a  local  battery  for  talk- 
ing and  also  a  magneto  generator  for  calling  the  central  office. 


262 


TELEPHONY 


The  central-office  equipment  consists  of  a  dial  connected  with 
an  impulse  wheel,  together  with  suitable  keys  by  which  the  various 
circuits  may  be  manipulated.  This  dial  and  its  associated  mechan- 
ism may  be  mounted  in  the  regular  switchboard  cabinet,  or  it  may 
be  furnished  in  a  separate  box  and  mounted 
alongside  of  the  cabinet  in  either  of  the 
positions  shown  at  1  or  2  of  Fig.  192. 

In  order  to  send  the  proper  number  of 
impulses  to  the  line  to  call  a  given  party, 
the  operator  places  her  finger  in  the  hole  in 
the  dial  that  bears  the  number  corresponding 
to  the  station  wanted  and  rotates  the  dial 
until  the  finger  is  brought  into  engagement 
with  the  fixed  stop  shown  at  the  bottom  of  the 
dial  in  Fig.  192.  The  dial  is  then  allowed  to 
return  by  the  action  of  a  spring  to  its  normal 
position,  and  in  doing  so  it  operates  a  switch 
within  the  box  to  make  and  break  the  battery 
circuit  the  proper  number  of  times. 

Operation.  A  complete  description  of  the 
operation  may  now  be  had  in  connection 
with  Fig.  193,  which  is  similar  to  Fig.  189, 
but  contains  the  details  of  the  calling  ar- 
rangement at  the  central  office  and  also  of 
the  talking  circuits  at  the  various  subscribers'  stations. 

Referring  to  the  central-office  apparatus  the  usual  ringing  key 
is  shown,  the  inside  contacts  of  which  lead  to  the  listening  key  and  to 
the  operator's  telephone  set  as  in  ordinary  switchboard  practice. 
Between  the  outside  contact  of  this  ringing  key  and  the  ringing 
generator  there  is  interposed  a  pair  of  contact  springs  8-8  and 
another  pair  9-9.  The  contact  springs  8  are  adapted  to  be  moved 
backward  and  forward  by  the  impulse  wheel  which  is  directly  con- 
trolled by  the  dial  under  the  manipulation  of  the  operator.  When 
these  springs  8  are  in  their  normal  position,  the  ringing  circuit  is 
continued  through  the  release-key  springs  9  to  the  ringing  generator. 
These  springs  8  occupy  their  normal  position  only  when  the  dial 
is  in  its  normal  position,  this  being  due  to  the  notch  10  in  the  con- 
tact wheel.  At  all  other  times,  i.  e.,  while  the  impulse  wheel  is  out 


Fig.  191.     K.  B.  Lock- 
Out  Station 


LOCK-OUT  PARTY-LINE  SYSTEMS 


263 


of  its  normal  position,  the  springs  8-8  are  either  depressed  so  as  to 
engage  the  lower  battery  contacts,  or  else  held  in  an  intermediate 
position  so  as  to  engage  neither  the  battery  contacts  nor  the  generator 
contacts. 

When  it  is  desired  to  call  a  given  station,  the  operator  pulls  the 
subscriber's  number  on  the  dial  and  holds  the  ringing  key  closed, 


Fig.  192.     Calling  Apparatus  K.  B.  System 

allowing  the  dial  to  return  to  normal.  This  connects  the  impulse 
battery  to  the  subscriber's  line  as  many  times  as  is  required  to  move 
the  subscriber's  sectors  to  the  proper  position,  and  in  such  direction 
as  to  cause  the  stepping  movement  of  the  various  relays.  As  the 
impulse  wheel  conies  to  its  normal  position,  the  springs  8,  associated 
with  it,  again  engage  their  upper  contacts,  by  virtue  of  the  notch  10 


264 


TELEPHONY 


in  the  impulse  wheel,  and  this  establishes  the  connection  between 
the  ringing  generator  and  the  subscriber's  line,  the  ringing  key 
being  still  held  closed.  The  pulling  of  the  transmitter  dial  and 
holding  the  ringing  key  closed,  therefore,  not  only  sends  the  stepping 
impulses  to  line,  but  also  follows  it  by  the  ringing  current.  The 
sending  of  five  impulses  to  line  moves  all  of  the  sectors  to  the  sixth 
notch,  and  this  corresponds  to  the  position  necessary  to  make  the 
fourth  station  operative.  Such  a  condition  is  shown  in  Fig.  193, 
it  being  assumed  that  the  subscriber  at  Station  C  originated  the 
call  and  pressed  his  own  button  so  as  to  prevent  his  sector  from  be- 


Fig.  193.     Circuit  K.  B.  System 

ing  moved  out  of  its  normal  position.  As  a  result  of  this,  the  talk- 
ing circuit  at  Station  C  is  left  closed,  and  the  talking  and  the  ringing 
circuit  of  Station  D,  the  called  station,  are  closed,  while  both  the 
talking  and  the  ringing  circuits  of  all  the  other  stations  are  left 
open.  Station  D  may,  therefore,  be  rung  and  may  communicate 
with  Station  C,  while  all  of  the  other  stations  on  the  line  are  locked 
out,  because  of  the  fact  that  both  their  talking  and  ringing  circuits 
are  left  open. 

When  conversation  is  ended,  the  operator  is  notified  by  the 
usual  clearing-out  signal,  and  she  then  depresses  the  release  button, 
which  brings  the  springs  9  out  of  engagement  with  the  generator 
contact  but  into  engagement  with  the  battery  contact  in  such  rela- 
tion as  to  send  a  battery  current  on  the  line  in  the  reverse  direction 
from  that  sent  out  by  the  impulse  wheel.  This  sends  current 
through  all  of  the  relays  in  such  direction  as  to  withdraw  both  the 
moving  and  the  holding  pawls  from  the  segments  and  thus  allow  all 
of  the  segments  to  return  to  their  normal  positions.  Of  course, 
in  thus  establishing  the  release  current,  it  is  necessary  for  the  opera- 
tor to  depress  the  ringing  key  as  well  as  the  release  key. 


LOCK-OUT  PARTY-LINE  SYSTEMS 


265 


A  one-half  microfarad  condenser  is  placed  in  the  receiver  cir- 
cuit at  each  station  so  that  the  line  will  not  be  tied  up  should  some 
subscriber  inadvertently  leave  his  receiver  off  its  hook.  This  per- 
mits the  passage  of  voice  currents,  but  not  of  the  direct  currents 
used  in  stepping  the  relays  or  in  releasing  them. 

The  circuit  of  Fig.  193  is  somewhat  simplified  from  that  in 
actual  practice,  and  it  should  be  remembered  that  the  hook  switch, 
which  is  not  shown  in  this  figure,  controls  in  the  usual  way  the  con- 
tinuity of  the  receiver  and  the  transmitter  circuits  as  well  as  of  the 
generator  circuits,  the  generator  being  attached  to  the  line  as  in  an 
ordinary  telephone. 

Broken=Line  System.  The  broken-line  method  of  accom- 
plishing selective  signaling  and  locking-out  on  telephone  party  lines 
is  due  to  Homer  Roberts  and  his  associates. 

To  understand  just  how  the  principles  illustrated  in  Figs.  186 
and  187  are  put  into  effect,  it  will  be  necessary  to  understand  the 
latching  relay  shown  diagrammatically  in  its  two  possible  positions 
in  Fig.  194,  and  in  perspective  in  Fig.  195.  Referring  to  Fig.  194, 
the  left-hand  cut  of  which  shows  the  line  relay  in  its  normal  position, 
it  is  seen  that  the  framework  of  the  device  resembles  that  of  an  ordi- 
nary polarized  ringer. 
Under  the  influence  of 
current  in  one  direction 
flowing  through  the  left- 
hand  coil,  the  armature 
of  this  device  depresses 
the  hard  rubber  stud  4> 
and  the  springs  1,  2,  and 
3  are  forced  downwardly 
until  the  spring  2  has 
passed  under  the  latch 

carried  on  the  spring  5.  When  the  operating  current  through  the 
coil  6  ceases,  the  pressure  of  the  armature  on  the  spring  1  is  relieved, 
allowing  this  spring  to  resume  its  normal  position  and  spring  3  to 
engage  with  spring  2.  The  spring  2  cannot  rise,  since  it  is  held  by 
the  latch  5,  and  the  condition  shown  in  the  right-hand  cut  of  Fig. 
194  exists.  It  will  be  seen  that  the  spring  2  has  in  this  operation 
carried  out  just  the  same  function  as  the  switch  lever  performed 


Fig.  194.     Roberts  Latching  Relay 


266 


TELEPHONY 


as  described  in  connection  with  Figs.  186  and  187.  An  analysis 
of  this  action  will  show  that  the,  normal  contact  between  the  springs 
1  and  2,  which  contact  controls  the  circuit  through  the  relay  coil 
and  the  bell,  is  not  broken  until  the  coil  6  is  de-energized,  which 
means  that  the  magnet  is  effective  until  it  has  accomplished  its  work. 
It  is  impossible,  therefore,  for  this  relay  to  cut  itself  out  of  circuit 
before  it  has  caused  the  spring  2  to  engage  under  the  latch  5.  If 


Fig.  195.     Koberts  Latching  Relay 

current  of  the  proper  direction  were  sent  through  the  coil  7  of  the 
relay,  the  opposite  end  of  the  armature  would  be  pulled  down 
and  the  hard  rubber  stud  at  the  left-hand  end  of  the  armature 
would  bear  against  the  bent  portion  of  the  spring  5  in  such  manner 
as  to  cause  the  latch  of  this  spring  to  release  the  spring  2  and  thus 
allow  the  relay  to  assume  its  normal,  or  unlatched,  position. 

A  good  idea  of  the  mechanical  construction  of  this  relay  may  be 
obtained  from  Fig.  195.  The  entire  selecting  function  of  the  Roberts 
system  is  performed  by  this  simple  piece  of  apparatus  at  each  station. 

The  diagram  of  Fig.  196  shows,  in  simplified  form,  a  four- 
station  line,  the  circuits  being  given  more  in  detail  than  in  the  dia- 
grams of  Chapter  XVI. 


LOCK-OUT  PARTY-LINE  SYSTEMS 


267 


It  will  be  noticed  that  the  ringer  and  the  relay  coil  6  at  the  first 
station  are  bridged  across  the  sides  of  the  line  leading  to  the  central 
office.  In  like  manner  the  bell  and  the  relay  magnets  are  bridged 
across  the  two  limbs  of  the  line  leading  into  each  succeeding  station, 
but  this  bridge  at  each  of  the  stations  beyond  Station  A  is  ineffective 
because  the  line  extension  Rx  is  open  at  the  next  station  nearest  the 
central  office. 

In  order  to  ring  Station  A  it  is  only  necessary  to  send  out  ring- 
ing current  from  the  central  office.  This  current  is  in  such  direc- 
tion as  not  to  cause  the  operation  of  the  relay,  although  it  passes 
through  the  coil  6.  If,  on  the  other  hand,  it  is  desired  to  ring  Station 
B,  a  preliminary  impulse  would  be  sent  over  the  metallic  circuit 
from  the  central  office,  which  impulse  would  be  of  such  direction  as 
to  operate  the  relay  at  Station  A,  but  not  to  operate  the  bell  at  that 


STXT/ON-A- 


•STAT/ON-B-         STAT/OH-C- 


STAT/OM  -D- 


Fig.  196.     Simplified  Circuits  of  Roberts  System 

station.  The  operation  of  the  relay  at  Station  A  causes  the  spring 
2  of  this  relay  to  engage  the  spring  3,  thus  extending  the  line  on  to 
the  second  station.  After  the  spring  2  at  Station  A  has  been  forced 
into  contact  with  the  spring  3,  it  is  caught  by  the  latch  of  the  spring 
5  and  held  mechanically.  When  the  impulse  from  the  central  office 
ceases,  the  spring  1  resumes  its  normal  position,  thus  breaking  the 
bridge  circuit  through  the  bell  at  that  station.  It  is  apparent  now 
that  the  action  of  coil  6  at  Station  A  has  made  the  relay  powerless 
to  perform  any  further  action,  and  at  the  same  time  the  line  has 
been  extended  on  to  the  second  station.  A  second  similar  impulse 
from  the  central  office  will  cause  the  relay  at  Station  B  to  extend  the 
line  on  to  Station  C,  and  at  the  same  time  break  the  circuit  through 
the  operating  coil  and  the  bell  at  Station  B.  In  this  way  any  sta- 
tion may  be  picked  out  by  sending  the  proper  number  of  impulses 
to  operate  the  line  relays  of  all  the  stations  between  the  station  de- 


268 


TELEPHONY 


sired  and  the  central  office,  and  having  picked  out  a  station  it  is 
only  necessary  to  send  out  ringing  current,  which  current  is  in  such 
direction  as  to  ring  the  bell  but  not  to  operate  the  relay  magnet  at 
that  station. 

In  Fig.  197,  a  four-station  line,  such  as  is  shown  in  Fig.  196,  is 
illustrated,  but  the  condition  shown  in  this  is  that  existing  when 
two  preliminary  impulses  have  been  sent  over  the  line,  which  caused 


S7AT/OM-A- 


STAT/ON-B- 


S7XT/OH-C- 


•STAT/0/y-D- 


Fig.  197.     Simplified  Circuits  of  Roberts  System 

the  line  relays  at  Station  A  and  Station  B  to  be  operated.  The  bell 
at  Station  C  is,  therefore,  the  only  one  susceptible  to  ringing  current 
from  the  central  office. 

Since  only  one  bell  and  one  relay  are  in  circuit  at  any  one 
time,  it  is  obvious  that  all  of  the  current  that  passes  over  the  line 
is  effective  in  operating  a  single  bell  or  relay  only.  There  is  no 
splitting  up  of  the  current  among  a  large  number  of  bells  as  in  the 
bridging  system  of  operating  step-by-step  devices,  which  method 
sometimes  so  greatly  reduces  the  effective  current  for  each  bell  that 
it  is  with  great  difficulty  made  to  respond.  All  the  energy  avail- 
able is  applied  directly  to  the  piece  of  apparatus  at  the  time  it  is 
being  operated.  This  has  a  tendency  toward  greater  surety  of  action, 
and  the  adjustment  of  the  various  pieces  of  apparatus  may  be  made 
with  less  delicacy  than  is  required  where  many  pieces  of  apparatus, 
each  having  considerable  work  to  do,  must  necessarily  be  operated 
in  multiple. 

The  method  of  unlatching  the  relays  has  been  'briefly  referred 
to.  After  a  connection  has  been  established  with  a  station  in  the 
manner  already  described,  the  operator  may  clear  the  line  when 
it  is  proper  to  do  so  by  sending  impulses  of  such  a  nature  as  to  cause 
the  line  relays  of  the  stations  beyond  the  one  chosen  to  operate,  thus 


LOCK-OUT  PARTY-LINE  SYSTEMS 


269 


continuing  the  circuit  to  the  end  of  the  line.  The  operation  of  the 
line  relay  at  the  last  station  brings  into  circuit  the  coil  8,  Figs.  196 
and  197,  of  a  grounding  device.  This  is  similar  to  the  line  relay, 
but  it  holds  its  operating  spring  in  a  normally  latched  position  so 
as  to  maintain  the  two  limbs  of  the  line  disconnected  from  the 
ground.  The  next  impulse  following  over  the  metallic  circuit 
passes  through  the  coil  8  and  causes  the  operation  of  this  ground- 
ing device  which,  by  becoming  unlatched,  grounds  the  limb  L  of 
the  line  through  the  coil  8.  This  temporary  ground  at  the  end  of 
the  line  makes  it  possible  to  send  an  unlocking  or  restoring  cur- 
rent from  the  central  office  over  the  limb  L,  which  current  passes 
through  all  of  the  unlocking  coils  7,  shown  in  Figs.  194,  196,  and 
197,  thus  causing  the  simultaneous  unlocking  of  all  of  the  line  relays 
and  the  restoration  of  the  line  to  its  normal  condition,  as  shown 
in  Fig.  196. 

As  has  been  stated,  the  windings  7  on  the  line  relays  are  the 
unlatching  windings.  In  Figs.  196  and  197,  for  the  purpose  of 
simplicity,  these  windings  are  not  shown  connected,  but  as  a  matter 
of  fact  each  of  them  is  included  in  series  in  the  continuous  limb  L 
of  the  line.  This  would  introduce  a  highly  objectionable  feature 
from  the  standpoint  of  talking 
over  the  line  were  it  not  for  the 
balancing  coils  71,  each  wound 
on  the  same  core  as  .the  corre- 
sponding winding  7,  and  each 
included  in  series  in  the  limb  R 
of  the  line,  and  in  such  direc- 
tion as  to  be  differential  thereto 
with  respect  to  currents  passing 
in  series  over  the  two  limbs  of 
the  line. 

The  windings  7  are  the 
true  unlocking  windings,  while 
the  windings  71  have  no  other  function  than  to  neutralize  the  in- 
ductive effects  of  these  unlocking  windings  necessarily  placed  in 
series  in  the  talking  circuit.  All  of  these  windings  are  of  low 
ohmic  resistance,  a  construction  which,  as  has  previously  been 
noted,  brings  about  the  desired  effect  without  introducing  any 


Pig.  198.     Details  of  Latching 
Relay  Connections 


TELEPHONY 


self-induction  in  the  line,  and  without  producing  any  appreciable 
effect  upon  the  transmission.  A  study  of  Fig.  198  will  make  clear 
the  connections  of  these  unlocking  and  balancing  windings  at  each 
station. 

The  statement  of  operation  so  far  given  discloses  the  general 
method  of  building  up  the  line  in  sections  in  order  to  choose  any 
party  and  of  again  breaking  it  up  into  sections  when  the  conversa- 
tion is  finished.  It  has  been  stated  that  the  same  operation  which 
selects  the  party  wanted  also  serves  to  give  that  party  the  use  of  the 
line  and  to  lock  the  others  off.  That  this  is  true  will  be  understood 
when  it  is  stated  that  the  ringer  is  of  such  construction  that  when 
operated  to  ring  the  subscriber  wanted,  it  also  operates  to  unlatch 
a  set  of  springs  similar  to  those  shown  in  Fig.  194,  this  unlatching 
causing  the  proper  connection  of  the  subscriber's  talking  circuit 
across  the  limbs  of  the  line,  and  also  closing  the  local  circuit  through 
his  transmitter.  The  very  first  motion  of  the  bell  armature  performs 
this  unlatching  operation  after  which  the  bell  behaves  exactly  as 
an  ordinary  polarized  biased  ringer. 


F3  I 


Fig.  199.     Broken-Back  Ringer 

The  construction  of  this  ringer  is  interesting  and  is  shown  in 
its  two  possible  positions  in  Fig.  199.  The  group  of  springs  car- 
ried on  its  frame  is  entirely  independent  of  the  movement  of  the 
armature  during  the  ringing  operation.  With  reversed  currents, 
however,  the  armature  is  moved  in  the  opposite  direction  from  that 
necessary  to  ring  the  bells,  and  this  causes  the  latching  of  the  springs 
into  their  normal  position.  In  order  that  this  device  may  perform 


LOCK-OUT  PARTY-LINE  SYSTEMS 


271 


the  double  function  of  ringer  and  relay  the  tapper  rod  of  the  bell 
is  hinged  on  the  armature  so  as  to  partake  of  the  movements  of  the 
armature  in  one  direction  only.     This  has  been  called  by  the  in- 
ventor and  engineers  of  the  Roberts  system  a  broken-back  ringer,, 
a  name  suggestive  of  the  movable 
relation  between  the  armature  and 
the  tapper  rod.    The  construction 
of   the  ringer  is  of  the  same  na- 
ture as  that  of  the  standard  polar- 
ized ringer  universally  employed, 
but  a  hinge   action  between   the 
armature  and  the  tapper  rod,  of 
such  nature  as  to  make  the  tap- 
per   partake     positively    of     the 
movements  of    the    armature    in 
one  direction,  but  to  remain  per- 
fectly quiescent  when  the  arma- 
ture moves  in  the  other  direction,      Fig  200     Details  of  Ringer  ConnectioQ 
is  provided. 

How  this  broken-back  ringer  controls  the  talking  and  the  lock- 
ing-out conditions  may  best  be  understood  in  connection  with  Fig. 
200.  The  ringer  springs  are  normally  latched  at  all  stations.  Un- 
der these  conditions  the  receiver  is  short-circuited  by  the  engagement 
of  springs  10  and  11,  the  receiver  circuit  is  open  between  springs  10 
and  12,  and  the  local-battery  circuit  is  open  between  springs  9  and 
12.  The  subscribers  whose  ringers  are  latched  are,  therefore,  locked 
out  in  more  ways  than  one. 

When  the  bell  is  rung,  the  first  stroke  it  makes  unlatches  the 
springs,  which  assume  the  position  shown  in  the  right-hand  cut 
of  Fig.  199,  and  this,  it  will  be  seen  from  Fig.  200,  establishes 
proper  conditions  for  enabling  the  subscriber  to  transmit  and  to 
receive  speech. 

The  hook  switch  breaks  both  transmitter  and  receiver  circuits 
when  down  and  in  raising  it  establishes  a  momentary  circuit  between 
the  ground  and  the  limb  L  of  the  line,  both  upper  and  lower  hook 
contacts  engaging  the  hook  lever  simultaneously  during  the  rising 
of  the  hook. 

The  mechanism  at  the  central  office  by  which  selection  of  the 


272 


TELEPHONY 


proper  station  is  made  in  a  rapid  manner  is  shown  in  Fig.  201.  It 
has  already  been  stated  that  the  selection  of  the  proper  subscriber 
is  brought  about  by  the  sending  of  a  predetermined  number  of 
impulses  from  the  central  office,  these  impulses  passing  in  one  di- 
rection only  and  over  the  metallic  circuit.  After  the  proper  party 
has  been  reached,  the  ringing  current  is  put  on  in  the  reverse  di- 
rection. 

The  operator  establishes  the  number  of  impulses  to  be  sent  by 
placing  the  pointer  opposite  the  number  on  the  dial  corresponding 


Fig.  201.     Central-Office  Impulse  Transmitter 

to  the  station  wanted.  The  ratchet  wheel  is  stepped  around  auto- 
matically by  each  impulse  of  current  from  an  ordinary  pole  changer 
such  as  is  employed  in  ringing  biased  bells.  When  the  required 
number  of  impulses  has  been  sent,  a  projection,  carried  on  a  group 
of  springs,  drops  into  a  notch  on  the  drum  of  the  selector  shaft, 
which  operation  instantly  stops  the  selecting  current  impulses  and  at 
the  same  time  throws  on  the  ringing  current  which  consists  of  im- 
pulses in  the  reverse  direction.  So  rapidly  does  this  device  operate 
that  it  will  readily  follow  the  impulses  of  an  ordinary  pole  changer, 
even  when  this  is  adjusted  to  its  maximum  rate  of  vibration. 

Operation.  Space  will  not  permit  a  full  discussion  of  the 
details  of  the  central-office  selective  apparatus,  but  a  general 
resume  of  the  operation  of  the  system  may  now  be  given,  with  the 
aid  of  Fig.  202,  which  shows  a  four-station  line  with  the  circuits  of 
three  of  the  stations  somewhat;  simplified.  In  this  figure  Station 


LOCK-OUT  PARTY-LINE  SYSTEMS 


273 


A,  Station  B,  and  Station  D  are  shown  in  their  locked-out  positions, 
A  and  B  having  been  passed  by  the  selection  and  ringing  of  Station 
C,  while  Station  D  is  inoperative  because  it  was  not  reached  in 
the  selection  and  the  line  is  still  broken  at  Station  C.  Station  C, 
therefore,  has  possession  of  the  line. 

When  the  subscriber  at  Station  C  raised  his  receiver  in  order  to 
call  central,  a  "flash"  contact  was  made  as  the  hook  moved  up,  which 
momentarily  grounded  the  limb  L  of  the  line.  (See  Fig.  200.)  This 
"flash"  contact  is  produced  by  the  arrangement  of  the  hook  which 
assures  that  the  lower  contact  shall,  by  virtue  of  its  flexibility, 
follow  up  the  hook  lever  until  the  hook  lever  engages  the  upper 
contact,  after  which  the  lower  contact  breaks.  This  results  in  the 
momentary  connection  of  both  the  upper  and  the  lower  contacts  of 
the  hook  with  the  lever,  and,  therefore,  the  momentary  grounding 
of  the  limb  L  of  the  line.  This  limb  always  being  continuous  serves, 
when  this  "flash"  contact  is  made,  to  actuate  the  line  signal  at  the 
central  office. 

Since,  however,  all  parties  on  the  line  are  normally  locked  out 
of  talking  circuits,  some  means  must  be  provided  whereby  the  oper- 
ator may  place  the  signaling  party  in  talking  connection  and  leave 
all  the  other  instruments  on  the  line  in  their  normally  locked-out  po- 
sition. In  fact,  the  operator  must  be  able  automatically  to  pick 
out  the  station  that  signaled  in,  and  operate  the  ringer  to  unlatch 


STAT/OH-A-        5TAT/OM-B-        STAT/ON-C- 


Fig.  202.     Circuits  of  Roberts  Line 

the  springs  controlling  the  talking  circuit  of  that  station.  Accord- 
ingly the  operator  sends  impulses  on  the  line,  from  a  grounded 
battery,  which  are  in  the  direction  to  operate  the  line  relays  and  to 
continue  the  line  circuit  to  the  station  calling.  When,  after  a  sufficient 
number  of  impulses,  this  current  reaches  that  station  it  finds  a  path 


274  TELEPHONY 

to  ground  from  the  limb  L.  This  path  is  made  possible  by  the  fact 
that  the  subscriber's  receiver  is  off  its  hook  at  that  station.  In  or- 
der to  understand  just  how  this  ground  connection  is  made,  it  must 
be  remembered  that  each  of  the  ringer  magnets  is  energized  with 
each  selecting  impulse,  but  in  such  a  direction  as  not  to  ring  the 
bells,  it  being  understood  that  all  of  the  ringer  mechanisms  are  nor- 
mally latched.  When  the  selecting  impulse  for  Station  C  arrives, 
it  passes  through  the  ringer  and  the  selecting  relay  coils  at  that 
station  and  starts  to  operate  the  remainder  of  the  ringers  sufficiently 
to  cause  the  spring  12  to  engage  the  spring  13.  This  establishes  the 
ground  connection  from  the  limb  L  of  the  line,  the  circuit  being 
traced  through  limb  L  through  the  upper  contact  of  the  switch,  thence 
through  springs  12  and  13  to  ground,  and  this,  before  the  line  relay 
has  time  to  latch,  operates  the  quick-acting  relay  at  the  central  office, 
which  acts  to  cut  off  further  impulses,  and  thus  automatically  stops 
at  the  calling  station.  Ringing  current  in  the  opposite  direction  is 
then  sent  to  line;  this  unlatches  the  ringer  springs  and  places  the 
calling  subscriber  in  talking  circuit.  When  the  operator  has  com- 
municated with  the  calling  subscriber,  and  found,  for  example, 
that  another  party  on  another  similar  line  is  desired,  she  turns  the 
dial  pointer  on  the  selector  to  the  number  corresponding  to  the 
called-for  party's  number  on  that  line,  and  presses  the  signal  key. 
Pressing  this  key  causes  impulses  to  "run  down  the  line,"  selecting 
the  proper  party  and  ringing  his  bell  in  the  manner  already  described. 
The  connection  between  the  two  parties  is  then  established,  and  no 
one  else  can  in  any  possible  way,  except  by  permission  of  the  oper- 
ator, obtain  access  to  the  line. 

It  is  obvious  that  some  means  must  be  provided  for  restoring 
the  selecting  relays  to  normal  after  a  conversation  is  finished.  By 
referring  to  Fig.  194  it  will  be  seen  that  the  upper  end  of  the  latch 
spring  5  is  bent  over  in  such  a  manner  that  when  the  armature  is 
attracted  by  current  flowing  through  the  coil  7,  the  knob  on  the 
left-hand  end  of  the  armature  on  rising  engages  with  the  bent  cam 
surface  and  forces  back  the  latch,  permitting  spring  2  to  return  to  its 
normal  position. 

To  restore  the  line  the  operator  sends  out  sufficient  additional 
selective  impulses  to  extend  the  circuit  to  the  end  of  the  line,  and 
thus  brings  the  grounder  into  circuit.  The  winding  of  the  grounder  is 


LOCK-OUT  PARTY-LINE  SYSTEMS  275 

connected  in  such  a  manner  that  the  next  passing  impulse  throws  off 
its  latch,  permitting  the  long  spring  to  contact  with  the  ground  spring. 
The  operator  now  sends  a  grounded  impulse  over  the  continuous 
limb  L  of  the  line  which  passes  through  the  restoring  coils  7  at  all  the 
stations  and  through  the  right-hand  coil  of  the  grounding  device 
to  ground.  The  selecting  relays  are,  therefore,  simultaneously 
restored  to  normal.  The  grounder  is  also  energized  and  restored  to 
its  normal  position  by  the  same  current. 

If  a  party  in  calling  finds  that  his  own  line  is  busy  and  he  cannot 
get  central,  he  may  leave  his  receiver  off  its  hook.  When  the  party 
who  is  using  the  line  hangs  up  his  receiver  the  fact  that  another  party 
desires  a  connection  is  automatically  indicated  to  the  operator,  who 
then  locks  out  the  instrument  of  the  party  who  has  just  finished 
conversation  and  passes  his  station  by.  When  the  operator  again 
throws  the  key,  the  waiting  subscriber  is  automatically  selected  in 
the  same  manner  as  was  the  first  party.  If  there  are  no  subscribers 
waiting  for  service,  the  stop  relay  at  central  will  not  ooerate  until 
the  grounder  end  of  the  line  is  unlatched,  the  selecting  relays  being 
then  restored  automatically  to  normal. 

The  circuits  are  so  organized  that  at  all  times  whether  the  line 
is  busy  or  not,  the  movement  up  and  down  of  the  switch  hook,  at 
any  sub-station,  operates  a  signal  before  the  operator.  Such  a  move- 
ment, when  made  slowly  and  repeatedly,  indicates  to  the  operator 
that  the  subscriber  has  an  emergency  call  and  she  may  use  her  judg- 
ment as  to  taking  the  line  away  from  the  parties  who  are  using  it, 
and  finding  out  what  the  emergency  call  is  for.  If  the  operator  finds 
that  the  subscriber  has  misused  this  privilege  of  making  the  emer- 
gency call,  she  may  restore  the  connection  to  the  parties  previously 
engaged  in  conversation. 

One  of  the  salient  points  of  this  Roberts  system  is  that  the 
operator  always  has  control  of  the  line.  A  subscriber  is  not  able 
even  to  use  his  own  battery  till  permitted  to  do  so.  A  subscriber 
who  leaves  his  receiver  off  its  hook  in  order  that  he  may  be  signaled 
by  the  operator  when  the  line  is  free,  causes  no  deterioration  of  the 
local  battery  because  the  battery  circuit  is  held  open  by  the  switch 
contacts  carried  on  the  ringer.  It  cannot  be  denied,  however,  that 
this  system  is  complicated,  and  that  it  has  other  faults.  For  instance, 
as  described  herein,  both  sides  of  the  line  must  be  looped  into  eacb 


276  TELEPHONY 

subscriber's  station,  thus  requiring  four  drop,  or  service,  wires  instead 
of  two.  It  is  possible  to  overcome  this  objection  by  placing  the 
line  relays  on  the  pole  in  a  suitably  protected  casing,  in  which  case  it 
is  sufficient  to  run  but  two  drop  wires  from  the  nearer  line  to  station. 
There  are  undoubtedly  other  objections  to  this  system,  and  yet  with 
all  its  faults  it  is  of  great  interest,  and  although  radical  in  many  re- 
spects, it  teaches  lessons  of  undoubted  value. 


CHAPTER  XVIII 
ELECTRICAL  HAZARDS 

All  telephone  systems  are  exposed  to  certain  electrical  hazards. 
When  these  hazards  become  actively  operative  as  causes,  harmful 
results  ensue.  The  harmful  results  are  of  two  kinds:  those  causing 
damage  to  property  and  those  causing  damage  to  persons.  The 
damage  to  persons  may  be  so  serious  as  to  result  in  death.  Dam- 
age to  property  may  destroy  the  usefulness  of  a  piece  of  apparatus 
or  of  some  portion  of  the  wire  plant.  Or  the  property  damage 
may  initiate  itself  as  a  harm  to  apparatus  or  wiring  and  may  result 
in  greater  and  extending  damage  by  starting  a  fire. 

Electrical  currents  which  endanger  life  and  property  may  be 
furnished  by  natural  or  artificial  causes.  Natural  electricity  which 
does  such  damage  usually  displays  itself  as  lightning.  In  rare  cases, 
currents  tending  to  flow  over  grounded  lines  because  of  extraordi- 
nary differences  of  potential  between  sections  of  the  earth's  surface 
have  damaged  apparatus  in  such  lines,  or  only  have  been  prevented 
from  causing  such  damage  by  the  operation  of  protective  devices. 

Telegraph  and  telephone  systems  have  been  threatened  by 
natural  electrical  hazards  since  the  beginning  of  the  arts  and  by 
artificial  electrical  hazards  since  the  development  of  electric  light 
and  power  systems.  At  the  present  time,  contrary  to  the  general 
supposition,  it  is  in  the  artificial,  and  not  in  the  natural  electrical 
hazards  that  the  greater  variety  and  degree  of  danger  lies. 

Of  the  ways  in  which  artificial  electricity  may  injure  a  telephone 
system,  the  entrance  of  current  from  an  external  electrical  power 
system  is  a  greater  menace  than  an  abnormal  flow  of  current  from 
a  source  belonging  to  the  telephone  system  itself.  Yet  modern 
practice  provides  opportunities  for  a  telephone  system  to  inflict 
damage  upon  itself  in  that  way.  Telephone  engineering  designs 
need  to  provide  means  for  protecting  all  parts  of  a  system  against 
damage,  from  external  ("foreign")  as  well  as  internal  ("domestic") 


278  TELEPHONY 

hazards,  and  to  cause  this  protection  to  be  inclusive  enough  to  pro- 
tect persons  against  injury  and  property  from  damage  by  any  form 
of  overheating  or  electrolytic  action. 

A  part  of  a  telephone  system  for  which  there  is  even  a  remote 
possibility  of  contact  with  an  external  source  of  electrical  power, 
whether  natural  or  artificial,  is  said  to  be  exposed  to  electrical  haz- 
ard. The  degree  or  character  of  possible  contact  or  other  interfer- 
ence often  is  referred  to  in  relative  terms  of  exposure.  The  same 
terms  are  used  concerning  inductive  relations  between  circuits.  The 
whole  tendency  of  design,  particularly  of  wire  plants,  is  to  arrange 
the  circuits  in  such  a  way  as  to  limit  the  exposure  as  greatly  as  possi- 
ble, the  intent  being  to  produce  a  condition  in  which  all  parts  of  the 
system  will  be  unexposed  to  hazards. 

Methods  of  design  are  not  yet  sufficiently  advanced  for  any 
plant  to  be  formed  of  circuits  wholly  unexposed,  so  that  protective 
means  are  required  to  safeguard  apparatus  and  circuits  in  case  the 
hazard,  however  remote,  becomes  operative. 

Lightning  discharges  between  the  clouds  and  earth  frequently 
charge  open  wires  to  potentials  sufficiently  high  to  damage  apparatus; 
and  less  frequently,  to  destroy  the  wires  of  the  lines  themselves. 
Lightning  discharges  between  clouds  frequently  induce  charges  in 
lines  sufficient  to  damage  apparatus  connected  with  the  lines.  Heavy 
rushes  of  current  in  lines,  from  lightning  causes,  occasionally  induce 
damaging  currents  in  adjacent  lines  not  sufficiently  exposed  to  the 
original  cause  to  have  been  injured  without  this  induction.  The 
lightning  hazard  is  least  where  the  most  lines  are  exposed.  In  a 
small  city  with  all  of  the  lines  formed  of  exposed  wires  and  all  of 
them  used  as  grounded  circuits,  a  single  lightning  discharge  may 
damage  many  switchboard  signals  and  telephone  ringers  if  there 
be  but  100  or  200  lines,  while  the  damage  might  have  been  nothing 
had  there  been  800  to  1,000  lines  in  the  same  area. 

Means  of  protecting  lines  and  apparatus  against  damage  by 
lightning  are  little  more  elaborate  than  in  the  earliest  days  of  tele- 
graph working.  They  are  adequate  for  the  almost  entire  protection 
of  life  and  of  apparatus. 

Power  circuits  are  classified  by  the  rules  of  various  governing 
bodies  as  high-potential  and  low-potential  circuits.  The  classifica- 
tion of  the  National  »Board  of  Fire  Underwriters  in  the  United  States 


ELECTRICAL  HAZARDS  279 

defines  low-potential  circuits  as  having  pressures  below  550  volts; 
high-potential  circuits  as  having  pressures  from  550  to  3,500  volts, 
and  extra  high-potential  circuits  as  having  pressures  above  3,500 
volts.  Pressures  of  100,000  volts  are  becoming  more  common. 
Where  power  is  valuable  and  the  distance  over  which  it  is  to  be 
transmitted  is  great,  such  high  voltages  are  justified  by  the  economics 
of  the  power  problem.  They  are  a  great  hazard  to  telephone  sys- 
tems, however.  An  unprotected  telephone  system  meeting  such  a 
hazard  by  contact  will  endanger  life  and  property  with  great  cer- 
tainty. A  very  common  form  of  distribution  for  lighting  and  power 
purposes  is  the  three-wire  system  having  a  grounded  neutral  wire, 
the  maximum  potential 'above  the  earth  being  about  115  volts. 

Telephone  lines  and  apparatus  are  subject  to  damage  by  any 
power  circuit  whether  of  high  or  low  potential.  The  cause  of  prop- 
erty damage  in  all  cases  is  the  flow  of  current.  Personal  damage, 
if  it  be  death  from  shock,  ordinarily  is  the  result  of  a  high  potential 
between  two  parts  of  the  body.  The  best  knowledge  indicates  that 
death  uniformly  results  from  shock  to  the  heart.  It  is  believed  that 
death  has  occurred  from  shock  due  to  pressure  as  low  as  100  volts. 
The  critical  minimum  voltage  which  can  not  cause  death  is  not 
known.  A  good  rule  is  never  willingly  to  subject  another  person  to 
personal  contact  with  any  electrical  pressure  whatever. 

Electricity  can  produce  actions  of  four  principal  kinds:  physio- 
logical, thermal,  chemical,  and  magnetic.  Viewing  electricity  as 
establishing  hazards,  the  physiological  action  may  injure  or  kill  liv- 
ing things;  the  thermal  action  may  produce  heat  enough  to  melt 
metals,  to  char  things  which  can  be  burned,  or  to  cause  them  actually 
to  burn,  perhaps  with  a  fire  which  can  spread;  the  chemical  action 
may  destroy  property  values  by  changing  the  state  of  metals,  as  by 
dissolving  them  from  a  solid  state  where  they  are  needed  into  a  state 
of  solution  where  they  are  not  needed;  the  magnetic  action  intro- 
duces no  direct  hazard.  The  greatest  hazard  to  which  property 
values  are  exposed  is  the  electro-thermal  action;  that  is,  the  same 
useful  properties  by  which  electric  lighting  and  electric  heating 
thrive  may  produce  heat  where  it  is  not  wanted  and  in  an  amount 
greater  than  can  safely  be  borne. 

The  tendency  of  design  is  to  make  all  apparatus  capable  of 
carrying  without  overheating  any  current  to  which  voltage  within 


280  TELEPHONY 

the  telephone  system  may  subject  it,  and  to  provide  the  system  so 
designed  with  specific  devices  adapted  to  isolate  it  from  currents 
originating  without.  Apparatus  which  is  designed  in  this  way, 
adapted  not  only  to  carry  its  own  normal  working  currents  but  to 
carry  the  current  which  would  result  if  a  given  piece  of  apparatus 
were  connected  directly  across  the  maximum  pressure  within  the 
telephone  system  itself,  is  said  to  be  self-protecting.  Apparatus 
amply  able  to  carry  its  maximum  working  current  but  likely  to  be 
overheated,  to  be  injured,  or  perhaps  to  destroy  itself  and  set  fire 
to  other  things  if  subjected  to  the  maximum  pressure  within  the 
system,  is  not  self-protecting  apparatus. 

To  make  all  electrical  devices  self-protecting  by  surrounding 
them  with  special  arrangements  for  warding  off  abnormal  currents 
from  external  sources,  is  not  as  simple  as  might  appear.  A  lamp, 
for  example,  which  can  bear  the  entire  pressure  of  a  central-office 
battery,  is  not  suitable  for  direct  use  in  a  line  several  miles  long 
because  it  would  not  give  a  practical  signal  in  series  with  that  line 
and  with  the  telephone  set,  as  it  is  required  to  do.  A  lamp  suitable 
for  use  in  series  with  such  a  line  and  a  telephone  set  would  burn 
out  by  current  from  its  own  normal  source  if  the  line  should  become 
short-circuited  in  or  near  the  central  office.  The  ballast  referred 
to  in  the  chapter  on  "Signals"  was  designed  for  the  very  purpose  of 
providing  rapidly-rising  resistance  to  offset  the  tendency  toward 
rapidly-rising  current  which  could  burn  out  the  lamp. 

As  another  example,  a  very  small  direct-current  electric  motor 
can  be  turned  on  at  a  snap  switch  and  will  gain  speed  quickly  enough 
so  that  its  armature  winding  will  not  be  overheated.  A  larger  motor 
of  that  kind  can  not  be  started  safely  without  introducing  resistance 
into  the  armature  circuit  on  starting,  and  cutting  it  out  gradually 
as  the  armature  gains  speed.  Such  a  motor  could  be  made  self- 
protecting  by  having  the  armature  winding  of  much  larger  wire  than 
really  is  required  for  mere  running,  choosing  its  size  great  enough 
to  carry  the  large  starting  current  without  overheating  itself  and  its 
insulation.  It  is  better,  and  for  long  has  been  standard  practice, 
to  use  starting  boxes,  frankly  admitting  that  such  motors  are  not 
self-protecting  until  started,  though  they  are  self-protecting  while 
running  at  normal  speeds.  Such  a  motor,  once  started,  may  be 
overloaded  so  as  to  be  slowed  down.  So  much  more  current  now 


ELECTRICAL  HAZARDS  281 

can  pass  through  the  armature  that  its  winding  is  again  in  danger. 
Overload  circuit-breakers  are  provided  for  the  very  purpose  of  tak- 
ing motors  out  of  circuit  in  cases  where,  once  up  to  speed,  they  are 
mechanically  brought  down  again  and  into  danger.  Such  a  circuit- 
breaker  is  a  device  for  protecting  against  an  internal  hazard;  that 
is,  internal  to  the  power  system  of  which  the  motor  is  a  part. 

Another  example:  In  certain  situations,  apparatus  intended 
to  operate  under  impulses  of  large  current  may  be  capable  of  carry- 
ing its  normal  impulses  successfully  but  incapable  of  carrying  cur- 
rents from  the  same  pressure  continuously.  Protective  means  may 
be  provided  for  detaching  such  apparatus  from  the  circuit  whenever 
the  period  in  which  the  current  acts  is  not  short  enough  to  insure 
safety.  This  is  cited  as  a  case  wherein  a  current,  normal  in  amount 
but  abnormal  in  duration,  becomes  a  hazard. 

The  last  mentioned  example  of  damage  from  internal  hazards 
brings  us  to  the  law  of  the  electrical  generation  of  heat.  The  greater 
the  current  or  the  greater  the  resistance  of  the  conductor  heated  or  the 
longer  the  time,  the  greater  will  be  the  heat  generated  in  that  conductor. 
But  this  generated  heat  varies  directly  as  the  resistance  and  as  the 
time  and  as  the  square  of  the  current,  that  is,  the  law  is 

Heat  generated  =  C2Rt 

in  which  C  =  the  current;  .R=the  resistance  of  the  conductor;  and 
t  =  the  time. 

It  is  obvious  that  a  protective  device,  such  as  an  overload  cir- 
cuit-breaker for  a  motor,  or  a  protector  for  telephone  apparatus, 
needs  to  operate  more  quickly  for  a  large  current  than  for  a  small 
one,  and  this  is  just  what  all  well-designed  protective  devices  are 
intended  to  do.  The  general  problem  which  these  heating  hazards 
present  with  relation  to  telephone-  apparatus  and  circuits  is:  To 
cause  all  parts  of  the  telephone  system  to  be  made  so  as  to  carry  suc- 
cessfully all  currents  which  may  flow  in  them  because  of  any  internal 
or  external  pressure,  or  to  supplement  them  by  devices  which  will  stop 
or  divert  currents  which  could  overheat  them. 

Electrolytic  hazards  depend  not  on  the  heating  effects  of  cur- 
rents but  on  their  chemical  effects.  The  same  natural  law  which 
enables  primary  and  secondary  batteries  to  be  useful  provides  a 
hazard  which  menaces  telephone-cable  sheaths  and  other  conductors. 


282  TELEPHONY 

When  a  current  leaves  a  metal  in  contact  with  an  electrolyte, 
the  metal  tends  to  dissolve  into  the  electrolyte.  In  the  processes  of 
electroplating  and  electrotyping,  current  enters  the  bath  at  the  anode, 
passes  from  the  anode  through  the  solution  to  the  cathode,  removing 
metal  from  the  former  and  depositing  it  upon  the  latter.  In  a  pri- 
mary battery  using  zinc  as  the  positive  element  and  the  negative 
terminal,  current  is  caused  to  pass,  within  the  cell,  from  the  zinc 
to  the  negative  element  and  zinc  is  dissolved.  Following  the  same 
law,  any  pipe  buried  in  the  earth  may  serve  to  carry  current  from 
one  region  to  another.  As  single-trolley  traction  systems  with 
positive  trolley  wires  constantly  are  sending  large  currents  through 
the  earth  toward  their  power  stations,  such  a  pipe  may  be  of 
positive  potential  with  relation  to  moist  earth  at  some  point  in 
its  length.  Current  leaving  it  at  such  a  point  may  cause  its  metal 
to  dissolve  enough  to  destroy  the  usefulness  of  the  pipe  for  its 
intended  purpose. 

Lead-sheathed  telephone  cables  in  the  earth  are  particularly 
exposed  to  such  damage  by  electrolysis.  The  reasons  are  that  such 
cables  often  are  long,  have  a  good  conductor  as  the  sheath-metal, 
and  that  metal  dissolves  readily  in  the  presence  of  most  aqueous 
solutions  when  electrolytic  differences  of  potential  exist.  The 
length  of  the  cables  enables  them  to  connect  between  points  of  con- 
siderable difference  of  potential.  It  is  lack  of  this  length  which 
prevents  electrolytic  damage  to  masses  of  structural  metal  in  the 
earth. 

Electrical  power  is  supplied  to  single-trolley  railroads  princi- 
pally in  the  form  of  direct  current.  Usually  all  the  trolley  wires  of 
a  city  are  so  connected  to  the  generating  units  as  to  be  positive  to 
the  rails.  This  causes  current  to  flow  from  the  cars  toward  the  power 
stations,  the  return  path  being  made  up  jointly  of  the  rails,  the  earth 
itself,  actual  return  wires  which  may  supplement  the  rails,  and  also 
all  other  conducting  things  in  the  earth,  these  being  principally  lead- 
covered  cables  and  other  pipes.  These  conditions  establish  definite 
areas  in  which  the  currents  tend  to  leave  the  cables  and  pipes,  i.  e.,  in 
which  the  latter  are  positive  to  other  things.  These  positive  areas 
usually  are  much  smaller  than  the  negative  areas,  that  is,  the  regions 
in  which  currents  tend  to  enter  the  cables  form  a  larger  total  than  the 
regions  in  which  the  currents  tend  to  hw>,  the  cables. 


ELECTRICAL  HAZARDS  283 

These  facts  simplify  the  ways  in  which  the  cables  may  be  pro- 
tected against  damage  by  direct  currents  leaving  them  and  also 
they  reduce  the  amount,  complication,  and  cost  of  applying  the 
corrective  and  preventive  measures. 

All  electric  roads  do  not  use  direct  current.  Certain  simpli- 
fications in  the  use  of  single-phase  alternating  currents  in  traction 
motors  have  increased  the  number  of  roads  using  a  system  of  alter- 
nating-current power  supply.  Where  alternating  current  is  used, 
the  electrolytic  conditions  are  different  and  a  new  problem  is  set, 
for,  as  the  current  flows  in  recurrently  different  directions,  an  area 
which  at  one  instant  is  positive  to  others,  is  changed  the  next  in- 
stant into  a  negative  area.  The  protective  means,  therefore,  must 
be  adapted  to  the  changed  requirements. 


CHAPTER  XIX 
PROTECTIVE  MEANS 

Any  of  the  heating  hazards  described  in  the  foregoing  chapter 
may  cause  currents  which  will  damage  apparatus.  All  devices  for 
the  protection  of  apparatus  from  such  damage,  operate  either  to 
stop  the  flow  of  the  dangerous  current,  or  to  send  that  flow  over 
some  other  path. 

Protection  Against  High  Potentials.  Lightning  is  the  most 
nearly  universal  hazard.  All  open  wires  are  exposed  to  it  in  some 
degree.  Damaging  currents  from  lightning  are  caused  by  extra- 
ordinarily high  potentials.  Furthermore,  a  lightning  discharge  is 
oscillatory;  that  is,  alternating,  and  of  very  high  frequency.  Drops, 
ringers,  receivers,  and  other  devices  subject  to  lightning  damage 
suffer  by  having  their  windings  burned  by  the  discharge.  The 
impedance  these  windings  offer  to  the  high  frequency  of  lightning 
oscillations  is  great.  The  impedance  of  a  few  turns  of  heavy  wire 
may  be  negligible  to  alternating  currents  of  ordinary  frequencies 
because  the  resistance  of  the  wire  is  low,  its  inductance  small,  and 
the  frequency  finite.  On  the  other  hand,  the  impedance  of  such  a 
coil  to  a  lightning  discharge  is  much  higher,  due  to  the  very  high 
frequency  of  the  discharge. 

Were  it  not  for  the  extremely  high  pressure  of  lightning  dis- 
charges, their  high  frequency  of  oscillation  would  enable  ordinary 
coils  to  be  self-protecting  against  them.  But  a  discharge  of  elec- 
tricity can  take  place  through  the  air  or  other  insulating  medium  if 
its  pressure  be  high  enough.  A  pressure  of  70,000  volts  can  strike 
across  a  gap  in  air  of  one  inch,  and  lower  pressures  can  strike 
across  smaller  distances.  When  lightning  encounters  an  impedance, 
the  discharge  seldom  takes  place  through  the  entire  winding,  as  an 
ordinary  current  would  flow,  usually  striking  across  whatever  short 
paths  may  exist.  Very  often  these  paths  are  across  the  insulation 
between  the  outer  turns  of  a  coil.  It  is  not  unusual  for  a  lightning 


PROTECTIVE  MEANS 


285 


discharge  to  plow  its  way  across  the  outer  layer  of  a  wound  spool, 
melting  the  copper  of  the  turns  as  it  goes.  Often  the  discharge  will 
take  place  from  inner  turns  directly  to  the  core  of  the  magnet.  This 
is  more  likely  when  the  core  is  grounded. 

Air-Gap  Arrester.  The  tendency  of  a  winding  to  oppose  light- 
ning discharges  and  the  ease  with  which  such  discharge  may  strike 
across  insulating  gaps,  points  the  way  to  protection  against  them. 
Such  devices  consist  of  two  conductors  separated  by  an  air  space 
or  other  insulator  and  are  variously  known  as  lightning  arresters, 
spark  gaps,  open-space  cutouts,  or  air-gap  arresters.  The  conductors 
between  which  the  gap  exists  may  be  both  of  metal,  may  be  one 
of  metal  and  one  of  carbon,  or  both  of  carbon.  One  combination 
consists  of  carbon  and  mercury,  a  liquid  metal.  The  space  between 
the  conductors  may  be  filled  with  either  air  or  solid  matter,  or  it 
may  be  a  vacuum.  Speaking  generally,  the  conductors  are  separated 
by  some  insulator.  Two  conductors  separated  by  an  insulator 
form  a  condenser.  The  insulator  of  an  open-space  arrester  often  is 
called  the  dielectric. 

Discharge  Across  Gaps: — Electrical  discharges  across  a  given 
distance  occur  at  lower  potentials  if  the  discharge  be  between  points 
than  if  between  smooth  surfaces.  Arresters, 
therefore,  are  provided  with  points.  Fig.  203 
shows  a  device  known  as  a  "saw-tooth"  arrester 
because  of  its  metal  plates  being  provided  with 
teeth.  Such  an  arrester  brings  a  ground  con- 
nection close  to  plates  connected  with  the  line  and 
is  adapted  to  protect  apparatus  either  connected 
across  a  metallic  circuit  or  in  series  with  a  single 
wire  circuit. 

Fig.  204  shows  another  form  of  metal  plate 
air-gap  arrester  having  the  further  possibility  of 
a  discharge  taking  place  from  one  line  wire  to  the  other.  Inserting 
a  plug  in  the  hole  between  the  two  line  plates  connects  the  line  wires 
directly  together  at  the  arrester.  This  practice  was  designed  for 
use  with  series  lines,  the  plug  short-circuiting  the  telephone  set  when 
in  place. 

A  defect  of  most  ordinary  types  of  metal  air-gap  lightning  ar- 
resters is  that  heavy  discharges  tend  to  melt  the  teeth  or  edges  of  the 


Fig.  203.     Saw-Tooth 
Arrester 


286 


TELEPHONY 


plates  and  often  to  weld  them  together,  requiring  special  attention 
to  re-establish  the  necessary  gap. 

Advantages  of  Carbon: — Solid  carbon  is  found  to  be  a  much 
better  material  than  metal  for  the  reasons  that  a  discharge  will  not 
melt  it  and  that  its  surface  is  composed  of  multitudes  of  points  from 
which  discharges  take  place  more  readily  than  from  metals. 


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UN£ 

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k 
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T  METAL  GKOUfiO 
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Fig.  204.     Saw-Tooth  Arrester 


Fig.  205.  Carbon  Block  Arrester 


Carbon  arresters  now  are  widely  used  in  the  general  form  shown 
in  Fig.  205.  A  carbon  block  connected  with  a  wire  of  the  line  is 
separated  from  a  carbon  block  connected  to  ground  by  some  form 
of  insulating  separator.  Mica  is  widely  used  as  such  a  separa- 
tor, and  holes  of  some  form  in  a  mica  slip  enable  the  discharge  to 
strike  freely  from  block  to  block,  while  preventing  the  blocks  from 
touching  each  other.  Celluloid  with  many  holes  is  used  as  a  sep- 
arator between  carbon  blocks.  Silk  and  various  special  compo- 
sitions also  have  their  uses. 

Dust  Between  Carbons: — Discharges  between  the  carbon  blocks 
tend  to  throw  off  particles  of  carbon  from  them.      The  separation 
between  the   blocks   being  small — from  .005   to 
.015  inch — the  carbon  particles  may  lodge  in  the 
air-gap,  on  the  edges  of  the  separator,  or  other- 
wise, so  as  to  leave  a  conducting  path  between 
the  two  blocks.    Slight  moisture  on  the  separator 
may   help   to   collect  this  dust,  thus  placing  a 
ground  on  that  wire  of  the  line.     This  ground 
may  be  of  very  high  resistance,  but  is  probably 
one  of  many  such — one  at  each  arrester  connected  to  the  line.    In 
special  forms  of  carbon  arresters  an  attempt  has  been  made  to  limit 


Fig.  206.    Arrester 
Separators 


PROTECTIVE  MEANS 


287 


this  danger  of  grounding  by  the  deposit  of  carbon  dust.  The  object 
of  the  U-shaped  separator  of  Fig.  206  is  to  enable  the  arrester  to  be 
mounted  so  that  this  opening  in  the  separator  is  downward,  in  the 
hope  that  loosened  carbon  particles  may  fall  out  of  the  space  between 
the  blocks.  The  deposit  of  carbon  on  the  inside  edges  of  the  U- 
shaped  separator  often  is  so  fine  and  clings  so  tightly  as  not  to  fall 
out.  The  separator  projects  beyond  the  blocks  so  as  to  avoid  the 
collection  of  carbon  on  the  outer  edges. 

Commercial  Types : — Fig.  207  is  a  commercial  form  of  the  ar- 
rangement shown  in  Fig.  205  and  is  one  of  the  many  forms  made 
by  the  American  Electric  Fuse  Company.  Line  wires  are  attached 
to  outside  binding  posts  shown  in  the  figure 
and  the  ground  wire  to  the  metal  binding  post 
a.t  the  front.  The  carbon  blocks  with  their 
separator  slide  between  clips  and  a  ground 
plate.  The  air-gap  is  determined  by  the  thick- 
ness of  the  separator  between  the  carbon  blocks. 

The  Roberts  carbon  arrester  is  designed 
with   particular   reference  to  the   disposal   of 
carbon  dust  and  is  termed  self-cleaning   for 
that  reason.      The  arrangement  of  carbons  and  dielectric  in  this 
device  is  shown  in  Fig.  208;  mica  is  cemented  to  the  line  carbon 
and  is  large   enough  to  provide  a  projecting  margin  all  around. 
The  spark  gap  is  not  uniform 
over  the  entire  surface  of  the 
block    but    is    made    wedge- 
shaped  by  grinding  away  the 
line  carbon  as  shown.      It  is 
claimed     that     a     continuous1 
arcing  fills   the  wedge-shaped 
chamber  with  heated  air  or  gas, 
converting   the  whole    of    the 
space  into  a  field  of  low  resist- 
ance to  ground,  and  that  this 
gas  in  expanding  drives    out 

every  particle  of  carbon  that  may  be  thrown  off.  It  seems  obvious  that 
the  wedge-shaped  space  offers  greater  freedom  for  carbon  dust  to  fall 
out  than  in  the  case  of  the  parallel  arrangement  of  the  block  faces. 


Fig.  207.    Carbon  Block 
Arrester 


MICA 


Fig.  208.     Roberts  "Self-Cleaning"  Arrester 


288 


TELEPHONY 


An  outdoor  arrester  for  metallic  circuits,  designed  by  F.  B. 
Cook,  is  shown  in  Fig.  209.  The  device  is  adapted  to  mount  on  a 
pole  or  elsewhere  and  to  be  covered  by  a  protecting  cap.  The  car- 
bons are  large  and  are  separated  by  a  special  compound  intended  to 


Fig.  209.     Cook  Air-Gap  Arrester 

assist  the  self-cleaning  feature.  The  three  carbons  being  grouped 
together  as  a  unit,  the  device  has  the  ability  to  care  for  discharges 
from  one  terminal  to  either  of  the  others  direct,  without  having  to 
pass  through  two  gaps.  In  this  particular,  the  arrangement  is  the 
same  as  that  of  Fig.  204. 

A  form  of  Western  Electric  arrester  particularly  adapted  for 
outside  use  on  railway  lines  is  shown  with  its  cover  in  Fig.  210. 


Fig.  210.     Western  Electric  Air-Gap  Arrester 

The  Kellogg  Company  regularly  equips  its  magneto  telephones 
with  air-gap  arresters  of  the  type  shown  in  Fig.  211.  The  two  line 
plates  are  semicircular  and  of  metal.  The  ground  plate  is  of  car- 


PROTECTIVE  MEANS 


289 


bon,  circular  in  form,  covering  both  line  plates  with  a  mica  separator. 
This  is  mounted  on  the  back  board  of  the  telephone  and  perma- 
nently wired  to  the  line  and  ground  binding  posts. 


Fig.  211.     Kellogg  Air-Gap  Arrester 

Vacuum  Arresters: — All  of  the  carbon  arresters  so  far  mentioned 
depend  on  the  discharge  taking  place  through 
air.  A  given  pressure  will  discharge  further 
in  a  fairly  good  vacuum  than  in  air.  The 
National  Electric  Specialty  Company  mounts 
three  conductors  in  a  vacuum  of  the  incan- 
descent lamp  type,  Fig.  212.  A  greater  sepa- 
ration and  less  likelihood  of  short-circuiting 
can  be  provided  in  this  way.  Either  carbon 
or  metal  plates  are  adapted  for  use  in  such 
vacuum  devices.  The  plates  may  be  further 
apart  for  a  given  discharge  pressure  if  the  sur- 
faces are  of  carbon. 

Introduction  of  Impedance: — It  has  been 
noted  that  the  existence  of  impedance  tends 
to  choke  back  the  passage  of  lightning  discharge 
through  a  coil  Fig.  213  suggests  the  relation 
between  such  an  impedance  and  air-gap  ar- 
rester. If  the  coil  shown  therein  be  considered  an  arrangement  of 
conductors  having  inductance,  it  will  be  seen  that  a  favorable  place 


Fig.  212.     Vacuum 
Arrester 


290 


TELEPHONY 


for  an  air-gap  arrester  is  between  that  impedance  and  the  line. 
This  fact  is  made  known  in  practice  by  frequent  damage  to  aerial 
cables  by  electricity  brought  into  them  over  long  open  wires,  the  dis- 
charge taking  place  at  the  first  turn  or  bend  in  the  aerial  cable; 
this  discharge  often  damages  both  core  and  sheath.  It  is  well  to 


1 
T 


TOWARDS  UrtE 


Fig.  214.     Holtzer-Cabot  Arrester 


Fig.  213.     Impedance  and  Air-Gap 

have  such  bends  as  near  the  end  of  the  cable  as  possible,  and  turns 
or  goosenecks  at  entrances  to-  terminals  have  that  advantage. 

This  same  principle  is  utilized  in  some  forms  of  arresters,  such 
as  the  one  shown  in  Fig.  214,  which  provides  an  impedance  of  its  own 

directly  in  the  arrester  element.  In 
this  device  an  insulating  base  carries 
a  grounded  carbon  rod  and  two  im- 
pedance coils.  The  impedance  coils 
are  wound  on  insulating  rods,  which 
hold  them  near,  but  not  touching,  the 
ground  carbon.  The  coils  are  ar- 
ranged so  that  they  may  be  turned 
when  discharges  roughen  the  surfaces  of  the  wires. 

Metallic  Electrodes : — Copper  or  other  metal  blocks  with 
roughened  surfaces  separated  by  an  insulating  slip  may  be  sub- 
stituted for  the  carbon  blocks  of  most  of  the  arresters  previously 
described.  Metal  blocks  lack 
the  advantage  of  carbon  in  that 
the  latter  allows  discharges  at 
lower  potentials  for  a  given  sep- 
aration, but  they  have  the  ad- 
vantage that  a  conducting  dust 
is  not  thrown  off  from  them. 

Provision  Against  Continuous  Arc: — For  the  purpose  of  short- 
circuiting  an  arc,  a  globule  of  low-melting  alloy  may  be  placed 
in  one  carbon  block  of  an  arrester.  This  feature  is  not  essential  in 
an  arrester  intended  solelv  to  divert  lightning  discharges.  Its  pur- 


Fig.  215.     Carbon  Air-Gap  Arrester 


PROTECTIVE  MEANS  291 

pose  is  to  provide  an  immediate  path  to  ground  if  an  arc  arising 
from  artificial  electricity  has  been  maintained  between  the  blocks 
long  enough  to  melt  the  globule.  Fig.  215  is  a  plan  and  section  of 
the  Western  Electric  Company's  arrester  used  as  the  high  potential 
element  in  conjunction  with  others  for  abnormal  currents  and  sneak 
currents;  the  latter  are  currents  too  small  to  operate  air-gap  arresters 
or  substantial  fuses. 

Protection  Against  Strong  Currents.  Fuses.  A  fuse  is  a  metal 
conductor  of  lower  carrying  capacity  than  the  circuit  with  which  it 
is  in  series  at  the  time  it  is  required  to  operate.  Fuses  in  use  in 
electrical  circuits  generally  are  composed  of  some  alloy  of  lead, 
which  melts  at  a  reasonably  low  temperature.  Alloys  of  lead  have 
lower  conductivity  than  copper.  A  small  copper  wire,  however,  may 
fuse  at  the  same  volume  of  current  as  a  larger  lead  alloy  wire. 

Proper  Functions: — A  fuse  is  not  a  good  lightning  arrester. 
As  lightning  damage  is  caused  by  current  and  as  it  is  current  which 
destroys  a  fuse,  a  lightning  discharge  can  open  a  circuit  over  which  it 
passes  by  melting  the  fuse  metal.  But  lightning  may  destroy  a  fuse 
and  at  the  same  discharge  destroy  apparatus  in  series  with  the  fuse. 
There  are  two  reasons  for  this:  One  is  that  lightning  discharges 
act  very  quickly  and  may  have  destroyed  apparatus  before  heating 
the  fuse  enough  to  melt  it;  the  other  reason  is  that  when  a  fuse  is 
operated  with  enough  current  even  to  vaporize  it,  the  vapor  serves 
as  a  conducting  path  for  an  instant  after  being  formed.  This  con- 
ducting path  may  be  of  high  resistance  and  still  allow  currents  to 
flow  through  it,  because  of  the  extremely  high  pressure  of  the  light- 
ning discharge.  A  comprehensive  protective  system  may  include 
fuses,  but  it  is  not  to  be  expected  that  they  always  will  arrest  light- 
ning or  even  assist  other  things  in  arresting  lightning.  They  should 
be  considered  as  of  no  value  for  that  purpose.  Furthermore,  fuses 
are  best  adapted  to  be  a  part  of  a  general  protective  system  when 
they  do  all  that  they  must  do  in  stopping  abnormal  currents  and  yet 
withstand  lightning  discharges  which  may  pass  through  them.  Other 
things  being  equal,  that  system  of  protection  is  best  in  which  all 
lightning  discharges  are  arrested  by  gap  arresters  and  in  which  no 
fuses  ever  are  operated  by  lightning  discharges. 

Mica  Fuse: — A  convenient  and  widely  used  form  of  fuse  is  that 
shown  in  Fig.  216.  A  mica  slip  has  metal  terminals  at  its  ends 


292 


TELEPHONY 


and  a  fuse  wire  joins  these  terminals.     The  fuse  is  inserted  in  the 
circuit  by  clamping  the   terminals   under  screws   or  sliding  them 


.METAL    TEfW/MAL 


METAL   TERM/MA  L 


FUSE   Vf/RE 

Fig.  216.     Mica  Slip  Fuse 


between  clips  as  in  Figs.  217  and  218.  Advantages  of  this  method 
of  fuse  mounting  for  protecting  circuits  needing  small  currents  are 
that  the  fuse  wire  can  be  seen,  the  fuses  are  readily  replaced  when 
blown,  and  their  mountings  may  be  made  compact.  As  elements  of 
a  comprehensive  protective  system,  however,  the  ordinary  types  of 
mica-slip  fuses  are  objectionable  because  too  short,  and  because  they 
have  no  means  of  their  own  for  extinguishing  an  arc  which  may 


Fig.  217.     Postal  Type  Mica 

Fuse 


Fig.  218.  Western  Union  Type 
Mica  Fuse 


follow  the  blowing  of  the  fuses.  As  protectors  for  use  in  distributing 
low  potential  currents  from  central-office  power  plants  they  are  ad- 
mirable. By  simple  means,  they  may  be  made  to  announce  audibly 
or  visibly  that  they  have  operated. 

Enclosed  Fuses: — If  a  fuse  wire  within  an  insulating  tube  be 
made  to  connect  metal  caps  on  that  tube  and  the  space  around  the 
tube  be  filled  with  a  non-conducting  powder,  the  gases  of  the  vap- 
orized fuse  metal  will  be  absorbed  more  quickly  than  when  formed 


Fig.  219.     Pair  of  Enclosed  Fuses 

without  such  imbedding  in  a  powder.  The  filling  of  such  a  tubular 
fuse  also  muffles  the  explosion  which  occurs  when  the  fuse  is  vapor- 
izet., 


PROTECTIVE  MEANS 


293 


Fuses  of  the  enclosed  type,  with  or  without  filling,  are  widely 
used  in  power  circuits  generally  and  are  recommended  by  fire  insur- 
ance bodies.  Fig.  219  illustrates  an  arrester  having  a  fuse  of  the 
enclosed  type,  this  example  being  that  of  the  H.  W.  Johns-Manville 
Company. 

In  telephony  it  is  frequently  necessary  to  mount  a  large  number 
of  fuses  or  other  protective  devices  together  in  a  restricted  space. 
In  Fig.  220  a  group  of  Western  Electric  tubular  fuses,  so  mounted, 
is  shown.  These  fuses 
have  ordinarily  a  carrying 
capacity  of  6  or  7  amperes. 
It  is  not  expected  that  this 
arrester  will  blow  because  6 
or  7  amperes  of  abnormal 
currents  are  flowing  through 
it  and  the  apparatus  to  be 
protected.  What  is  intended 
is  that  the  fuse  shall  with- 
stand lightning  discharges 
and  when  a  foreign  current 
passes  through  it,  other  ap- 
paratus will  increase  that 
current  enough  to  blow  the 
fuse.  It  will  be  noticed 
that  the  fuses  of  Fig.  220 
are  open  at  the  upper  end, 
which  is  the  end  connected 
to  the  exposed  wire  of  the 

line  The  fuses  are  closed  at  the  lower  end,  which  is  the  end  con- 
nected to  the  apparatus.  When  the  fuse  blows,  its  discharge  is  some- 
what muffled  by  the  lining  of  the  tube,  but  enough  explosion  remains 
so  that  the  heated  gases,  in  driving  outward,  tend  to  break  the  arc 
which  is  established  through  the  vaporized  metal. 

A  pair  of  Cook  tubular  fuses  in  an  individual  mounting  is  shown 
in  Fig.  221  Fuses  of  this  type  are  not  open  at  one  end  like  a  gun, 
but  opportunity  for  the  heated  gases  to  escape  exists  at  the  caps. 
The  tubes  are  made  of  wood,  of  lava,  or  of  porcelain. 

Fig.  222  is  another  tubular  fuse,  the  section  showing  the  arrange- 


Pig.  220.     Bank  of  Enclosed  Puses 


294  TELEPHONY 

ment  of  asbestos  lining  which  serves  the  two  purposes  of  muffling  the 

sound  of  the  discharge  and  absorbing  and  cooling  the  resulting  gases. 

Air-Gap  vs.  Fuse  Arresters.     It  is  hoped  that  the  student  grasps 

clearly  the  distinction  between   the  purposes  of  air-gap  and  fuse 


Fig.  221.     Pair  of  Wooden  Tube  Fuses 

arresters.  The  air-gap  arrester  acts  in  response  to  high  voltages, 
either  of  lightning  or  of  high-tension  power  circuits.  The  fuse  acts  in 
recponse  to  a  certain  current  value  flowing  through  it  and  this  , 
minimum  current  in  well-designed  protectors  for  telephone  lines  is 
not  very  small.  Usually  it  is  several  times  larger  than  the  maximum 
current  apparatus  in  the  line  can  safely  carry.  Fuses  can  be  made 
so  delicate  as  to  operate  on  the  very  smallest  current  which  could 
injure  apparatus  and  the  earlier  protective  systems  depended  on 
such  an  arrangement.  The  difficulty  with  such  delicate  fuses  is 
that  they  are  not  robust  enough  to  be  reliable,  and,  worse  still,  they 
change  their  carrying  capacity  with  age  and  are  not  uniform  in  opera- 
tion in  different  surroundings  and  at  different  temperatures.  They 
are  also  sensitive  to  lightning  discharges,  which  they  have  no  power 
to  stop  or  to  divert. 

Protection  Against  Sneak  Currents.     For  these  reasons,  a  sys- 
tem containing  fuses  and  air-gap  arresters  only,  does  not  protect 

against  abnormal  currents  which 
are  continuous  and  small,  though 
large  enough  to  injure  apparatus 
because  continuous.  These  cur- 
rents have  come  to  be  known  as 

Fig.  2227  Tubular  Fuse  with  AsbesMiiing  sneak  currents,  a  term  more  de- 

scriptive    than    elegant.      Sneak 

currents  though  small,  may,  when  allowed  to  flow  for  a  long  time 
through  the  winding  of  an  electromagnet  for  instance,  develop  enough 
heat  to  char  01  injure  the  insulation.  They  are  the  more  dangerous 
because  insidious. 


PROTECTIVE  MEANS 


295 


TO  APPARATUS 


Sneak-Current  Arresters.  As  typical  of  sneak-current  arresters, 
Fig.  223  shows  the  principle,  though  not  the  exact  form,  of  an 
arrester  once  widely  used  in  telephone  and  signal  lines.  The  nor- 
mal path  from  the  line  to  the  apparatus  is  through  a  small  coil  of 
fine  wire  imbedded  in  sealing  wax.  A  spring  forms  a  branch  path 
from  the  line  and  has  a  tension  which  would  cause  it  to  bear  against 
the  ground  contact  if  it  were  allowed  to  do  so.  It  is  prevented 
from  touching  that  contact  normally  by  a  string  between  itself  and 
a  rigid  support.  The  string  is 
cut  at  its  middle  and  the  knotted 
ends  as  thus  cut  are  imbedded  in 
the  sealing  wax  which  contains 
the  coil. 

A  small  current  through  the 
little  coil  will  warm  the  wax 
enough  to  allow  the  string  to 

rpi  •  ,1  -ii  Fig.  223.     Principle  of  Sneak-Current 

part.        I  he    spring    then    will  Arrester 

ground  the  line.  Even  so  sim- 
ple an  apparatus  as  this  operates  with  considerable  accuracy.  All 
currents  below  a  certain  critical  amount  may  flow  through  the 
heating  coil  indefinitely,  the  heat  being  radiated  rapidly  enough 
to  keep  the  wax  from  softening  and  the  string  from  parting. 
All  currents  above  this  critical  amount  will  operate  the  arrester; 
the  larger  the  current,  the  shorter  the  time  of  operating.  It  will  be 
remembered  that  the  law  of  these  heating  effects  is  that  the  heat 
generated  =  C2Rt,  so  that  if  a  certain  current  operates  the  arrester 
in,  say  40  seconds,  twice  as  great  a  current  should  operate  the  arres- 
ter in  10  seconds.  In  other  words,  the  time  of  operation  varies 
inversely  as  the  square  of  the  current  and  inversely  as  the  resistance. 
To  make  the  arrester  more  sensitive  for  a  given  current — i.  e.,  to 
operate  in  a  shorter  time — one  would  increase  the  resistance  of  the 
coil  in  the  wax  either  by  using  more  turns  or  finer  wire,  or  by  making 
the  wire  of  a  metal  having  higher  specific  resistance. 

The  present  standard  sneak-current  arrester  embodies  the  two 
elements  of  the  devices  of  Fig.  223:  a  resistance  material  to  trans- 
form the  dangerous  sneak  current  into  localized  heat;  and  a,  fusible 
material  softened  by  this  heat  to  release  some  switching  mechan- 
ism. 


EASILY  MELT/M6  -SOLO£R 


296  TELEPHONY 

The  resistance  material  is  either  a  resistance  wire  or  a  bit  of 
carbon,  the  latter  being  the  better  material,  although  both  are  good. 
The  fusible  material  is  some  alloy  melting  at  a  low  temperature. 
Lead,  tin,  bismuth,  and  cadmium  can  be  combined  in  such  proportions 
as  will  enable  the  alloy  to  melt  at  temperatures  from  140°  to  180°  F. 
Such  an  alloy  is  a  solder  which,  at  ordinary  temperatures,  is  firm 
enough  to  resist  the  force  of  powerful  springs ;  yet  it  will  melt  so  as 
to  be  entirely  fluid  at  a  temperature  much  less  than  that  of  boiling 
water. 

Heat  Coil.  Fig.  224  shows  a  practical  way  of  bringing  the 
heating  and  to-be-heated  elements  together.  A  copper  spool  is 
wound  with  resistance  wire.  A  metal  pin  is 
soldered  in  the  bore  of  the  spool  by  an  easily 
melting  alloy.  When  current  heats  the  spool 
Fig  224  Heat  Coil  enough,  the  pin  may  slide  or  turn  in  the  spool. 
It  may  slide  or  turn  in  many  ways  and  this 
happily  enables  many  types  of  arresters  to  result.  For  example,  the 
pin  may  pull  out,  or  push  in,  or  push  through,  or  rotate  like  a 
shaft  in  a  bearing,  or  the  spool  may  turn  on  it  like  a  hub  on  an 
axle.  Messrs.  Hayes,  Rolfe,  Cook,  McBerty,  Kaisling,  and  many 
other  inventors  have  utilized  these  combinations  and  motions  in  the 
production  of  sneak-current  arresters.  All  of  them  depend  on  one 
action:  the  softening  of  a  low-melting  alloy  by  heat  generated  in  a 
resistance. 

When  a  heat  coil  is  associated  with  the  proper  switching  springs, 
it  becomes  a  sneak-current  arrester.  The  switching  springs  always 
are  arranged  to  ground  the  line  wire.  In  some  arresters,  the  line 
wire  is  cut  off  from  the  wire  leading  toward  the  apparatus  by  the 
same  movement  which  grounds  it.  In  others,  the  line  is  not  broken 
at  all,  but  merely  grounded.  Each  method  has  its  advantages. 

Complete  Line  Protection.  Fig.  225  shows  the  entire  scheme 
of  protectors  in  an  exposed  line  and  their  relation  to  apparatus  in 
the  central-office  equipment  and  at  the  subscriber's  telephone.  The 
central-office  equipment  contains  heat  coils,  springs,  and  carbon 
arresters.  At  some  point  between  the  central  office  and  the  sub- 
scriber's premises,  each  wire  contains  a  fuse.  At  the  subscriber's 
premises  each  wire  contains  other  fuses  and  these  are  associated 
with  carbon  arresters.  The  figure  shows  a  central  battery  equip- 


PROTECTIVE  MEANS  297 

ment,  in  which  the  ringer  of  the  telephone  is  in  series  with  a  con- 
denser. A  sneak-current  arrester  is  not  required  at  the  subscriber's 
station  with  such  equipment. 

Assume  the  line  to  meet  an  electrical  hazard  at  the  point  X. 
If  this  be  lightning,  it  will  discharge  to  ground  at  the  central  office 
or  at  the  subscriber's  instrument  or  at  both  through  the  carbon  ar- 
resters connected  to  that  side  of  the  line.  If  it  be  a  high  potential 
from  a  power  circuit  and  of  more  than  350  volts,  it  will  strike  an  arc 
at  the  carbon  arrester  connected  to  that  wire  of  the  line  in  the  central 
office  or  at  the  subscriber's  telephone  or  at  both,  if  the  separation  of 
the  carbons  in  those  arresters  is  .005  inch  or  less.  If  the  carbon 
arresters  are  separated  by  celluloid,  it  will  burn  away  and  allow  the 
carbons  to  come  together,  extinguishing  the  arc.  If  they  are  sep- 
arated by  mica  and  one  of  the  carbons  is  equipped  with  a  globule  of 
low-melting  alloy,  the  heat  of  the  arc  will  melt  this,  short-circuiting 


Ifi    tH£  CENTRAL    OfF/CE 

PFIEM/SES 

Fig.  225.     Complete  Line  Protection 

the  gap  and  extinguishing  the  arc.  The  passage  of  current  to  ground 
at  the  arrester,  however,  will  be  over  a  path  containing  nothing  but 
wire  and  the  arrester.  The  resulting  current,  therefore,  may  be 
very  large.  The  voltage  at  the  arrester  having  been  350  volts  or 
more,  in  order  to  establish  the  arc,  short-circuiting  the  gap  will  make 
the  current  7  amperes  or  more,  unless  the  applied  voltage  miracu- 
lously falls  to  50  volts  or  less.  The  current  through  the  fuse  being 
more  than  7  amperes,  it  will  blow  promptly,  opening  the  line  and 
isolating  the  apparatus.  It  will  be  noted  that  this  explanation  ap- 
plies to  equipment  at  either  end  of  the  line,  as  the  fuse  lies  between 
the  point  of  contact  and  the  carbon  arrester. 

Assume,  on  the  other  hand,  that  the  contact  is  made  at  the 
point  Y.  The  central-office  carbon  arrester  will  operate,  grounding 
the  line  and  increasing  the  amount  of  current  flowing.  There  being 


298  TELEPHONY 

no  fuse  to  blow,  a  worse  thing  will  befall,  in  the  overheating  of  the 
line  wire  and  the  probable  starting  of  a  fire  in  the  central  office.  It 
is  obvious,  therefore,  that  a  fuse  must  be  located  between  the  carbon 
arrester  and  any  part  of  the  line  which  is  subject  to  contact  with  a 
potential  which  can  give  an  abnormal  current  when  the  carbon 
arrester  acts. 

Assume,  as  a  third  case,  that  the  contact  at  the  point  X  either 
is  with  a  low  foreign  potential  or  is  so  poor  a  contact  that  the  difference 
of  potential  across  the  gap  of  the  carbon  arrester  is  lower  than  its 
arcing  point.  Current  will  tend  to  flow  by  the  carbon  arrester  with- 
out operating  it,  but  such  a  current  must  pass  through  the  winding 
of  the  heat  coil  if  it  is  to  enter  the  apparatus.  The  sneak  current 
may  be  large  enough  to  overheat  the  apparatus  if  allowed  to  .flow 
long  enough,  but  before  it  has  flowed  long  enough  it  will  have  warmed 
the  heat-coil  winding  enough  to  soften  its  fusible  alloy  and  to  release 
springs  which  ground  the  line,  just  as  did  the  carbon  arrester  in  the 
case  last  assumed.  Again  the  current  will  become  large  and  will 
blow  the  fuse  which  lies  between  the  sneak-current  arrester  and  the 
point  of  contact  with  the  source  of  foreign  current.  In  this  case, 
also,  contact  at  the  point  F  would  have  operated  mechanism  to 
ground  the  line  at  the  central  office,  and,  no  fuse  interposing,  the 
wiring  would  have  been  overheated. 

Exposed  and  Unexposed  Wiring.  Underground  cables,  cables 
formed  of  rubber  insulated  wires,  and  interior  wiring  which  is  prop- 
erly done,  all  may  be  considered  to  be  wiring  which  is  unexposed, 
that  is,  not  exposed  to  foreign  high  potentials,  discharges,  sneak,  or 
abnormal  currents.  All  other  wiring,  such  as  bare  wires,  aerial 
cables,  etc.,  should  be  considered  as  exposed  to  such  hazards  and 
a  fuse  should  exist  in  each  wire  between  its  exposed  portion  and  the 
central  office  or  subscriber's  instrument.  The  rule  of  action,  there- 
fore, becomes: 

The  proper  position  of  the  fuse  is  between  exposed  and  unexposed 
wiring. 

It  may  appear  to  the  student  that  wires  in  an  aerial  cable  with  a 
lead  sheath — that  sheath  being  either  grounded  or  ungrounded — are 
not  exposed  to  electrical  hazards;  in  the  case  of  the  grounded  sheath, 
this  would  presume  that  a  contact  between  the  cable  and  a  high  po- 
tential wire  would  result  merely  in  the  foreign  currents  going  to 


PROTECTIVE  MEANS  299 

ground  through  the  cable  sheath,  the  arc  burning  off  the  high-po- 
tential wire  and  allowing  the  contact  to  clear  itself  by  the  falling  of 
the  wire.  If  the  assumption  be  that  the  sheath  is  not  grounded, 
then  the  student  may  say  that  no  current  at  all  would  flow  from  the 
high-potential  wire. 

Both  assumptions  are  wrong.  In  the  case  of  the  grounded 
sheath,  the  current  flows  to  it  at  the  contact  with  the  high-potential 
wire;  the  lead  sheath  is  melted,  arcs  strike  to  the  wires  within,  and 
currents  are  led  directly  to  the  central  office  and  to  subscribers' 
premises.  In  the  case  of  the  ungrounded  sheath,  the  latter  charges 
at  once  through  all  its  length  to  the  voltage  of  the  high-potential 
wire;  at  some  point,  a  wire  within  the  cable  is  close  enough  to  the 
sheath  for  an  arc  to  strike  across,  and  the  trouble  begins.  All 
the  wires  in  the  cable  are  endangered  if  the  cross  be  with  a  wire  of 
the  primary  circuit  of  a  high-tension  transmission  line.  Any  series 
arc-light  circuit  is  a  high-potential  menace.  Even  a  450-volt  trolley 
wire  or  feeder  can  burn  a  lead-covered  cable  entirely  in  two  in  a  few 
seconds.  The  authors  have  seen  this  done  by  the  wayward  trolley 
pole  of  a  street  car,  one  side  of  the  pole  touching  the  trolley  wire  and 
the  extreme  end  just  touching  the  telephone  cable. 

The  answer  lies  in  the  foregoing  rule.  Place  the  fuse  between 
the  wires  which  can  and  the  wires  which  can  not  get  into  contact  with 
high  potentials.  In  application,  the  ru-le  has  some  flexibility.  In 
the  case  of  a  cable  which  is  aerial  as  soon  as  it  leaves  the  central  office, 
place  the  fuses  in  the  central  office;  in  a  cable  wholly  underground, 
from  central  office  to  subscriber — as,  for  example,  the  feed  for  an 
office  building — use  no  fuses  at  all;  in  a  cable  which  leaves  the  cen- 
tral office  underground  and  becomes  aerial,  fuse  the  wires  just  where 
they  change  from  underground  to  aerial.  The  several  branches 
of  an  underground  cable  into  aerial  ones  should  be  fused  as  they 
branch. 

Wires  properly  installed  in  subscribers'  premises  are  considered 
unexposed.  The  position  of  the  fuse  thus  is  at  or  near  the  point  of 
entrance  of  the  wires  into  that  building  if  the  wires  of  the  subscrib- 
er's line  outside  the  premises  are  exposed,  as  determined  by  the 
definitions  given.  If  the  line  is  unexposed,  by  those  definitions, 
no  protector  is  required.  If  one  is  indicated,  it  should  be  used,  as 
compliance  with  the  best-knowTn  practice  is  a  clear  duty.  Less  than 


300 


TELEPHONY 


HEAT  CO/L 
AFTER   OPEftAT/MG 


what  is  known  to  be  best  is  not  honest  practice  in  a  matter  which 
involves  life,  limb,  and  indefinite  degrees  of  property  values. 

Protectors  in  central-battery  subscribers'  equipments  need  no 
sneak-current  arresters,  as  the  condenser  reduces  that  hazard  to  a 
negligible  amount.  Magneto  subscribers'  equipments  usually  lack 
condensers  in  ringer  circuits,  though  they  may  have  them  in  talking 
circuits  on  party  lines.  The  ringer  circuit  is  the  only  path  through  the 
telephone  set  for  about  98  per  cent  of  the  time.  Sneak-current  ar- 
resters, therefore,  should  be  a  part  of  subscribers'  station  protectors 
in  magneto  equipment,  except  in  such  rural  districts  as  may  have  no 
lighting  or  power  wires.  When  sneak-current  arresters  are  so  used 
the  arrangement  of  the  parts  then  is  the  same  as  in  the  central-office 
portion  of  Fig.  225. 

Types  of  Central=0ff  ice  Protectors.  A  form  of  combined  heat 
coil  and  air-gap  arrester,  widely  used  by  Bell  companies  for  central- 
office  protection,  is  shown  in  Fig. 
226.  The  two  inner  springs  form 
the  terminals  for  the  two  limbs 
of  the  metallic-circuit  line,  while 
the  two  outside  springs  are  ter- 
minals for  the  continuation  of 
the  line  leading  to  the  switch- 
board. The  heat  coils,  one  on 
each  side,  are  supported  between 
the  inner  and  outer  springs. 
High-tension  currents  jump  to 
ground  through  the  air-gap  ar- 
rester, while  sneak  currents  per- 
mit the  pin  of  the  heat  coil  to 
slide  within  the  sleeve,  thus 
grounding  the  outside  line  and 
the  line  to  the  switchboard. 

Self-Soldering  Heat  Coils.  Another  form  designed  by  Kaisling 
and  manufactured  by  the  American  Electric  Fuse  Company  is  shown 
in  Fig.  227.  In  this  the  pin  in  the  heat  coil  projects  unequally 
from  the  ends  of  the  coil,  and  under  the  action  of  a  sneak  current 
the  melting  of  the  solder  which  holds  it  allows  the  outer  spring  to 
push  the  pin  through  the  coil  until  it  presses  the  line  spring  against 


Fig.  226. 


ro  L//VE 


Sneak-Current  and  Air-Gap 
Arrester 


PROTECTIVE  MEANS 


301 


HEAT  CO/L 
BEFORE  OPERAT/HG 


TO  L/ME 


TO    SW/TCHBOAXD 


the  ground  plate  and  at  the  same  time  opens  the  path  to  the  switch- 
board. When  the  heat-coil  pin  assumes  this  new  position  it  cools 
off,  due  to  the  cessation  of  the 
current,  and  resolders  itself,  and 
need  only  be  turned  end  for  end 
by  the  attendant  to  be  reset. 
Many  are  the  variations  that  have 
been  made  on  this  self-soldering 
idea,  and  there  has  been  much 
controversy  as  to  its  desirability. 
It  is  certainly  a  feature  of  con- 
venience. 

Instead  of  using  a  wire- wound 
resistance  element  in  heat-coil 
construction  some  manufacturers 
employ  a  mass  of  high-resistance 
material,  interposed  in  the  path  Fig.227.  Self-Soldering  Heat-Coil  Arrester 
of  the  current.  The  Kellogg 

Company  has  long  employed  for  its  sneak-current  arrester  a  short 
graphite  rod,  which  forms  the  resistance  element.  The  ends  of 
this  rod  are  electroplated  with 
copper  to  which  the  brass  ter- 
minal heads  are  soldered.  These 
heads  afford  means  for  making 
the  connection  with  the  proper 
retaining  spdngs. 

Another  central-office  pro- 
tector, which  uses  a  mass  of  spe- 
cial metal  composition  for  its  heat 
producing  element  is  that  de- 
signed by  Frank  B.  Cook  and 
shown  in  Fig.  228.  In  this  the 
carbon  blocks  are  cylindrical  in 
form  and  specially  treated  to  make 
them  "self-cleaning."  Instead  of 
employing  a  self- soldering  feature 

in  the  sneak-current  arrester  of  this  device,  Cook  provides  for  elec- 
trically resoldering  them  after  operation,  a  clip  being  designed  for 


HEAT    CO/L 
AFTEH  Of>EftAT/H6 


TO 

SW/TCH60A/W 


Fig.  228.     Cook  Arrester 


302 


TELEPHONY 


holding  the  elements  in  proper  position  and  passing  a  battery  current 
through  them  to  remelt  the  solder. 

In  small  magneto  exchanges  it  is  not  uncommon  to  employ 
combined  fuse  and  air-gap  arresters  for  central-office  line  protection, 
the  fuses  being  of  the  mica-mounted  type  already  referred  to.  A 
group  of  such  arresters,  as  manufactured  by  the  Dean  Electric  Com- 
pany, is  shown  in  Fig.  229. 


Fig.  229.     Mica  Fuse  and  Air-Gap  Arresters 

Types  of  Subscribers'  Station  Protectors.  Figs.  230  and  231 
show  types  of  subscribers'  station  protectors  adapted  to  the  require- 
ments of  central-battery  and  magneto  systems.  These,  as  has  been 
said,  should  be  mounted  at  or  near  the  point  of  entrance  of  the  sub- 
scriber's line  into  the  premises,  if  the  line  is  exposed  outside  of  the 
premises.  It  is  possible  to  arrange  the  fuses  so  that  they  will  be  safe 
arid  suitable  for  their  purposes  if  they  are  mounted  out-of-doors 
near  the  point  of  entrance  to  the  premises.  The  sneak-current 
arrester,  if  one  exists,  and  the  carbon  arrester  also,  must  be  mounted 
inside  of  the  premises  or  in  a  protecting  case,  if  outside,  on  account 
of  the  necessity  of  shielding  both  of  these  devices  from  the  weather. 
Speaking  generally,  the  wider  practice  is  to  put  all  the  elements  of 
the  subscriber's  station  protector  inside  of  the  house.  It  is  nearer 
to  the  ideal  arrangement  of  conditions  if  the  protector  be  placed 
immediately  at  the  point  of  entrance  of  the  outside  wires  into  the 
building. 


PROTECTIVE  MEANS 


303 


Ribbon  Fuses.  A  point  of  interest  with  relation  to  tubular 
fuses  is  that  in  some  of  the  best  types  of  such  fuses,  the  resistance 
material  is  not  in  the  form  of  a  round  wire  but  in  the  form  of  a  flat 


Fig.  230.     Western  Electric  Station  Arrester 

ribbon.  This  arrangement  disposes  the  necessary  amount  of  fusible 
metal  in  a  form  to  give  the  greatest  amount  of  surface,  while  a 
round  wire  offers  the  least  surface  for  a  given  weight  of  metal — a  circle 
encloses  its  area  with  less  periphery  than  any  other  figure.  The 
reason  for  giving  the  fuse  the  largest  possible  surface  area  is  to 
decrease  the  likelihood  of  the  fuse  being  ruptured  by  lightning.  The 
fact  that  such  fuses  do  withstand  lightning  discharges  much  more 
thoroughly  than  round  fuses  of  the  same  rating  is  an  interesting 
proof  of  the  oscillating  nature  of  lightning  discharges,  for  the  density 
of  the  current  of  those  discharges  is  greater  on  and  near  the  sur- 
face of  the  conductor  than  within  the  metal  and,  therefore,  flattening 


Fig.  231.     Cook  Arrester  for  Magneto  Stations 

the  fuse  increases  its  carrying  capacity  for  high-frequency  currents, 
without  appreciably  changing  its  carrying  capacity  for  direct  cur- 
rents. The  reason  its  capacity  for  direct  currents  is  increased  at 


304  TELEPHONY 

all  by  flattening  it,  is  that  the  surface  for  the  radiation  of  heat  is 
increased.  However,  when  enclosed  in  a  tube,  radiation  of  heat  is 
limited,  so  that  for  direct  currents  the  carrying  capacity  of  fuses 
varies  closely  with  the  area  of  cross-section. 

City=Exchange  Requirements.  The  foregoing  has  set  down  the 
requirements  of  good  practice  in  an  average  city-exchange  system. 
Nothing  short  of  the  general  arrangement  shown  in  Fig.  225  meets  the 
usual  assortment  of  hazards  of  such  an  exchange.  It  is  good  modern 
practice  to  distribute  lines  by  means  of  cables,  supplemented  in  part 
by  short  insulated  drop  wires  twisted  in  pairs.  Absence  of  bare  wires 
reduces  electrical  hazards  enormously.  Nevertheless,  hazards  remain. 

Though  no  less  than  the  spirit  of  this  plan  of  protection  should 
be  followed,  additional  hazards  may  exist,  which  may  require  addi- 
tional elements  of  protection.  At  the  end  of  a  cable,  either  aerial 
or  underground,  long  open  wires  may  extend  into  the  open  country 
as  rural  or  long-distance  circuits.  If  these  be  longer  than  a  mile  or 
two,  in  most  regions  they  will  be  subjected  to  lightning  discharges. 
These  may  be  subjected  to  high-potential  contacts  as  well. 

If  a  specific  case  of  such  exposure  indicates  that  the  cables  may 
be  in  danger,  the  long  open  lines  then  are  equipped  with  additional 
air-gap  arresters  at  the  point  of  junction  of  those  open  lines  with  the 
cable.  Practice  varies  as  to  the  type.  Maintenance  charges  are 
increased  if  carbon  arresters  separated  .005  inch  are  used,  because  of 
the  cost  of  sending  to  the  end  of  the  long  cable  to  clear  the  blocks  from 
carbon  dust  after  each  slight  discharge.  Roughened  metal  blocks 
do  not  become  grounded  as  readily  as  do  carbon  blocks.  The  oc- 
casions of  visit  to  the  arresters,  therefore,  usually  follow  actual 
heavy  discharges  through  them. 

The  recommendations  and  the  practice  of  the  American  Tele- 
phone and  Telegraph  Company  differ  on  this  point,  while  the  practice 
of  other  companies  varies  with  the  temperaments  of  the  engineers. 
The  American  Company  specifies  copper-block  arresters  where  long 
country  lines  enter  cables,  if  those  lines  are  exposed  to  lightning  dis- 
charges only.  The  exposed  line  is  called  long  if  more  than  one- 
half  mile  in  length.  If  it  is  exposed  to  high-potential  hazards,  car- 
bon blocks  are  specified  instead  of  copper.  Other  specifications  of 
that  company  have  called  for  the  use  of  copper-block  arresters  on 
lines  exposed  to  hazards  above  2,500  volts. 


PROTECTIVE  MEANS 


305 


The  freedom  of  metal- block  arresters  from  dust  troubles  gives 
them  a  large  economical  advantage  over  carbon.  For  similar  sep- 
arations, the  ratio  of  striking  voltages  between  carbon  blocks  and 
metal  blocks  respectively  is  as  7  to  16.  In  certain  regions  of  the 
Pacific  Coast  where  the  lightning  hazard  is  negligible  and  the  high 
tension  hazard  is  great,  metal-block  arresters  at  the  outer  ends  of 
cables  give  acceptable  protection. 

High  winds  which  drive  snow  or  dust  against  bare  wires  of  a 
long  line,  create  upon  or  place  upon  those  wires  a  charge  of  static 
electricity  which  makes  its  way  from  the  line  in  such  ways  as  it  can. 
Usually  it  discharges  across  arresters  and  when  this  discharge  takes 
place,  the  line  is  disturbed  in  its  balance  and  loud  noises  are  heard 
in  the  telephones  upon  it. 

A  telephone  line  which  for  a  long  distance  is  near  a  high-tension 
transmission  line  may  have  electrostatic  or  electromagnetic  poten- 


Fig.  232.     Drainage  Coils 

tials,  or  both,  induced  upon  it.  If  the  line  be  balanced  in  its  prop- 
erties, including  balance  by  transposition  of  its  wires,  the  electro- 
static induction  may  neutralize  itself.  The  electromagnetic  induction 
still  may  disturb  it. 

Drainage  Coils.  The  device  shown  in  Fig.  232,  which  amounts 
merely  to  an  inductive  leak  to  earth,  is  intended  to  cure  both  the 
snowstorm  and  electromagnetic  induction  difficulties.  It  is  required 
that  its  impedance  be  high  enough  to  keep  voice-current  losses  low, 
while  being  low  enough  to  drain  the  line  effectively  of  the  disturbing 
charges.  Such  devices  are  termed  "drainage  coils." 

Electrolysis.  The  means  of  protection  against  the  danger  due 
to  chemical  action,  set  forth  in  the  preceding  chapter,  form  such 
a  distinct  phase  of  the  subject  of  guarding  property  against  electrical 
hazards  as  to  warrant  treatment  in  a  separate  chapter  devoted  to 
the  subject  of  electrolysis. 


CHAPTER  XX 
GENERAL  FEATURES  OF  THE  TELEPHONE  EXCHANGE 

Up  to  this  point  only  those  classes  of  telephone  service  which 
could  be  given  between  two  or  more  stations  on  a  single  line  have 
been  considered.  Very  soon  after  the  practical  conception  of  the 
telephone,  came  the  conception  of  the  telephone  exchange;  that  is. 
the  conception  of  centering  a  number  of  lines  at  a  common  point  and 
there  terminating  them  in  apparatus  to  facilitate  their  intercon- 
nection, so  that  any  subscriber  on  any  line  could  talk  with  any  sub- 
scriber on  any  other  line. 

The  complete  equipment  of  lines,  telephone  instruments,  and 
switching  facilities  by  which  the  telephone  stations  of  the  community 
are  given  telephone  service  is  called  a  telephone  exchange. 

The  building  where  a  group  of  telephone  lines  center  for  inter- 
connection is  called  a  central  office,  and  its  telephonic  equipment  the 
central-office  equipment.  The  terms  telephone  office  and  telephone 
exchange  are  frequently  confused.  Although  a  telephone  office 
building  may  be  properly  referred  to  as  a  telephone  exchange  build- 
ing, it  is  hardly  proper  to  refer  to  the  telephone  office  as  a  telephone 
exchange,  as  is  frequently  done.  In  modern  parlance  the  telephone 
exchange  refers  not  only  to  the  central  office  and  its  equipment  but 
to  the  lines  and  instruments  connected  therewith  as  well;  further- 
more, a  telephone  exchange  may  embrace  a  number  of  telephone 
offices  that  are  interconnected  by  means  of  so-called  trunk  lines  for 
permitting  the  communication  of  subscribers  whose  lines  terminate 
in  one  office  with  those  subscribers  whose  lines  terminate  in  any 
other  office. 

Since  a  given  telephone  exchange  may  contain  one  or  more 
central  offices,  it  is  proper  to  distinguish  between  them  by  referring  to 
an  exchange  which  contains  but  a  single  central  office  as  a  single 
office  exchange,  and  to  an  exchange  which  contains  a  plurality  of 
central  offices  as  a  multi-office  exchange. 


308  TELEPHONY 

In  telephone  exchange  working,  three  classes  of  lines  are  dealt 
with — subscribers'  lines,  trunk  lines,  and  toll  lines. 

Subscribers'  Lines.  The  term  subscriber  is  commonly  applied  to 
the  patron  of  the  telephone  service.  His  station  is,  therefore,  referred 
to  as  a  subscriber's  station,  and  the  telephone  equipment  at  any  sub- 
scriber's station  is  referred  to  as  a  subscriber's  station  equipment. 
Likewise,  a  line  leading  from  a  central  office  to  one  or  more  subscribers' 
stations  is  called  a  subscriber's  line.  A  subscriber's  line  may,  as 
has  been  shown  in  a  previous  chapter,  be  an  individual  line  if  it 
serves  but  one  station,  or  a  party  line  if  it  serves  to  connect  more 
than  one  station  with  the  central  office. 

Trunk  Lines.  A  trunk  line  is  a  line  which  is  not  devoted  to  the 
service  of  any  particular  subscriber,  but  which  may  form  a  con- 
necting link  between  any  one  of  a  group  of  subscribers'  lines  which 
terminate  in  one  place  and  any  one  of  a  group  of  subscribers'  lines 
which  terminate  in  another  place.  If  the  two  groups  of  subscribers' 
lines  terminate  in  the  same  building  or  in  the  same  switchboard,  so 
that  the  trunk  line  forming  the  connecting  link  between  them  is  en- 
tirely within  the  central-office  building,  it  is  called  a  local  trunk  line, 
or  a  local  trunk.  If,  on  the  other  hand,  the  trunk  line  is  for  con- 
necting groups  of  subscribers'  lines  which  terminate  in  different 
central  offices,  it  is  called  an  inter-office  trunk. 

Toll  Lines.  A  toll  line  is  a  telephone  line  for  the  use  of  which  a 
special  fee  or  toll  is  charged;  that  is,  a  fee  that  is  not  included  in  the 
charges  made  to  the  subscriber  for  his  regular  local  exchange  service. 
Toll  lines  extend  from  one  exchange  district  to  another,  more  or  less 
remote,  and  they  are  commonly  termed  local  toll  and  long-distance 
toll  lines  according  to  the  degree  of  remoteness.  A  toll  line,  whether 
local  or  long-distance,  may  be  looked  upon  in  the  nature  of  an  inter- 
exchange  trunk. 

Districts.  The  district  in  a  given  community  which  is  served 
by  a  single  central  office  is  called  an  office  district.  Likewise,  the 
district  which  is  served  by  a  complete  exchange  is  called  an 
exchange  district.  An  exchange  district  may,  therefore,  consist  of 
a  number  of  central-office  districts,  just  as  an  exchange  may  com- 
prise a  number  of  central  offices.  To  illustrate,  the  entire  area  served 
by  the  exchange  of  the  Chicago  Telephone  Company  in  Chicago, 
embracing  the  entire  city  and  some  of  its  suburbs,  is  the  Chicago 


GENERAL  FEATURES  OF  THE  EXCHANGE  309 

exchange  district.  The  area  served  by  one  of  the  central  offices, 
such  as  the  Hyde  Park  office,  the  Oakland  office,  the  Harrison  office, 
or  any  of  the  others,  is  an  office  district. 

Switchboards.  The  apparatus  at  the  central  office  by  which  the 
telephone  lines  are  connected  for  conversation  and  afterwards  dis- 
connected, and  by  which  the  various  other  functions  necessary  to 
the  giving  of  complete  telephone  service  are  performed,  is  called  a 
switchboard.  This  may  be  simple  in  the  case  of  small  exchanges, 
or  of  vast  complexity  in  the  case  of  the  larger  exchanges. 

Sometimes  the  switchboards  are  of  such  nature  as  to  require  the 
presence  of  operators,  usually  girls,  to  connect  and  disconnect  the 
line  and  perform  the  other  necessary  functions,  and  such  switchboards, 
whether  large  or  small,  are  termed  manual. 

Sometimes  the  switchboards  are  of  such  a  nature  as  not  to  re- 
quire the  presence  of  operators,  the  various  functions  of  connection, 
disconnection,  and  signaling  being  performed  by  the  aid  of  special 
forms  of  apparatus  which  are  under  the  control  of  the  subscriber 
who  makes  the  call.  Such  switchboards  are  termed  automatic. 

Of  recent  years  there  has  appeared  another  class  of  switch- 
boards, employing  in  some  measure  the  features  of  the  automatic 
and  in  some  measure  those  of  the  manual  switchboard.  These  boards 
are  commonly  referred  to  as  semi-automatic  switchboards,  pre- 
sumably because  they  are  supposed  to  be  half  automatic  and  half 
manual. 

Manual.  Manual  switchboards  may  be  subdivided  into  two 
classes  according  to  the  method  of  distributing  energy  for  talking 
purposes.  Thus  we  may  have  magneto  switchboards,  which  are 
those  capable  of  serving  lines  equipped  with  magneto  telephones, 
local  batteries  being  used  for  talking  purposes.  On  the  other  hand, 
we  may  have  common-battery  switchboards,  adapted  to  connect 
lines  employing  common-battery  telephones  in  which  all  the  cur- 
rent for  both  talking  and  signaling  is  furnished  from  the  central  office. 
In  still  another  way  we  may  classify  manual  switchboards  if  the 
method  of  distributing  the  energy  for  talking  and  signaling  pur- 
poses is  ignored.  Thus,  entirely  irrespective  of  whether  the  switch- 
boards are  adapted  to  serve  common-battery  or  local-battery  lines, 
we  may  have  non-multiple  switchboards  and  multiple  switch- 
boards. 


310  TELEPHONY 

The  term  multiple  switchboard  is  applied  to  that  class  of  switch- 
boards in  which  the  connection  terminals  or  jacks  for  all  the  lines 
are  repeated  at  intervals  along  the  face  of  the  switchboard,  so  that 
each  operator  may  have  within  her  reach  a  terminal  for  each  line  and 
may  thus  be  able  to  complete  by  herself  any  connection  between  two 
lines  terminating  in  the  switchboard. 

The  term  non-multiple  switchboard  is  applied  to  that  class 
of  boards  where  the  provision  for  repeating  the  line  terminals  at  in- 
tervals along  the  face  of  the  board  is  not  employed,  but  where,  as  a 
consequence,  each  line  has  but  a  single  terminal  on  the  face  of  the 
board.  Non-multiple  switchboards  have  their  main  use  in  small 
exchanges  where  not  more  than  a  few  hundred  lines  terminate. 
Where  such  is  the  case,  it  is  an  easy  matter  to  handle  all  the  traffic 
by  one,  two,  or  three  operators,  and  as  all  of  these  operators  may 
reach  all  over  the  face  of  the  switchboard,  there  is  no  need  for  giving 
any  line  any  more  than  one  connection  terminal.  Such  boards  may 
be  called  simple  switchboards. 

There  is  another  type  of  non-multiple  switchboard  adaptable 
for  use  in  larger  exchanges  than  the  simple  switchboard.  A  cor- 
rect idea  of  the  fundamental  principle  involved  in  these  may  be  had 
by  imagining  a  row  of  simple  switchboards  each  containing  terminals 
or  jacks  for  its  own  group  of  lines.  In  order  to  provide  for  the  con- 
nection of  a  line  in  one  of  these  simple  switchboards  with  a  line  in 
another  one,  out  of  reach  of  the  operator  at  the  first,  short  connecting 
lines  extending  between  the  twro  switchboards  are  provided,  these 
being  called  transfer  or  trunk  lines.  In  order  that  connections  may 
be  made  between  any  two  of  the  simple  boards,  a  group  of  transfer 
lines  is  run  from  each  board  to  every  other  one. 

In  such  switchboards  an  operator  at  one  of  the  boards  or  posi- 
tions may  complete  the  connection  herself  between  any  two  lines 
terminating  at  her  own  board.  If,  however,  the  line  called  for  ter- 
minates at  another  one  of  the  boards,  the  operator  makes  use  of  the 
transfer  or  trunk  line  extending  to  that  board,  and  the  operator  at  this 
latter  board  completes  the  connection,  so  that  the  two  subscribers'  lines 
are  connected  through  the  trunk  or  transfer  line.  A  distinguishing 
feature,  therefore,  in  the  operation  of  so-called  transfer  switchboards, 
is  that  an  operator  can  not  always  complete  a  connection  herself,  the 
connection  frequently  requiring  the  attention  of  two  operators. 


GENERAL  FEATURES  OF  THE  EXCHANGE     311 

Transfer  systems  are  not  now  largely  used,  the  multiple  switch- 
board having  almost  entirely  supplanted  them  in  manual  exchanges 
of  such  size  as  to  be  beyond  the  limitation  of  the  simple  switchboard. 
At  multi-office  manual  exchanges,  however,  where  there  are  a  num- 
ber of  multiple  switchboards  employed  at  various  central  offices,  the 
same  sort  of  a  requirement  exists  as  that  which  was  met  by  the  pro- 
vision of  trunk  lines  between  the  various  simple  switchboards  in  a 
transfer  system.  Obviously,  the  lines  in  one  central  office  must  be 
connected  to  those  of  another  in  order  to  give  universal  service  in 
the  community  in  which  the  exchange  operates.  For  this  purpose 
inter-office  trunk  lines  are  used,  the  arrangement  being  such  that 
when  an  operator  at  one  office  receives  a  call  for  a  subscriber  in 
another  office,  she  will  proceed  to  connect  the  calling  subscriber's 
line,  not  directly  with  the  line  of  the  called  subscriber  because  that 
particular  line  is  not  within  her  reach,  but  rather  with  a  trunk  line 
leading  to  the  office  in  which  the  called-for  subscriber's  line  termi- 
nates; having  done  this  she  will  then  inform  an  operator  at  that 
second  office  of  the  connection  desired,  usually  by  means  of  a  so- 
called  order- wire  circuit.  The  connection  between  the  trunk  line  so 
used  and  the  line  of  the  called-for  subscriber  will  then  be  com- 
pleted by  the  connecting  link  or  trunk  line  extending  between  the 
two  offices. 

In  such  cases  the  multiple  switchboard  at  each  office  is  divided 
into  two  portions,  termed  respectively  the  A  board  and  the  B  board. 
Each  of  these  boards,  with  the  exception  that  will  be  pointed  out  in  a 
subsequent  chapter,  is  provided  with  a  full  complement  of  multiple 
jacks  for  all  of  the  lines  entering  that  office.  At  the  A  board  are 
located  operators,  called  A  operators,  who  answer  all  the  calls  from 
the  subscribers  whose  lines  terminate  in  that  office.  In  the  case 
of  calls  for  lines  in  that  same  office,  they  complete  the  connection 
themselves  without  the  assistance  of  the  other  operators.  On  the 
other  hand,  the  calls  for  lines  in  another  office  are  handled  through 
trunk  lines  leading  to  that  other  office,  as  before  described,  and  these 
trunk  lines  always  terminate  in  the  B  board  at  that  office.  The 
B  operators  are,  therefore,  those  operators  who  receive  the  calls  over 
trunk  lines  and  complete  the  connection  with  the  line  of  the  sub- 
scriber desired. 

To  define  these  terms  more  specifically,  an  A  board  is  a  multi- 


312  TELEPHONY 

pie  switchboard  in  which  the  subscriber's  lines  of  a  given  office  dis- 
trict terminate.  For  this  reason  the  A  board  is  frequently  referred 
to  as  a  subscribers'  board,  and  the  operators  who  work  at  these 
boards  and  who  answer  the  calls  of  the  subscribers  are  called  A 
operators  or  subscribers'  operators.  B  boards  are  switchboards  in 
which  terminate  the  incoming  ends  of  the  trunk  lines  leading  from 
other  offices  in  the  same  exchange.  These  boards  are  frequently 
called  incoming  trunk  boards,  or  merely  trunk  boards,  and  the 
operators  who  work  at  them  and  who  receive  the  directions  from  the 
A  operators  at  the  other  boards  are  called  B  operators,  or  incoming 
trunk  operators. 

The  circuits  which  are  confined  wholly  to  the  use  of  operators 
and  over  which  the  instructions  from  one  operator  to  another  are 
sent,  as  in  the  case  of  the  A  operator  giving  an  order  for  a  connec- 
tion to  a  B  operator  at  another  switchboard,  are  designated  call 
circuits  or  order  wire  circuits. 

Sometimes  trunk  lines  are  so  arranged  that  connections  may 
be  originated  at  either  of  their  ends.  In  other  cases  they  are  so  ar- 
ranged that  one  group  of  trunk  lines  connecting  two  offices  is  for 
the  traffic  in  one  direction  only,  while  another  group  leading  between 
the  same  two  offices  is  for  handling  only  the  traffic  in  the  other 
direction.  Trunk  lines  are  called  one-way  or  two-way  trunks,  ac- 
cording to  whether  they  handle  the  traffic  in  one  direction  or  in  two. 
A  trunking  system,  where  the  same  trunks  handle  traffic  both  ways, 
is  called  a  single-track  system;  and,  on  the  other  hand,  a  system  in 
which  there  are  two  groups  of  trunks,  one  handling  traffic  in  one 
direction  and  the  other  in  the  other,  is  called  a  double-track  system. 
This  nomenclature  is  obviously  borrowed  from  railroad  practice. 

There  is  still  another  class  of  manual  switchboards  called  the 
toll  board  of  which  it  will  be  necessary  to  treat.  Telephone  calls 
made  by  one  person  for  another  within  the  limits  of  the  same 
exchange  district  are  usually  charged  for  either  by  a  flat  rate  per 
month,  or  by  a  certain  charge  for  each  call.  This  is  usually 
regardless  of  the  duration  of  the  conversation  following  the  call.  On 
the  other  hand,  where  a  call  is  made  by  one  party  for  another  out- 
side of  the  limits  of  the  exchange  district  and,  therefore,  in  some 
other  exchange  district,  a  charge  is  usually  made,  based  on  the  time 
that  the  connecting  long-distance  line  is  employed.  Such  calls  and 


GENERAL  FEATURES  OF  THE  EXCHANGE     313 

their  ensuing  conversations  are  charged  for  at  a  very  much  higher 
rate  than  the  purely  local  calls,  this  rate  depending  on  the  distance 
between  the  stations  involved.  The  making  up  of  connections  be- 
tween a  long-distance  and  a  local  line  is  usually  done  by  means  of 
operators  other  than  those  employed  in  handling  the  local  calls, 
who  work  either  by  means  of  special  equipment  located  on  the  local 
board,  or  by  means  of  a  separate  board.  Such  equipments  for 
handling  long-distance  or  toll  traffic  are  commonly  termed  toll 
switchboards. 

They  differ  from  local  boards  (a)  in  that  they  are  arranged  for  a 
very  much  smaller  number  of  lines;  (6)  in  that  they  have  facilities 
by  which  the  toll  operator  may  make  up  the  connections  with  a  mini- 
mum amount  of  labor  on  the  part  of  the  assisting  local  operators;  and 
(c)  in  that  they  have  facilities  for  recording  the  identification  of 
the  parties  and  timing  the  conversations  taking  place  over  the  toll 
lines,  so  that  the  proper  charge  may  be  made  to  the  proper  sub- 
scriber. 


CHAPTER  XXI 
THE  SIMPLE  MAGNETO  SWITCHBOARD 

Definitions.  As  already  stated  those  switchboards  which  are 
adapted  to  work  in  conjunction  with  magneto  telephones  are  called 
magneto  switchboards.  The  signals  on  such  switchboards  are 
electromagnetic  devices  capable  of  responding  to  the  currents  of  the 
magneto  generators  at  the  subscribers'  stations.  Since,  as  a  rule, 
magneto  telephones  are  equipped  with  local  batteries,  it  follows  that 
the  magneto  switchboard  does  not  need  to  be  arranged  for  supplying 
'the  subscribers'  stations  with  talking  current.  This  fact  is  ac- 
countable for  magneto  switchboards  often  being  referred  to  as  local- 
battery  switchboards,  in  contradistinction  to  common-battery 
switchboards  which  are  equipped  so  as  to  supply  the  connected 
subscribers'  stations  with  talking  current. 

The  term  simple  as  applied  in  the  headings  of  this  and  the  next 
chapter,  is  employed  to  designate  switchboards  adapted  for  so  small 
a  number  of  lines  that  they  may  be  served  by  a  single  or  a  very  small 
group  of  operators;  each  line  is  provided  with  but  a  single  con- 
nection terminal  and  all  of  them,  without  special  provision,  are 
placed  directly  within  the  reach  of  the  operator,  or  operators  if  there 
are  more  than  one.  This  distinction  will  be  more  apparent  under 
the  discussion  of  transfer  and  multiple  switchboards. 

Mode  of  Operation.  The  cycle  of  operation  of  any  simple  man- 
ual switchboard  may  be  briefly  outlined  as  follows:  The  subscriber 
desiring  a  connection  transmits  a  signal  to  the  central  office,  the  op- 
erator seeing  the  signal  makes  connection  with  the  calling  line  and 
places  herself  in  telephonic  communication  with  the  calling  sub- 
scriber to  receive  his  orders;  the  operator  then  completes  the  con- 
nection with  the  line  of  the  called  subscriber  and  sends  ringing  cur- 
rent out  on  that  line  so  as  to  ring  the  bell  of  that  subscriber;  the  two 
subscribers  then  converse  over  the  connected  lines  and  when  the 
conversation  is  finished  either  one  or  both  of  them  may  send  a  signal 


SIMPLE  MAGNETO  SWITCHBOARD  315 

to  the  central  office  for  disconnection,  this  signal  being  called  a 
clearing-out  signal;  upon  receipt  of  the  clearing-out  signal,  the  oper- 
ator disconnects  the  two  lines  and  restores  all  of  the  central-office 
apparatus  involved  in  the  connection  to  its  normal  position. 

Component  Parts.  Before  considering  further  the  operation  of 
manual  switchboards  it  will  be  well  to  refer  briefly  to  the  component 
pieces  of  apparatus  which  go  to  make  up  a  switchboard. 

Line  Signal.  The  line  signal  in  magneto  switchboards  is  prac- 
tically always  in  the  form  of  an  electromagnetic  annunciator  or  drop. 
It  consists  in  an  electromagnet  adapted  to  be 
included  in  the  line  circuit,  its  armature  con- 
trolling a  latch,  which  serves  to  hold  the  drop 
or  shutter  or  target  in  its  raised  position  when 

the  magnet  is  not  energized,  and  to  release 

,  '  ,  .         Fig.  233.     Drop  Symbol 

the  drop  or  shutter  or  target  so  as  to  permit 

the  display  of  the  signal  when  the  magnet  is  energized.  The  sym- 
bolic representation  of  such  an  electromagnetic  drop  is  shown  in 
Fig.  233. 

Jacks  and  Plugs.  Each  line  is  also  provided  with  a  connection 
terminal  in  the  form  of  a  switch  socket.  This  assumes  many  forms, 
but  always  consists  in  a  cylindrical  opening  behind  which  are  ar- 
ranged one  or  more  spring  contacts.  The  opening  forms  a  recep- 
tacle for  plugs  which  have  one  or  more  metallic  terminals  for  the 
conductors  in  the  flexible  cord  in  which  the  plug  terminates.  The 
arrangement  is  such  that  when  a  plug  is  inserted  into  a  jack  the 
contacts  on  the  plug  will  register  with  certain  of  the  contacts  in  the 
jack  and  thus  continue  the  line  conductors,  which  terminate  in  the 
jack  contacts,  to  the 
cord  conductors,  which 
terminate  in  the  plug 
contacts.  Usually  also 
when  a  plug  is  inserted 
certain  of  the  spring  con- 

r  Fig.  234      Spring  Jack 

tacts  in  the  jack  are  made 

to  engage  with  or  disengage  other  contacts  in  the  jack  so  as  to  make 

or  break  auxiliary  circuits. 

A  simple  form  of  spring  jack  is  shown  in  section  in  Fig.  234. 
In  Fig.  235  is  shown  a  sectional  view  of  a  plug  adapted  to  co-operate 


316 


TELEPHONY 


with  the  jack  of  Fig.  234.     In  Fig.  236  the  plug  is  shown  inserted 
into    the   jack.     The  cylindrical  portion  of  the  jack  is  commonly 


called  the  sleeve  or  thimble  and  it  usually  forms  one  of  the  main  ter- 
minals of  the  jack;  the  spring,  forming  the  other  principal  terminal, 


Fig.  236.     Plug  and  Jack 

is  called  the  tip  spring,  since  it  engages  the  tip  of  the  plug.  The  tip 
spring  usually  rests  on  another  contact  which  may  be  termed  the 
anvil.  When  the  plug  is  inserted  into  the  jack  as  shown  in  Fig.  236, 
the  tip  spring  is  raised  from  contact  with  this  anvil  and  thus  breaks 
the  circuit  leading  through  it.  It  will  be  understood  that  spring  jacks 
are  not  limited  to  three  contacts  such  as  shown  in  these  figures  nor 
are  plugs  limited  to  two  contacts.  Sometimes  the  plugs  have  three, 

and  even  more,  contacts,  and 
frequently  the  jacks  correspond- 
ing to  such  plugs  have  not  only  a 
contact  spring  adapted  to  register 
with  each  of  the  contacts  of  the 
plug,  but  several  other  auxiliary 
contacts  also,  which  will  be  made 
or  broken  according  to  whether 
the  plug  is  inserted  or  withdrawn 
from  the  jack.  Symbolic  repre- 
sentations of  plugs  and  jacks  are  shown  in  Fig.  237.  These  are  em- 
ployed in  diagrammatic  representations  of  circuits  and  are  supposd  to 
represent  the  essential  elements  of  the  plugs  and  jacks  in  such  a  way 
as  to  be  suggestive  of  their  operation.  It  will  be  understood  that 
such  symbols  may  be  greatly  modified  to  express  the  various  pecu- 
liarities of  the  plugs  and  jacks  which  they  represent. 


Fig.  237.     Jack  and  Plug  Symbols 


SIMPLE  MAGNETO  SWITCHBOARD 


317 


Keys.  Other  important  elements  of  manual  switchboards  are 
ringing  and  listening  keys.  These  are  the  devices  by  means  of  which 
the  operator  may  switch  the  central-office  generator  or  her  telephone 
set  into  or  out  of  the  circuit  of  the  connected  lines.  The  details  of  a 
simple  ringing  and  listening  key  are  shown  in  Fig.  238.  This  con- 


Fig.  238.    Ringing  and  Listening  Key 

sists  of  two  groups  of  springs,  one  of  four  and  one  of  six,  the  springs 
in  each  group  being  insulated  from  each  other  at  their  points  of 
mounting.  Two  of  these  springs  1  and  2  in  one  group — the  ringing 
group — are  longer  than  the  others,  and  act  as  movable  levers  en- 
gaging the  inner  pair  of  springs  3  and  4  when  in  their  normal  posi- 
tions, and  the  outer  pair  5  and  6  when  forced  into  their  alternate 
positions.  Movement  is  imparted  to  these  springs  by  the  action  of 
a  cam  which  is  mounted  on  a  lever,  manipulated  by  the  operator. 
When  this  lever  is  moved  in  one  direction  the  cam  presses  the 
two  springs  1  and  2  apart,  thus  causing 
them  to  disengage  the  springs  3  and  4 
and  to  engage  the  springs  5  and  6. 

The  springs  of  the  other  group  con- 
stitute the  switching  element  of  the  listen- 
ing key  and  are  very  similar  in  their 
action  to  those  of  the  ringing  key,  differ- 
ing in  the  fact  that  they  have  no  inner 
pair  of  springs  such  as  3  and  4~  The 
two  long  springs  7  and  8,  therefore,  nor- 
mally do  not  rest  against  anything,  but 
when  the  key  lever  is  pressed,  so  as  to 
force  the  cam  between  them,  they  are  made  to  engage  the  two 
outer  springs  9  and  10. 

The  design  and  construction  of  ringing  and  listening  keys  as- 
sume many  different  forms.     In  general,  however,  they  are  adapted 


Fig.  239.     Ringing-  and  Lis- 
tening-Key Symbols 


318  TELEPHONY 

to  do  exactly  the  same  sort  of  switching  operations  as  that  of  which 
the  device  of  Fig.  238  is  capable.  Easily  understood  symbols  of 
ringing  and  listening  keys  are  shown  in  Fig.  239;  the  cam  member 
which  operates  on  the  two  long  springs  is  usually  omitted  for  ease 
of  illustration.  It  will  be  understood  in  considering  these  symbols, 
therefore,  that  the  two  long  curved  springs  usually  rest  against  a  pair 
of  inner  contacts  in  case  of  the  ringing  key  or  against  nothing  at  all 
in  case  of  the  listening  key,  and  that  when  the  key  is  operated  the 
two  springs  are  assumed  to  be  spread  apart  so  as  to  engage  the  outer 
pair  of  contacts  with  which  they  are  respectively  normally  discon- 
nected. 

Line  and  Cord  Equipments.  The  parts  of  the  switchboard 
that  are  individual  to  the  subscriber's  line  are  termed  the  line  equip- 
ment; this,  in  the  case  of  a  magneto  switchboard,  consists  of  the  line 
drop  and  the  jack  together  with  the  associated  wiring  necessary 
to  connect  them  properly  in  the  line  circuit.  The  parts  of  the  switch- 
board that  are  associated  with  a  connecting  link — consisting  of  a  pair 
of  plugs  and  associated  cords  with  their  ringing  and  listening  keys 
and  clearing-out  drop — are  referred  to  as  a  cord  equipment.  The 
circuit  of  a  complete  pair  of  cords  and  plugs  with  their  associated 
apparatus  is  called  a  cord  circuit.  In  order  that  there  may  be  a  num- 
ber of  simultaneous  connections  between  different  pairs  of  lines 
terminating  in  a  switchboard,  a  number  of  cord  circuits  are  provided, 
this  number  depending  on  the  amount  of  traffic  at  the  busiest  time 
of  the  day. 

Operator's  Equipment.  A  part  of  the  equipment  that  is  not 
individual  to  the  lines  or  to  the  cord  circuits,  but  which  may,  as 
occasion  requires,  be  associated  with  any  of  them  is  called  the  oper- 
ator's equipment.  This  consists  of  the  operator's  transmitter  and 
receiver,  induction  coil,  and  battery  connections  together  with  .the 
wiring  and  other  associated  parts  necessary  to  co-ordinate  them  with 
the  rest  of  the  apparatus.  Still  another  part  of  the  equipment  that 
is  not  individual  to  the  lines  nor  to  the  cord  circuits  is  the  calling- 
current  generator.  This  may  be  common  to  the  entire  office  or  a 
separate  one  may  be  provided  for  each  operator's  position. 

Operation  in  Detail.  With  these  general  statements  in  mind 
we  may  take  up  in  some  detail  the  various  operations  of  a  telephone 
system  wherein  the  lines  center  in  a  magneto  switchboard.  This 


SIMPLE  MAGNETO  SWITCHBOARD 


319 


may  best  be  done  by  considering  the  circuits  involved,  without  spe- 
cial regard  to  the  details  of  the  apparatus. 

The  series  of  figures  showing  the  cycle  of  operations  of  the  mag- 
neto switchboard  about  to  be  discussed  are  typical  of  this  type  of 
switchboard  almost  regardless  of  make.  The  apparatus  is  in  each 
case  represented  symbolically,  the  representations  indicating  type 
rather  than  any  particular  kind  of  apparatus  within  the  general 
class  to  which  it  belongs. 

Normal  Condition  of  Line.  In  Fig.  240  is  shown  the  circuit  of 
an  ordinary  magneto  line.  The  subscriber's  sub-station  apparatus, 
shown  at  the  left,  consists  of  the  ordinary  bridging  telephone  but 
might  with  equal  propriety  be  indicated  as  a  series  telephone.  The 
subscriber's  station  is  shown  connected  with  the  central  office  by  the 
two  limbs  of  a  metallic-circuit  line.  One  limb  of  the  line  terminates 


Pig.  240.     Normal  Condition  of  Line 

in  the  spring  1  of  the  jack,  and  the  other  limb  in  the  sleeve  or  thimble 
2  of  4;he  jack.  The  spring  1  normally  rests  on  the  third  contact  or 
anvil  3  in  the  jack,  its  construction  being  such  that  when  a  plug  is 
inserted  this  spring  will  be  raised  by  the  plug  so  as  to  break  contact 
with  the  anvil  3.  It  is  understood,  of  course,  that  the  plug  associated 
with  this  jack  has  two  contacts,  referred  to  respectively  as  the  tip 
and  the  sleeve;  the  tip  makes  contact  with  the  tip  spring  1  and  the 
sleeve  with  the  sleeve  or  thimble  2. 

The  drop  or  line  signal  is  permanently  connected  between  the 
jack  sleeve  and  the  anvil  3.  As  a  result,  the  drop  is  normally  bridged 
across  the  circuit  of  the  line  so  as  to  be  in  a  receptive  condition  to 
signaling  current  sent  out  by  the  subscriber.  It  is  evident,  however, 
that  when  the  plug  is  inserted  into  the  jack  this  connection  between 
the  line  and  the  drop  will  be  broken. 


320 


TELEPHONY 


In  this  normal  condition  of  the  line,  therefore,  the  drop  stands 
ready  at  the  central  office  to  receive  the  signal  from  the  subscriber 
and  the  generator  at  the  sub-station  stands  ready  to  be  bridged  across 
the  circuit  of  the  line  as  soon  as  the  subscriber  turns  its  handle. 
Similarly  the  ringer — the  call-receiving  device  at  the  sub-station — is 
permanently  bridged  across  the  line  so  as  to  be  responsive  to  any 
signal  that  may  be  sent  out  from  the  central  office  in  order  to  call  the 
subscriber.  The  subscriber's  talking  apparatus  is,  in  this  normal 
condition  of  the  line,  cut  out  of  the  circuit  by  the  switch  hook. 

Subscriber  Calling.  Fig.  241  shows  the  condition  of  the  line 
when  the  subscriber  at  the  sub-station  is  making  a  call.  In  turning 
his  generator  the  two  springs  which  control  the  connection  of  the 
generator  with  the  line  are  brought  into  engagement  with  each  other 


CALL/M6   STATION 


Pig.  241      Subscriber  Calling 

so  that  the  generator  currents  may  pass  out  over  the  line.  The  con- 
dition at  the  central  office  is  the  same  as  that  of  Fig.  240  except  that 
the  drop  is  shown  with  its  shutter  fallen  so  as  to  indicate  a  call* 

Operator  Answering.  The  next  step  is  for  the  operator  to  an- 
swer the  call  and  this  is  shown  in  Fig.  242.  The  subscriber  has  re- 
leased the  handle  of  his  generator  and  the  generator  has,  therefore, 
been  automatically  cut  out  of  the  circuit.  He  also  has  removed  his 
receiver  from  its  hook,  thus  bringing  his  talking  apparatus  into 
the  line  circuit.  The  operator  on  the  other  hand  has  inserted  one  of 
the  plugs  Pa  into  the  jack.  This  action  has  resulted  in  the  breaking 
of  the  circuit  through  the  drop  by  the  raising  of  the  spring  1  from 
the  anvil  3,  and  also  in  the  continuance  of  the  line  circuit  through  the 
conductors  of  the  cord  circuits.  Thus,  the  upper  limb  of  the  line  is 
continued  by  means  of  the  engagement  of  the  tip  spring  1  with  the 
tip  4  of  the  plug  to  the  conducting  strand  6  of  the  cord  circuit;  like- 


SIMPLE  MAGNETO  SWITCHBOARD 


321 


wise  the  lower  limb  of  the  line  is  continued  by  the  engagement  of  the 
thimble  2  of  the  jack  with  the  sleeve  contact  5  of  the  plug  Pa  to  the 
strand  7  of  the  cord  circuit.  The  operator  has  also  closed  her  listen- 
ing key  L.K.  In  doing  so  she  has  brought  the  springs  8  and  9  into 
engagement  with  the  anvils  10  and  11  and  has  thus  bridged  her  head 
telephone  receiver  with  the  secondary  of  her  induction  coil  across  the 
two  strands  6  and  7  of  the  cord.  Associated  with  the  secondary 
winding  of  her  receiver  is  a  primary  circuit  containing  a  transmitter, 
battery,  and  the  primary  of  the  induction  coil.  It  will  be  seen  that 
the  conditions  are  now  such  as  to  permit  the  subscriber  at  the  calling 
station  to  converse  with  the  operator  and  this  conversation  consists 
in  the  familiar  "Number  Please"  on  the  part  of  the  operator  and  the 
response  of  the  subscriber  giving  the  number  of  the  line  that  is  desired. 


CALL/KG    STAT/OM 


SET 


Pig.  242.     Operator  Answering 


Neither  the  plug  PC,  nor  the  ringing  key  R.K.,  shown  in  Fig.  242, 
is  used  in  this  operation.  The  clearing-out  drop  C.O.  is  bridged 
permanently  across  the  strands  6-7  of  the  cord,  but  is  without  func- 
tion at  this  time;  the  fact  that  it  is  wound  to  a  high  resistance  and 
impedance  prevents  its  having  a  harmful  effect  on  the  transmission. 
It  may  be  stated  at  this  point  that  the  two  plugs  of  an  associated 
pair  are  commonly  referred  to  as  the  answering  and  calling  plugs. 
The  answering  plug  is  the  one  which  the  operator  always  uses  in 
answering  a  call  as  just  described  in  connection  with  Fig.  242.  The 
calling  plug  is  the  one  which  she  next  uses  in  connecting  with  the 
line  of  the  called  subscriber.  It  lies  idle  during  the  answering  of  a 
call  and  is  only  brought  into  play  after  the  order  of  the  calling  sub- 
scriber has  been  given,  in  which  case  it  is  used  in  establishing  con- 
nection with  the  called  subscriber. 


322 


TELEPHONY 


Operator  Calling.  We  may 
now  consider  how  the  operator 
calls  the  called  subscriber.  The 
condition  existing  for  this  opera- 
tion is  shown  in  Fig.  243.  The 
operator  after  receiving  the  order 
from  the  calling  subscriber  inserts 
the  calling  plug  Pc  into  the  jack 
of  the  line  of  the  called  station. 
This  act  at  once  connects  the 
limbs  of  the  line  with  the  strands 
6  and  7  of  the  cord  circuit,  and 
also  cuts  out  the  line  drop  of  the 
called  station,  as  already  ex- 
plained. The  operator  is  shown 
in  this  figure  as  having  opened  her 
listening  key  L.K.  and  closed  her 
ringing  key  R.K.  As  a  result, 
ringing  current  from  the  central- 
office  generator  will  flow  out  over 
the  two  ringing  key  springs  12 
and  13  to  the  tip  and  sleeve  con- 
tacts of  the  calling  plug  Pc,  then 
to  the  tip  spring  1  and  the  sleeve 
or  thimble  2  of  the  jack,  and  then 
to  the  two  sides  of  the  metallic- 
circuit  line  to  the  sub-station  and 
through  the  bell  there.  This 
causes  the  ringing  of  the  called 
subscriber's  bell,  after  which  the 
operator  releases  the  ringing 
key  and  thereby  allows  the  two 
springs  12  and  13  of  that  key 
to  again  engage  their  normal 
contacts  14  and  15,  thus  making 
the  twro  strands  6  and  7  of  the 
cord  circuit  continuous  from 
the  contacts  of  the  answering 


SIMPLE  MAGNETO  SWITCHBOARD 


323 


plug  Pa  to  the  contacts  of  the  calling  plug 
Pc.  This  establishes  the  condition  at  the 
central  office  for  conversation  between  the 
two  subscribers. 

Subscribers  Conversing.  The  only  other 
thing  necessary  to  establish  a  complete  set 
of  talking  conditions  between  the  two  sub- 
scribers is  for  the  called  subscriber  to  remove 
his  receiver  from  its  hook,  which  he  does  as 
soon  as  he  responds  to  the  call.  The  condi- 
tions for  conversation  between  the  two  sub- 
scribers are  shown  in  Fig.  244.  It  is  seen 
that  the  two  limbs  of  the  calling  line  are 
connected  respectively  to  the  two  limbs  of 
the  called  line  by  the  two  strands  of  the  cord 
circuit,  both  the  operator's  receiver  and  the 
central-office  generator  being  cut  out  by 
the  listening  and  ringing  keys,  respectively. 
Likewise  the  two  line  drops  are  cut  out 
of  circuit  and  the  only  thing  left  asso- 
ciated with  the  circuit  at  the  central  office 
is  the  clearing-out  drop  C.  O.,  which 
remains  bridged  across  the  cord  circuit. 
This,  like  the  two  ringers  at  the  respec- 
tive connected  stations,  which  also  remain 
bridged  across  the  circuit  when  bridging 
instruments  are  used,  is  of  such  high  re- 
sistance and  impedance  that  it  offers  prac- 
tically no  path  to  the  rapidly  fluctuating 
voice  currents  to  leak  from  one  side  of  the 
line  circuit  to  the  other.  Fluctuating  cur- 
rents generated  by  the  transmitter  at  the 
calling  station,  for  instance,  are  converted 
by  means  of  the  induction  coil  into  alter- 
nating currents  flowing  in  the  secondary  of 
the  induction  coil  at  that  station.  Consider- 
ing a  momentary  current  as  passing  up 
through  the  secondary  winding  of  the  induc- 


324 


TELEPHONY 


1 

^ 

L 

X 

F 

.\ 

1 

=» 

1 

°L 


tion  coil  at  the  calling  station,  it  passes 
through  the  receiver  of  that  station  through 
the  upper  limb  of  the  line  to  the  spring  /  of 
the  line  jack  belonging  to  that  line  at  the 
central  office;  thence  through  the  tip  4  of  the 
answering  plug  to  the  conductor  G  of  the 
cord;  thence  through  the  pair  of  contacts  l/t 
and  12  forming  one  side  of  the  ringing  key 
to  the  tip  4  °f  the  calling  plug;  thence  to 
the  tip  spring  /  of  the  jack  of  the  called  sub- 
scriber's line;  thence  over  the  upper  limb  of 
his  line  through  his  receiver  and  through 
the  secondary  of  the  induction  to  one  of 
the  upper  switch-hook  contacts;  thence 
through  the  hook  lever  to  the  lower  side  of 
the  line,  back  to  the  central  office  and 
through  the  sleeve  contact  2  of  the  jack 
and  the  sleeve  contact  5  of  the  plug; 
thence  through  the  other  ringing  key  con- 
tacts 13  and  15,  thence  through  the  strand 
7  of  the  cord  to  the  sleeve  contact  5  and 
the  sleeve  contact  2  of  the  answering 
plug  and  jack,  respectively;  thence  through 
the  lower  limb  of  the  calling  subscriber's 
line  to  the  hook  lever  at  his  station; 
thence  through  one  of  the  upper  contacts 
of  this  hook  to  the  secondary  of  the  in- 
duction coil,  from  which  point  the  current 
started. 

Obviously,  when  the  called  subscriber  is 
talking  to  the  calling  subscriber  the  same 
path  is  followed.  It  will  be  seen  that  at 
any  time  the  operator  may  press  her  listen- 
ing key  L.K.,  bridge  her  telephone  set 
across  the  circuit  of  the  two  connected  lines, 
and  listen  to  the  conversation  or  converse 
with  either  of  the  subscribers  in  case  of 
necessity. 


SIMPLE  MAGNETO  SWITCHBOARD  325 

Clearing  Out.  At  the  close  of  the  conversation,  either  one  or 
both  of  the  subscribers  may  send  a  clearing-out  signal  by  turning 
their  generators  after  hanging  up  their  receivers.  This  condition 
is  shown  in  Fig.  245.  The  apparatus  at  the  central  office  remains 
in  exactly  the  same  position  during  conversation  as  that  of  Fig. 
244,  except  that  the  clearing-out  drop  shutter  is  shown  as  having 
fallen.  The  two  subscribers  are  shown  as  having  hung  up  their 
receivers,  thus  cutting  out  their  talking  apparatus,  and  as  operat- 
ing their  generators  for  the  purpose  of  sending  the  clearing-out 
signals.  In  response  to  this  act  the  operator  pulls  down  both  the 
calling  and  the  answering  plug,  thus  restoring  them  to  their 
normal  seats,  and  bringing  both  lines  to  the  normal  condition  as 
shown  in  Fig.  240.  The  line  drops  are  again  brought  into  opera- 
tive relation  with  their  respective  lines  so  as  to  be  receptive 
to  subsequent  calls  and  the  calling  generators  at  the  sub-stations 
are  removed  from  the  bridge  circuits  across  the  line  by  the  open- 
ing of  the  automatic  switch  contacts  associated  with  those  genera- 
tors. 

Essentials  of  Operation.  The  foregoing  sequence  of  opera- 
tions while  described  particularly  with  respect  to  magneto  switch- 
boards is,  with  certain  modifications,  typical  of  the  operation  of 
nearly  all  manual  switchboards.  In  the  more  advanced  types  of 
manual  switchboards,  certain  of  the  functions  described  are  some- 
times done  automatically,  and  certain  other  functions,  not  necessary 
in  connection  with  the  simple  switchboard,  are  added.  The  essential 
mode  of  operation,  however,  remains  the  same  in  practically  all 
manual  switchboards,  and  for  this  reason  the  student  should  thor- 
oughly familiarize  himself  with  the  operation  and  circuits  of  the 
simple  switchboard  as  a  foundation  for  the  more  complex  and  con- 
sequently more-difficult-to-understand  switchboards  that  will  be  de- 
scribed later  on. 

Commercial  Types  of  Drops  and  Jacks.  Early  Drops.  Coming 
now  to  the  commercial  types  of  switchboard  apparatus,  the  first 
subject  that  presents  itself  is  that  of  magneto  line  signals  or  drops. 
The  very  early  forms  of  switchboard  drops  had,  in  most  cases,  two- 
coil  magnets,  the  cores  of  which  were  connected  at  their  forward 
ends  by  an  iron  yoke  and  the  armature  of  which  was  pivoted  op- 
posite the  rear  end  of  the  two  cores.  To  the  armature  was  attached 


326  TELEPHONY 

a  latch  rod  which  projected  forwardly  to  the  front  of  the  device  and 
was  there  adapted  to  engage  the  upper  edge  of  the  hinged  shutter, 
so  as  to  hold  it  in  its  raised  or  undisplayed  position  when  the  arma- 
ture   was    unattracted.      Such   a 
drop,  of  Western   Electric  manu- 
facture, is  shown  in  Fig.  246. 

Liability  to  Cross-Talk:— This 
type  of  drop  is  suitable  for  use 
only  on  small  switchboards  where 
space  is  not  an  important  con- 
sideration, and  even  then  only 

when  the  drop  is  entirely  cut  out 
Fig.  246.     Old-Style  Drop  » 

of  the  circuit  during  conversa- 
tion. The  reason  for  this  latter  requirement  will  be  obvious 
when  it  is  considered  that  there  is  no  magnetic  shield  around 
the  winding  of  the  magnet  and  no  means  for  preventing  the 
stray  field  set  up  by  the  talking  currents  in  one  of  the  magnets 
from  affecting  by  induction  the  windings  of  adjacent  magnets 
contained  in  other  talking  circuits.  Unless  the  drops  are  entirely 
cut  out  of  the  talking  circuit,  therefore,  they  are  very  likely  to 
produce  cross-talk  between  adjacent  circuits.  Furthermore,  such 
form  of  drop  is  obviously  not  economical  of  space,  two  coils  placed 
side  by  side  consuming  practically  twice  as  much  room  as  in  the  case 
of  later  drops  wherein  single  magnet  coils  have  been  made  to  answer 
the  purpose. 

Tubular  Drops.  In  the  case  of  line  drops,  which  usually  can 
readily  be  cut  out  of  the  circuit  during  conversation,  this  cross-talk 
feature  is  not  serious,  but  sometimes  the  line  drops,  and  always  the 
clearing-out  drops  must  be  left  in  connection  with  the  talking 
circuit.  On  account  of  economy  in  space  and  also  on  account  of 
this  cross-talk  feature,  there  has  come  into  existence  the  so-called  tubu- 
lar or  iron-clad  drop,  one  of  which  is  shown  in  section  in  Fig.  247. 
This  was  developed  a  good  many  years  ago  by  Mr.  E.  P.  Warner  of 
the  Western  Electric  Company,  and  has  since,  with  modifications, 
become  standard  with  practically  all  the  manufacturing  companies. 
In  this  there  is  but  a  single  bobbin,  and  this  is  enclosed  in  a  shell  of 
soft  Norway  iron,  which  is  closed  at  its  front  end  and  joined  to  the 
end  of  the  core  as  indicated,  so  as  to  form  a  complete  return  mag- 


SIMPLE  MAGNETO  SWITCHBOARD 


netic  path  for  the  lines  of  force  generated  in  the  coil.  The  rear  end 
of  the  shell  and  core  are  both  cut  off  in  the  same  plane  and  the  arma- 
ture is  made  in  such  form  as  to  practically  close  this  end  of  the  shell. 
The  armature  carries  a  latch  rod  extending  the  entire  length  of  the 


Pig.  247.     Tubular  Drop 

shell  to  the  front  portion  of  the  structure,  where  it  engages  the  upper 
edge  of  the  pivoted  shutter;  this,  when  released  by  the  latch  upon 
the  attraction  of  the  armature,  falls  so  as  to  display  a  target  be- 
hind it. 

These  drops  may  be  mounted  individually  on  the  face  of  the 
switchboard,  but  it  is  more  usual  to  mount  them  in  strips  of  five  or 
ten.  A  strip  of  five  drops,  as  manufactured  by  the  Kellogg  Switch- 
board and  Supply  Company,  is  shown  in  Fig.  248.  The  front 
strip  on  which  these  drops  are  mounted  is  usually  of  brass  or  steel, 
copper  plated,  and  is  sufficiently  heavy  to  provide  a  rigid  support  for 
the  entire  group  of  drops  that  are  mounted  on  it.  This  construction 


Fig.  248.     Strip  of  Tubular  Drops 


greatly  facilitates  the  assembling  of  the  switchboard  and  also  serves 
to  economize  space — obviously,  the  thing  to  economize  on  the  face 
of  a  switchboard  is  space  as  defined  by  vertical  and  horizontal  di- 
mensions. These  tubular  drops,  having  but  one  coil,  are  readily 


328 


TELEPHONY 


mounted  on  1-inch  centers,  both  vertically  and  horizontally.  Some- 
times even  smaller  dimensions  than  this  are  secured.  The  greatest 
advantage  of  this  form  of  construction,  however,  is  in  the  absolute 
freedom  from  cross-talk  between  two  adjacent  drops.  So  com- 
pletely is  the  magnetic  field  of  force  kept  within  the  material  of  the 
shell,  that  there  is  practically  no  stray  field  and  two  such  drops 
may  be  included  in  two  different  talking  circuits  and  the  drops 
mounted  immediately  adjacent  to  each  other  without  producing  -any 
cross-talk  whatever. 

Night  Alarm.  Switchboard  drops  in  falling  make  but  little 
noise,  and  during  the  day  time,  while  the  operator  is  supposed 
to  be  needed  continually  at  the  board,  the  visual  signal  which 
they  display  is  sufficient  to  attract  her  attention.  In  small  ex- 
changes, however,  it  is  frequently  not  practicable  to  keep  an 
operator  at  the  switchboard  at  night  or  during  other  comparatively 
idle  periods,  and  yet  calls  that  do  arrive  during  such  periods  must 
be  attended  to.  For  this  reason  some  other  than  a  visual  signal 
is  necessary,  and  this  need  is  met  by  the  so-called  night-alarm  at- 
tachment. This  is  merely  an  arrangement  by  which  the  shutter 
in  falling  closes  a  pair  of  contacts  and  thus  completes  the  circuit 
of  an  ordinary  vibrating  bell  or  buzzer  which  will  sound  until  the 
shutter  is  restored  to  its  normal  position.  Such  contacts  are  shown 


Fig.  249.     Drop  with  Night-Alarm  Contacts 

in  Fig.  249  at  1  and  2.  Night-aiarm  contacts  have  assumed  a 
variety  of  forms,  some  of  which  will  be  referred  to  in  the  discussion 
of  other  types  of  drops  and  jacks. 

Jack  Mounting.  Jacks,  like  drops,  though  frequently  individually 
mounted  are  more  often  mounted  in  strips.  An  individually  mounted 
jack  is  shown  in  Fig.  250,  and  a  strip  of  ten  jacks  in  Fig.  251.  In 
such  a  strip  of  jacks,  the  strips  supporting  the  metallic  parts  of  the  var- 


SIMPLE  MAGNETO  SWITCHBOARD  329 

ious  jacks  are  usually  of  hard  rubber  reinforced  by  brass  so  as  to  give 
sufficient  strength.  Various  forms  of  supports  for  these  strips  are 
used  by  different  manufacturers,  the  means  for  fastening  them  in 
the  switchboard  frame  usually  consisting  of  brass  lugs  on  the  end  of 


Fig.  250.     Individual  Jack 

the  jack  strip  adapted  to  be  engaged  by  screws  entering  the  sta- 
tionary portion  of  the  iron  framework;  or  sometimes  pins  are  fixed  in 
the  framework,  and  the  jack  is  held  in  place  by  nuts  engaging  screw- 
threaded  ends  on  such  pins. 

Methods  of  Associating  Jacks  and  Drops.  There  are  two  gen- 
eral methods  of  arranging  the  drops  and  jacks  in  a  switchboard. 
One  of  these  is  to  place  all  of  the  jacks  in  a  group  together  at 
the  lower  portion  of  the  panel  in  front  of  the  operator  and  all  of 
the  drops  together  in  another  group  above  the  group  of  jacks. 
The  other  way  is  to  locate  each  jack  in  immediate  proximity  to 


Fig.  251.     Strip  of  Jacks 

the  drop  belonging  to  the  same  line  so  that  the  operator's  atten- 
tion will  always  be  called  immediately  to  the  jack  into  which  she 
must  insert  her  plug  in  response  to  the  display  of  a  drop.  This 
latter  practice  has  several  advantages  over  the  former.  Where  the 
drops  are  all  mounted  in  one  group  and  the  jacks  in  another,  an 
operator  seeing  a  drop  fall  must  make  mental  note  of  it  and  pick 


330  TELEPHONY 

out  the  corresponding  jack  in  the  group  of  jacks.  On  the  other 
hand,  where  the  jacks  and  drops  are  mounted  immediately  adja- 
cent to  each  other,  the  falling  of  a  drop  attracts  the  attention  of 
the  operator  to  the  corresponding  jack  without  further  mental  effort 
on  her  part. 

The  immediate  association  of  the  drops  and  jacks  has  another 
advantage — it  makes  possible  such  a  mechanical  relation  between 
the  drop  and  its  associated  jack  that  the  act  of  inserting  the  plug  into 
the  jack  in  making  the  connection  will  automatically  and  mechan- 
ically restore  the  drop  to  its  raised  position.  Such  drops  are  termed 
self-restoring  drops,  and,  since  a  drop  and  jack  are  often  made  struc- 
turally a  unitary  piece  of  apparatus,  they  are  frequently  called  com- 
bined drops  and  jacks. 

Manual  vs.  Automatic  Restoration.  There  has  been  much 
difference  of  opinion  on  the  question  of  manual  versus  automatic 
restoration  of  drops.  Some  have  contended  that  there  is  no 
advantage  in  having  the  drops  restored  automatically,  claim- 
ing that  the  operator  has  plenty  of  time  to  restore  the  drops  by 
hand  while  receiving  the  order  from  the  calling  subscriber  or  per- 
forming some  of  her  other  work.  Those  who  think  this  way  have 
claimed  that  the  only  place  where  an  automatically  restored  drop 
is  really  desirable  is  where,  on  account  of  the  lack  of  space  on  the 
front  of  the  switchboard,  the  drops  are  placed  on  such  a  portion 
of  the  board  as  to  be  not  readily  reached  by  the  operator.  This 
resulted  in  the  electrically  restored  drop,  mention  of  which  will  be 
made  later. 

Others  have  contended  that  even  though  the  drop  is  mounted 
within  easy  reach  of  the  operator,  it  is  advantageous  that  the  operator 
should  be  relieved  of  the  burden  of  restoring  it,  claiming  that  even 
though  there  are  times  in  the  regular  performance  of  the  operator's 
duties  when  she  may  without  interfering  with  other  work  restore  the 
drops  manually,  such  requirement  results  in  a  double  use  of  her 
attention  and  in  a  useless  strain  on  her  which  might  better  be  de- 
voted to  the  actual  making  of  connections. 

Until  recently  the  various  Bell  operating  companies  have  ad- 
hered, in  their  small  exchange  work,  to  the  manual  restoring  method, 
while  most  of  the  so-called  independent  operating  companies  have 
adhered  to  the  automatic  self-restoring  drops. 


SIMPLE  MAGNETO  SWITCHBOARD  331 

Methods  of  Automatic  Restoration.  Two  general  methods 
present  themselves  for  bringing  about  the  automatic  restoration  of 
the  drop.  First,  the  mechanical  method,  which  is  accomplished  by 
having  some  moving  part  of  the  jack  or  of  the  plug  as  it  enters  the 
jack  force  the  drop  mechanically  into  its  restored  position.  This 
usually  means  the  mounting  of  the  drop  and  the  corresponding 
jack  in  juxtaposition,  and  this,  in  turn,  has  usually  resulted  in 
the  unitary  structure  containing  both  the  drop  and  the  jack. 
Second,  the  electrical  method  wherein  the  plug  in  entering  the 
jack  controls  a  restoring  circuit,  which  includes  a  battery  or  other 
source  of  energy  and  a  restoring  coil  on  the  drop,  the  result  being 
that  the  insertion  of  the  plug  into  the  jack  closes  this  auxiliary 
circuit  and  thus  energizes  the  restoring  magnet,  the  armature  of 
which  pulls  the  shutter  back  into  its  restored  position.  This  prac- 
tice has  been  followed  by  Bell  operating  companies  whenever  con- 
ditions require  the  drop  to  be  mounted  out  of  easy  reach  of  the 
operator;  not  otherwise. 

Mechanical — Direct  Contact  with  Plug.  One  widely  used 
method  of  mechanical  restoration  of  drops,  once  employed  by  the 
Western  Telephone  Construction  Company  with  considerable  suc- 
cess, was  to  hang  the  shutter  in  such  position  that  it  would  fall  im- 
mediately in  front  of  the  jack  so  that  the  operator  in  order  to  reach 
the  jack  with  the  plug  would  have  to  push  the  plug  directly  against 
the  shutter  and  thus  restore  it  to  its  normal  or  raised  position.  In 
this  construction  the  coil  of  the  drop  magnet  was  mounted  directly 
behind  the  jack,  the  latch  rod  controlled  by  the  armature  reaching 
forward,  parallel  with  the  jack,  to  the  shutter,  which,  as  stated, 
was  hung  in  front  of  the  jack.  This  resulted  in  a  most  compact 
arrangement  so  far  as  the  space  utilization  on  the  front  of  the 
board  was  concerned  and  such  combined  drops  and  jacks  were 
mounted  on  about  1-inch  centers,  so  that  a  bank  of  one  hundred 
combined  drops  and  jacks  occupied  a  space  only  a  little  over  10 
inches  -square. 

A  modification  of  this  scheme,  as  used  by  the  American  Electric 
Telephone  Company,  was  to  mount  the  drop  immediately  over  the 
jack  so  that  its  shutter,  when  down,  occupied  a  position  almost  in 
front  of,  but  above,  the  jack  opening.  The  plug  was  provided  with 
a  collar,  which,  as  it  entered  the  jack,  engaged  a  cam  on  the  base 


332 


TELEPHONY 


of  the  shutter  and  forced  the  latter  mechanically  into  its  raised  po- 
sition. 

Neither  of  these  methods  of  restoring — i.  e,  by  direct  contact  be- 
tween the  shutter  or  part  of  it  and  the  plug  or  part  of  it — is  now  as 
widely  used  as  formerly.  It  has  been  found  that  there  is  no  real 
need  in  magneto  switchboards  for  the  very  great  compactness  which 
the  hanging  of  the  shutter  directly  in  front  of  the  drop  resulted  in, 
and  the  tendency  in  later  years  has  been  to  make  the  combined  drops 
and  jacks  more  substantial  in  construction  at  the  expense  of  some 
space  on  the  face  of  the  switchboard. 

Kellogg  Type : — A  very  widely  used  scheme  of  mechanical  res- 
toration is  that  employed  in  the  Miller  drop  and  jack  manufactured 
by  the  Kellogg  Switchboard  and  Supply  Company,  the  principles 

of    which    may   be    understood 
in  connection  with  Fig,  252.     In 
this  figure  views  of  one  of  these 
combined   drops    and    jacks   in 
three     different     positions     are 
shown.      The  jack  is  composed 
of   the   framework    B    and    the 
hollow  screw  A,  the  latter  form- 
ing the  sleeve  or  thimble  of  the 
jack  and  being  externally  screw- 
threaded   so    as   to   engage  and 
bind   in   place   the  front  end  of 
the  framework  B.    The  jack  is 
mounted   on   the   lower   part  of 
the   brass    mounting  strip  C  but 
insulated    therefrom. 
The   tip  spring  of  the 
jack   is   bent  down   as 
usual  to  engage  the  tip 
of    the   plug,  as   better 
shown  in  the  lower  cut 

of  Fig.  252,  and  then  continues  in  an  extension  D,  which  passes 
through  a  hole  in  the  mounting  plate  C.  This  tip  spring  in  its  normal 
position  rests  against  another  spring  as  shown,  which  latter  spring 
forms  one  terminal  of  the  drop  winding. 


SHUTTEf?  UPBtrOffE  CALL/NG 


Sttl/TTfff  DOWM  AFTER  CALL/NG 


SHt/77&r  ffE-S  TOffEDAFTEff  PLL/GG/VG  W 


Fig.  252.     Kellogg  Drop  and' Jack 


SIMPLE  MAGNETO  SWITCHBOARD  333 

The  drop  or  annunciator  is  of  tubular  form,  and  the  shutter  is 
so  arranged  on  the  front  of  the  mounting  strip  C  as  to  fall  directly 
above  the  extension  D  of  the  tip  spring.  As  a  result,  when  the  plug 
is  inserted  into  the  jack,  the  upward  motion  of  the  tip  spring  forces 


Fig.  253.     Strip  of  Kellogg  Drops  and  Jacks 

the  drop  into  its  restored  position,  as  indicated  in  the  lower  cut  of 
the  figure.  These  drops  and  jacks  are  usually  mounted  in  banks  of 
five,  as  shown  in  Fig.  253. 

Western  Electric  Type: — The  combined  drop  and  jack  of 
the  Western  Electric  Company  recently  put  on  the  market  to 
meet  the  demands  of  the  independent  trade,  differs  from  others 
principally  in  that  it  employs  a  spherical  drop  or  target  instead 
of  the  ordinary  flat  shutter.  This  piece  of  apparatus  is  shown 
in  its  three  possible  positions  in  Fig.  254.  The  shutter  or  target 
normally  displays  a  black  surface  through  a  hole  in  the  mount- 
ing plate.  The  sphere  forming  the  target  is  out  of  balance, 
and  when  the  latch  is  withdrawn  from  it  by  the  action  of  the 
electromagnet  it  falls  into  the  position  shown  in  the  middle  cut 
of  Fig.  254,  thus  displaying  a  red  instead  of  a  black  surface  to 
the  view  of  the  operator.  When  the  operator  plugs  in,  the  plug 
engages  the  lower  part  of  an  S-shaped  lever  which  acts  on  the 
pivoted  sphere  to  restore  it  to  its  normal  position.  A  perspective 
view  of  one  of  these  combined  line  signals  and  jacks  is  shown  in 
Fig.  255. 


334 


TELEPHONY 


A  feature  that  is  made  much  of  in  recently  designed  drops  and 
jacks  for  magneto  service  is  that  which  provides  for  the  ready  re- 
moval of  the  drop  coil,  from  the  rest  of 
the  structure,  for  repair.    The  drop  and 
jack  of  the  Western  Electric  Company, 
just  described,  embodies  this  feature,  a 
single  screw  being  so  arranged  that  its 
removal  will  permit  the  withdrawal  of 
the  coil  without  disturbing  any  of  the 
other  parts  or  connections.     The  coil 
windings  terminate   in  two  projections 
on  the  front  head  of  the  spool,  and  these 
register  with  spring  clips  on  the  inside 
of  the  shell  so  that  the  proper  connec- 
tions for  the  coil  are  au- 
tomatically made  by  the 
mere  insertion  of  the  coil 
into  the  shell. 

Dean  Type:— The 
combined  drop  and  jack 
of  the  Dean  Electric 
Company  is  illustrated 
in  Figs.  256  and  257.  The  two  perspective  views  show  the  general 
features  of  the  drop  and  jack  and  the  method  by  which  the  magnet 


TARGET    OPERATED  AFTER 
SUBSCRIBER    HAS  CALLED 


TARGET    RESTORED   AFTER 
OPERATOR    HAS   IMSE*RTED   PLUG 

Fig.  254.     Western  Electric  Drop  and  Jack 


Fig.  255.     Western  Electric  Drop  and  Jack 

coil  may  be  withdrawn  from  the  shell.  As  will  be  seen  the  magnet 
is  wound  on  a  hollow  core  which  slides  over  the  iron  core,  the  latter 
remaining  permanently  fixed  in  the  shell,  even  though  the  coil  be 
withdrawn. 

Fig.  258  shows  the  structural  details  of  the  jack  employed  in  this 
combination  and  it  will  be  seen  that  the  restoring  spring  for  the 
drop  is  not  the  tip  spring  itself,  but  another  spring  located  above  and 
insulated  from  it  and  mechanically  connected  therewith. 


SIMPLE  MAGNETO  SWITCHBOARD 


335 


Monarch  Type : — Still  another  combined  drop  and  jack  is  that 
of  the  Monarch  Telephone   Manufacturing   Company  of   Chicago, 
shown   in  sectional  view  in  Fig.  259. 
This  differs  from  the  usual    type   in 
that  the  armature  is  mounted  on  the 
front  end  of  the  electromagnet,  its  latch 
arm  retaining  the  shutter  in  its  normal 
position  when  raised,  and  releasing  it 
when  depressed  by  the  attraction  of 
the  armature.     As  is  shown,  there  is 
within  the  core  of  the  magnet  an  ad- 
justable  spiral   spring   which  presses         Fig.  256.    Dean  Drop  and  Jack 
forward    against    the    armature    and 
which  spring  is  compressed  by  the  attraction  of  the  armature  of  the 


Fig.  257.     Dean  Drop  and  Jack 


magnet.     The  night-alarm  contact  is  clearly   shown    immediatelv 
below  the  strip  which  supports  the  drop,  this  consisting  of  a  spring 


Fig.  258.     Details  of  Dean  Jack 


adapted  to  be  engaged  by  a  lug  on  the  shutter  and  pressed  upwardly 
against  a  stationary  contact  when  the  shutter  falls.     The  method  of 


336 


TELEPHONY 


restoration  of  the  shutter  in  this  case  is  by  means  of  an  auxiliary 
spring  bent  up  so  as  to  engage  the  shutter  and  restore  it  when  the 
spring  is  raised  by  the  insertion  of  a  plug  into  the  jack. 


Fig.  259.    Monarch  Drop  and  Jack 

Code  Signaling.  On  bridging  party  lines,  where  the  sub- 
scribers sometimes  call  other  subscribers  on  the  same  line  and  some- 
times call  the  switchboard  so  as  to  obtain  a  connection  with  another 
line,  it  is  not  always  easy  for  the  operator  at  the  switchboard  to  dis- 
tinguish whether  the  call  is  for  her  or  for  some  other  party  on  the 
line.  On  such  lines,  of  course,  code  ringing  is  used  and  in  most 
cases  the  operator's  only  way  of  distinguishing  between  calls  for  her 

and  those  for  some  sub-station 
parties  on  the  line  is  by  listening 
to  the  rattling  noise  which  the 
drop  armature  makes.  In  the 
case  of  tile  Monarch  drop  the 
adjustable  spring  tension  on  the 
armature  is  intended  to  provide 
for  such  an  adjustment  as  will 
permit  the  armature  to  give  a 
satisfactory  buzz  in  response  to 
the  alternating  ringing  currents, 
whether  the  line  be  long  or 
short. 

The  Monarch  Company  provides  in  another  way  for  code  sig- 
naling at  the  switchboard.  In  some  cases  there  is  a  special  attach- 
ment, shown  in  Fig.  260,  by  means  of  which  the  code  signals  are 
repeated  on  the  night-alarm  bell.  This  is  in  the  nature  of  a  special 


Fig.  260.     Code  Signal  Attachment 


SIMPLE  MAGNETO  SWITCHBOARD  337 

attachment  placed  on  the  drop,  which  consists  of  a  light,  flat  spring 
attached  to  the  armature  and  forming  one  side  of  a  local  circuit.  The 
other  side  of  the  circuit  terminates  in  a  fixture  which  is  mounted  on 
the  drop  frame  and  is  provided  with  a  screw,  having  a  platinum 
point  forming  the  other  contact  point;  this  allows  of  considerable 
adjustment.  At  the  point  where  the  screw  comes  in  contact  with  the 
spring  there  is  a  platinum  rivet.  When  an  operator  is  not  always  in 
attendance,  this  code-signaling  attachment  has  some  advantages 


Fig.  261.     Combined  Drop  and  Ringer 

over  the  drop  as  a  signal  interpreter,  in  that  it  permits  the  code  sig- 
nals to  be  heard  from  a  distance.  Of  course,  the  addition  of  spring 
contacts  to  the  drop  armature  tends  to  complicate  the  structure  and 
perhaps  to  cut  down  the  sensitiveness  of  the  drop,  which  are  offsetting 
disadvantages. 

For  really  long  lines,  this  code  signaling  by  means  of  the  drop 
is  best  provided  for  by  employing  a  combined  drop  and  ringer,  al- 
though in  this  case  whatever  advantages  are  secured  by  the  mechan- 
ical restoration  of  the  shutter  upon  plugging  in  are  lost.  Such  a  de- 
vice as  manufactured  by  the  Dean  Electric  Company  is  shown  in 
Fig.  261.  In  this  the  ordinary  polarized  ringer  is  used,  but  in  ad- 
dition the  tapper  rod  carries  a  latch  which,  when  vibrated  by  the 
ringing  of  the  bell,  releases  a  shutter  and  causes  it  to  fall,  thus  giv- 
ing a  visual  as  well  as  an  audible  signal. 

Electrical.  Coming  now  to  the  electrical  restoration  of  drop 
shutters,  reference  is  made  to  Fig.  262,  which  shows  in  side  section 
the  electrical  restoring  drop  employed  by  the  Bell  companies  and 
manufactured  by  the  Western  Electric  Company.  In  this  the  coil  1  is 
a  line  coil,  and  it  operates  on  the  armature  2  to  raise  the  latch  lever 


338 


TELEPHONY 


3  in  just  the  same  manner  as  in  the  ordinary  tubular  drop.  The 
latch  lever  3  acts,  however,  to  release  another  armature  4  instead 
of  a  shutter.  This  armature  4  'ls  pivoted  at  its  lower  end  at  the 


Fig.  262.     Electrically  Restored  Drop 

opposite  end  of  the  device  from  the  armature  2  and,  by  falling  out- 
wardly when  released,  it  serves  to  raise  the  light  shutter  5.  The 
restoring  coil  of  this  device  is  shown  at  6,  and  when  energized  it 
attracts  the  armature  4  SQ  as  to  pull  it  back  under  the  catch  of  the 
latch  lever  3  and  also  so  as  to  allow  the  shutter  5  to  fall  into  its  nor- 
mal position.  The  method  of  closing  the  restoring  circuit  is  by 
placing  coil  6  in  circuit  with  a  local  battery  and  with  a  pair  of  con- 
tacts in  the  jack,  which 
latter  contacts  are  nor- 
mally open  but  are 
bridged  across  by  the 
plug  when  it  enters 
the  jack,  thus  ener- 
gizing the  restoring 
coil  and  restoring  the 
shutter. 

A  perspective  view 

of  this  Western  Electric  electrical  restoring  drop  is  shown  in  Fig. 
263,  a  more  complete  mention  being  made  of  this  feature  under  the 
discussion  of  magneto  multiple  switchboards,  wherein  it  found  its 
chief  use.  It  is  mentioned  here  to  round  out  the  methods  that  have 
been  employed  for  accomplishing  the  automatic  restoration  of  shut- 
ters by  the  insertion  of  the  plug. 

Switchboard  Plugs.  A  switchboard  plug  such  as  is  commonly 
used  in  simple  magneto  switchboards  is  shown  in  Fig.  264  and  also 
in  Fig.  235.  The  tip  contact  is  usually  of  brass  and  is  connected 


Fig.  263.     Electrically  Restored  Drop 


SIMPLE  MAGNETO  SWITCHBOARD  339 

to  a  slender  steel  rod  which  runs  through  the  center  of  the  piug  and 
terminates  near  the  rear  end  of  the  plug  in  a  connector  for  the  tip 
conductor  of  the  cord.  This  central  core  of  steel  is  carefully  insu- 
lated from  the  outer  shell  of  the  plug  by  means  of  hard  rubber  bush- 
ings, the  parts  being  forced  tightly  together.  The  outer  shell,  of 
course,  forms  the  other  conductor  of  the  plug,  called  the  sleeve  con- 
tact. A  handle  of  tough  fiber  tubing  is  fitted  over  the  rear  end  of 
the  plug  and  this  also 
serves  to  close  the  open- 
ing formed  by  cutting 
away  a  portion  of  the 
plug  shell,  thus  exposing 
the  connector  for  the  tip 

Fig.  264.     Switchboard  Plug 

conductor. 

Cord  Attachment.  The  rear  end  of  the  plug  shell  is  usually 
bored  out  just  about  the  size  of  the  outer  covering  of  the  switch- 
board cord,  and  it  is  provided  with  a  coarse  internal  screw  thread, 
as  shown.  The  cord  is  attached  by  screwing  it  tightly  into  this 
screw-threaded  chamber,  the  screw  threads  in  the  brass  being  suffi- 
ciently coarse  and  of  sufficiently  small  internal  diameter  to  afford  a 
very  secure  mechanical  connection  between  the  outer  braiding  of 
the  cord  and  the  plug.  The  connection  between  the  tip  conductor 
of  the  cord  and  the  tip  of  the  plug  is  made  by  a  small  machine 
screw  connection  as  shown,  while  the  connection  between  the 
sleeve  conductor  of  the  plug  and  the  sleeve  conductor  of  the 
cord  is  made  by  bending  back  the  latter  over  the  outer  braiding 
of  the  cord  before  it  is  screwed  into  the  shank  of  the  plug. 
This  results  in  the  close  electrical  contact  between  the  sleeve  con- 
ductor of  the  cord  and  the  inner  metal  surface  of  the  shank  of  the 
plug. 

Switchboard  Cords.  A  great  deal  of  ingenuity  has  been 
exerted  toward  the  end  of  producing  a  reliable  and  durable  switch- 
board cord.  While  great  improvement  has  resulted,  the  fact  remains 
that  the  cords  of  manual  switchboards  are  today  probably  the  most 
troublesome  element,  and  they  need  constant  attention  and  repairs. 
While  no  two  manufacturers  build  their  cords  exactly  alike,  descrip- 
tions of  a  few  commonly  used  and  successful  cords  may  be  here 
given. 


340  TELEPHONY 

Concentric  Conductors.  In  one  the  core  is  made  from  a  double 
strand  of  strong  lock  stitch  twine,  over  which  is  placed  a  linen  braid. 
Then  the  tip  conductor,  which  is  of  stranded  copper  tinsel,  is 
braided  on.  This  is  then  covered  with  two  layers  of  tussah  silk,  laid 
in  reverse  wrappings,  then  there  is  a  heavy  cotton  braid,  and  over  the 
latter  a  linen  braid.  The  sleeve  conductor,  which  is  also  of  copper 
tinsel,  is  then  braided  over  the  structure  so  formed,  after  which  two 
reverse  wrappings  of  tussah  silk  are  served  on,  and  this  is  covered 
by  a  cotton  braid  and  this  in  turn  by  a  heavy  linen  or  polished 


Fig.  265.     Switchboard  Cord 

cotton  braid.  The  plug  end  of  the  cord  is  reinforced  for  a  length  of 
from  12  to  18  inches  by  another  braiding  of  linen  or  polished  cot- 
ton, and  the  whole  cord  is  treated  with  melted  beeswax  to  make  it 
moisture-proof  and  durable. 

Steel  Spiral  Conductors.  In  another  cord  that  has  found  much 
favor  the  two  conductors  are  formed  mainly  by  two  concentric  spiral 
wrappings  of  steel  wire,  the  conductivity  being  reinforced  by  adja- 
cent braidings  of  tinsel.  The  structure  of  such  a  cord  is  well  shown 
in  Fig.  265.  Beginning  at  the  right,  the  different  elements  shown  are, 
in  the  order  named,  a  strand  of  lock  stitch  twine,  a  linen  braiding, 
into  the  strands  of  which  are  intermingled  tinsel  strands,  the  inner 
spiral  steel  wrapping,  a  braiding  of  tussah  silk,  a  linen  braiding,  a 
loose  tinsel  braiding,  the  outer  conductor  of  round  spiral  steel, 
a  cotton  braid,  and  an  outside  linen  or  polished  cotton  braid. 
The  inner  tinsel  braiding  and  the  inner  spiral  together  form  the  tip 
conductor  while  the  outer  braiding  and  spiral  together  form  the  sleeve 
conductor.  The  cord  is  reinforced  at  the  plug  end  for  a  length  of 
about  14  inches  by  another  braiding  of  linen.  The  tinsel  used  is,  in 
each  case,  for  the  purpose  of  cutting  down  the  resistance  of  the  main 
steel  conductor.  These  wrappings  of  steel  wire  forming  the  tip 
and  sleeve  conductors  respectively,  have  the  advantage  of  affording 
great  flexibility,  and  also  of  making  it  certain  that  whatever  strain 
the  cord  is  subjected  to  will  fall  on  the  insulated  braiding  rather 


SIMPLE  MAGNETO  SWITCHBOARD 


341 


than  on  the  spiral  steel  which  has  in  itself  no  power  to  resist  tensile 
strains. 

Parallel  Tinsel  Conductors.  Another  standard  two-conductor 
switchboard  cord  is  manufactured  as  follows:  One  conductor  is 
of  very  heavy  copper  tinsel  insulated  with  one  wrapping  of  sea  island 
cotton,  which  prevents  broken  ends  of  the  tinsel  or  knots  from  piercing 
through  and  short-circuiting  with  the  other  conductor.  Over  this 
is  placed  one  braid  of  tussah  silk  and  an  outer  braid  of  cotton.  This 
combines  high  insulation  with  considerable  strength.  The  other 
conductor  is  of  copper  tinsel,  not  insulated,  and  this  is  laid  parallel 
to  the  thrice  insulated  conductor  already  described.  Around  these 
two  conductors  is  placed  an  armor  of  spring  brass  wire  in  spiral  form, 
and  over  this  a  close,  stout  braid  of  glazed  cotton.  This  like  the 
others  is  reinforced  by  an  extra  braid  at  the  plug  end. 

Ringing  and  Listening  Keys.  The  general  principles  of  the 
ringing  key  have  already  been  referred  to.  Ringing  keys  are  of 
two  general  types,  one  having  horizontal  springs  and  the  other 
vertical. 

Horizontal  Spring  Type.  Various  Bell  operating  companies 
have  generally  adhered  to  the  horizontal  spring  type  except  in 


Fig.  266.     Horizontal-Spring  Listening  and  Ringing  Key 

individual  and  four-party-line  keys.  The  construction  of  a  West- 
ern Electric  Company  horizontal  spring  key  is  shown  in  Fig. 
266.  In  this  particular  key,  as  illustrated,  there  are  two  cam  levers 


342 


TELEPHONY 


operating  upon  three  sets  of  springs.  The  cam  lever  at  the  left 
operates  the  ordinary  ringing  and  listening  set  of  springs  according 
to  whether  it  is  pushed  one  way  or  the  other.  In  ringing  on 
single-party  lines  the  cam  lever  at  the  left  is  the  one  to  be  used; 
while  on  two-party  lines  the  lever  at  the  left  serves  to  ring  the  first 
party  and  the  ringing  key  at  the  right  the  second  party. 

In  order  that  the  operator  may  have  an  indication  as  to  which 
station  on  a  two-party  line  she  has  called,  a  small  target  1  carried 
on  a  lever  2  is  provided.  This  target  may  display  a  black  or  a 
white  field,  according  to  which  of  its  positions  it  occupies.  The 
lever  2  is  connected  by  the  links  3  and  4  with  the  two  key  levers 
and  the  target  is  thus  moved  into  one  position  or  the  other,  accord- 
ing to  which  lever  was  last  thrown  into  ringing  position. 

It  will  be  noticed  that  the  springs  are  mounted  horizontally  and 
on  edge.  This  on-edge  feature  has  the  advantage  of  permitting 

ready  inspection  of  the  con- 
tacts and  of  avoiding  the  lia- 
bility of  dust  gathering  be- 
tween the  contacts.  As  will  be 
seen,  at  the  lower  end  of  each 
switch  lever  there  is  a  roller 
of  insulating  material  which 
serves  as  a  wedge,  when  forced 
between  the  two  long  springs 
of. any  set,  to  force  them  apart 
and  in  to  engagement  wi  th  their 
respective  outer  springs. 

Vertical  Spring  Type.  The 
other  type  of  ringing  and  lis- 
tening key  employing  vertical 
springs  is  almost  universally 
used  by  the  various  independ- 
ent manufacturing  companies.  A  good  example  of  this  is  shown  in 
Fig.  267,  which  shows  partly  in  elevation  and  partly  in  section  a  double 
key  of  the  Monarch  Company.  The  operation  of  this  is  obvious 
from  its  mode  of  construction.  The  right-hand  set  of  springs  of 
the  right-hand  key  in  this  cut  are  the  springs  of  the  listening 
key,  while  the  left-hand  set  of  the  right-hand  key  are  those  of 


Fig.  267.     Vertical-Spring  Listening  and 
Ringing  Key 


SIMPLE  MAGNETO  SWITCHBOARD 


343 


Fig.  268.     Vertical  Listening  and 
Ringing  Key 


the  calling-plug  ringing  key.  The  left-hand  set  of  the  left-hand 
key  may  be  those  of  a  ring-back  key  on  the  answering  plug,  while 
the  right-hand  set  of  the  left-hand  key  may  be  for  any  special  pur- 
pose. It  is  obvious  that  these  groups  of  springs  may  be  grouped  in 
different  combinations  or  omitted 
in  part,  as  required.  This  same 
general  form  of  key  is  also  manu- 
factured by  the  Kellogg  Company 
and  the  Dean  Company,  that  of 
the  Kellogg  Company  being  illus- 
trated in  perspective,  Fig.  268. 
The  keys  of  this  general  type  have 
the  same  advantages  as  those  of 
the  horizontal  on-edge  arrangement 
with  respect  to  the  gathering  of 
dust,  and  while  perhaps  the  con- 
tacts are  not  so  readily  get-at-able 
for  inspection,  yet  they  have  the 
advantage  of  being  somewhat  more 
simple,  and  of  taking  up  less  horizontal  space  on  the  key  shelf. 

Party-Line  Ringing  Keys.  For  party-line  ringing  the  key 
matter  becomes  somewhat  more  complicated.  Usually  the  arrange- 
ment is  such  that  in  connection  with  each  calling  plug  there  are  a 
number  of  keys,  each  ar- 
ranged with  respect  to  the 
circuits  of  the  plug  so  as  to 
send  out  the  proper  combi- 
nation and  direction  of  cur- 
rent, if  the  polarity  system 
is  used;  or  the  proper  fre- 
quency of  current  if  the  har- 
monic system  is  used;  or  the 
proper  number  of  impulses  if 
the  step-by-step  or  broken- 
line  system  is  used.  The 

number  of  different  kinds  of  arrangements  and  combinations  is 
legion,  and  we  will  here  illustrate  only  an  example  of  a  four-party 
line  ringing  key  adapted  for  harmonic  ringing.  A  Kellogg  party- 


Fig.  269.     Four-Party  Listening  and 
Ringing  Key 


344  TELEPHONY 

line  listening  and  ringing  key  is  shown  in  Fig.  269.  In  this,  besides 
the  regular  listening  key,  are  shown  four  push-button  keys,  each 
adapted,  when  depressed,  to  break  the  connection  back  of  the  key, 
and  at  the  same  time  connect  the  proper  calling  generator  with  the 
calling  plug. 

Self-Indicating  Keys.  A  complication  that  has  given  a  good 
deal  of  trouble  in  the  matter  of  party-line  ringing  is  due  to  the  fact 
that  it  is  sometimes  necessary  to  ring  a  second  or  a  third  time  on  a 
party-line  connection,  because  the  party  called  may  not  respond 
the  first  time.  The  operator  is  not  always  able  to  remember  which 
one  of  the  four  keys  associated  with  the  plug  connected  with  the 
desired  party  she  has  pressed  on  the  first  occasion  and,  therefore, 
when  it  becomes  necessary  to  ring  again,  she  may  ring  the  wrong 
party.  This  is  provided  for  in  a  very  ingenious  way  in  the  key 
shown  in  Fig.  269,  by  making  the  arrangement  such  that  after  a  given 
key  has  been  depressed  to  its  full  extent  in  ringing,  and  then  re- 
leased, it  does  not  come  quite  back  to  its  normal  position  but  re- 
mains slightly  depressed.  This  always  serves  as  an  indication  to 
the  operator,  therefore,  as  to  which  key  she  depressed  last,  and  in 
the  case  of  a  re-ring,  she  merely  presses  the  key  that  is  already  down 
a  little  way.  On  the  next  call  if  she  is  required  to  press  another  one 
of  the  four  keys,  the  one  which  remained  down  a  slight  distance  on 
the  last  call  will  be  released  and  the  one  that  is  fully  depressed  will 
be  the  one  that  remains  down  as  an  indication. 

Such  keys,  where  the  key  that  was  last  used  leaves  an  indica- 
tion to  that  effect,  are  called  indicating  ringing  keys.  In  other  forms 
the  indication  is  given  by  causing  the  key  lever  to  move  a  little  tar- 
get which  remains  exposed  until  some  other  key  in  the  same  set  is 
moved.  The  key  shown  in  Fig.  266  is  an  example  of  this  type. 

NOTE.  The  matter  of  automatic  ringing  and  other  special  forms  of  ring- 
ing will  be  referred  to  and  discussed  at  their  proper  places  in  this  work,  but  at 
this  point  they  are  not  pertinent  as  they  are  not  employed  in  simple  switch- 
boards. 

Operator's  Telephone  Equipment.  Little  need  be  said  con- 
cerning the  matter  of  the  operator's  talking  apparatus,  i.  e.,  the  oper- 
ator's transmitter  and  receiver,  since  as  transmitters  and  receivers 
they  are  practically  the  same  as  those  in  ordinary  use  for  other 
purposes.  The  watch-case  receiver  is  nearly  always  employed  for 


SIMPLE  MAGNETO  SWITCHBOARD 


345 


operators'  purposes  on  account  of  its  lightness  and  compactness.  It 
is  used  in  connection  with  a  head  band  so  as  to  be  held  continually 
at  the  operator's  ear,  allowing  both  of  her  hands  to  be  free. 

The  transmitter  used  by  operators  does  not  in  itself  differ  from 
the  transmitters  employed  by  subscribers,  but  the  methods  by  which 
it  is  supported  differ,  two  general  practices  being  followed.  One  of 
these  is  to  suspend  the  transmitter  by  flexible  conducting  cords  so 
as  to  be  adjustable  in  a  vertical  direction.  A  good  illustration  of 
this  is  given  in  Fig.  270.  The  other  method,  and  one  that  is 
coming  into  more  and  more  favor,  is  to  mount  the  transmitter  on  a 


Fig.  270.     Operator's  Transmitter  Suspension 

light  bracket  suspended  by  a  flexible  band  from  the  neck  of  the 
operator,  a  breast  plate  being  furnished  so  that  the  transmitter  will 
rest  on  her  breast  and  be  at  all  times  within  proper  position  to  receive 
her  speech.  To  facilitate  this,  a  long  curved  mouthpiece  is  commonly 
employed,  as  shown  clearly  in  Fig.  47. 

Cut-in  Jack.  It  is  common  to  terminate  that  portion  of  the 
apparatus  which  is  worn  on  the  operator's  person — that  is,  the  re- 
ceiver only  if  the  suspended  type  of  transmitter  is  employed,  and 
the  receiver  and  transmitter  if  the  breast  plate  type  of  transmitter 
is  employed — in  a  plug,  and  a  flexible  cord  connecting  the  plug  ter- 


346 


TELEPHONY 


Pig.  271.     Operator's  Cut-In  Jack 


minates  with  the  apparatus.  The  portions  of  the  operator's  talking 
circuit  that  are  located  permanently  in  the  switchboard  cabinet  are 
in  such  cases  terminated  in  a  jack,  called  an  operator's  cut-in  jack. 

This  is  usually  mounted  on  the  front 
rail  of  the  switchboard  cabinet  just 
below  the  key  shelf.  Such  a  cut-in 
jack  is  shown  in  Fig.  271  and  it  is 
merely  a  specialized  form  of  spring 
jack  adapted  to  receive  the  short, 
stout  plug  in  which  the  operator's 
transmitter,  or  transmitter  and  re- 
ceiver, terminate.  By  this  arrange- 
ment the  operator  is  enabled  readily 
to  connect  or  disconnect  her  talking 
apparatus,  which  is  worn  on  her 
person,  whenever  she  comes  to  the 
board  for  work  or  leaves  it  at  the 
end  of  her  work.  A  complete  operator's  telephone  set,  or  that 
portion  that  is  carried  on  the  person  of  the  operator,  together 
with  the  cut-in  plug,  is  shown  in  Fig.  272. 

Circuits  of  Complete  Switch= 
board.  We  may  now  discuss  the 
circuits  of  a  complete  simple 
magneto  switchboard.  The  one 
shown  in  Fig.  273  is  typical.  Be- 
fore going  into  the  details  of  this, 
it  is  well  to  inform  the  student 
that  this  general  form  of  circuit 
representation  is  one  that  is  com- 
monly employed  in  showing  the 
complete  circuits  of  any  switch- 
board. Ordinarily  two  subscrib- 
ers' lines  are  shown,  these  con- 
necting their  respective  subscrib- 
ers' stations  with  two  different 
line  equipments  at  the  central  office.  The  jacks  and  signals  of 
these  line  equipments  are  turned  around  so  as  to  face  each  other, 
in  order  to  clearly  represent  how  the  connection  between  them 


Fig.  272.     Operator's  Talking  Set 


SIMPLE  MAGNETO  SWITCHBOARD 


347 


may  be  made  by  means  of  the  cord  circuit.  The  elements  of  the 
cord  circuit  are  also  spread  out,  so  that  the  various  parts  occupy 
relative  positions  which  they  do  not  assume  at  all  in  practice.  In 
other  words  it  must  be  remembered  that,  in  circuit  diagrams,  the 
relative  positions  of  the  parts  are  sacrificed  in  order  to  make  clear 
the  circuit  connections.  However,  this  does  not  mean  that  it  is  often 
not  possible  to  so  locate  the  pieces  of  apparatus  that  they  will  in 
a  certain  way  indicate  relative  positions,  as  may  be  seen  in  the  case 


CALL/HG    S  TAT /Off 


CALLED   STAT/O77 


OPERATORS 
TELEPHONE  5ET 


COMB/MED  DROP 
AM?  JACK 


Fig.  273.     Circuit  of  Simple  Magneto  Switchboard 

of  the  drop  and  jack  in  Fig.  273,  the  drop  being  shown  immediately 
above  the  jack,  which  is  the  position  in  which  these  parts  are  located 
in  practice. 

Little  need  be  said  concerning  this  circuit  in  view  of  what  has 
already  been  said  in  connection  with  Figs.  240  to  245.  It  will  be 
seen  in  the  particular  sub-station  circuit  here  represented,  that  the 
talking  apparatus  is  arranged  in  the  usual  manner  and  that  the 
ringer  and  generator  are  so  arranged  that  when  the  generator  is 
operated  the  ringer  will  be  cut  out  of  circuit,  while  the  generator 
will  be  placed  across  the  circuit;  while,  when  the  generator  is  idle,  the 
ringer  is  bridged  across  the  circuit  and  the  generator  is  cut  out. 

The  line  terminates  in  each  case  in  the  tip  and  sleeve  contacts 


348 


TELEPHONY 


of  the  jack,  and  in  the  normal  condition  of  the  jack  the  line  drop  is 
bridged  across  the  line.  The  arrangement  by  which  the  drop  is 
restored  and  at  the  same  time  cut  out  of  circuit  when  the  operator 
plugs  in  the  jack,  is  obvious  from  the  diagrammatic  illustration. 
The  cord  circuit  is  the  same  as  that  already  discussed,  with  the 
exception  that  two  ringing  keys  are  provided,  one  in  connection 
with  the  calling  plug,  as  is  universal  practice,  and  the  other  in  con- 
nection with  the  answering  plug  as  is  sometimes  practiced  in  order 
that  the  operator  may,  when  occasion  requires,  ring  back  the  calling 
subscriber  without  the  necessity  of  changing  the  plug  in  the  jack. 
The  outer  contacts  of  these  two  ringing  keys  are  connected  to  the 
terminals  of  the  ringing  generator  and,  when  either  key  is  operated, 
the  connection  between  the  plug,  on  which  the  ringing  is  to  be  done, 

and  the  rest  of  the  cord  circuit  will  be 
broken,  while  the  generator  will  be 
connected  with  the  terminals  of  the 
plug.  The  listening  key  and  talking 
apparatus  need  no  further  explanation, 
it  being  obvious  that  when  the  key  is 
operated  the  subscriber's  telephone  set 
will  be  bridged  across  the  cord  circuit 
and,  therefore,  connected  with  either  or 
both  of  the  talking  subscribers. 

Night=Alarm  Circuits.  The  circuit 
of  Fig.  273,  while  referred  to  as  a  com- 
plete circuit,  is  not  quite  that.  The 
night-alarm  circuit  is  not  shown.  In 
order  to  clearly  indicate  how  a  single 
battery  and  bell,  or  buzzer,  may  serve 
in  connecting  a  number  of  line  drops, 
reference  is  made  to  Fig.  274  which 
shows  the  connection  between  three 

different  line  drops  and  the  night-alarm  circuit.  The  night-alarm 
apparatus  consists  in  the  battery  1  and  the  buzzer,  or  bell,  2.  A 
switch  3  adapted  to  be  manually  operated  is  connected  in  the  cir- 
cuit with  the  battery  and  the  buzzer  so  as  to  open  this  circuit  when 
the  night  alarm  is  not  needed,  thus  making  it  inoperative.  During 
the  portions  of  the  day  when  the  operator  is  needed  constantly  at 


Fig.  274.    Night- Alarm  Circuit 


SIMPLE  MAGNETO  SWITCHBOARD 


349 


the  board  it  is  customary  to  leave  this  switch  3  open,  but  during  the 
night  period  when  she  is  not  required  constantly  at  the  board  this 
switch  is  closed  so  that  an  audible  signal  will  be  given  whenever 
a  drop  falls.  The  night-alarm  contact  4  on  each  of  the  drops  will 
be  closed  whenever  a  shutter  falls,  and  as  the  two  members  of  this 
contact,  in  the  case  of  each  drop,  are  connected  respectively  with 
the  two  sides  of  the  night-alarm  circuit,  any  one  shutter  falling  will 
complete  the  necessary  conditions  for  causing  the  buzzer  to  sound, 
assuming  of  course  that  the  switch  3  is  closed. 

Night  Alarm  with  Relay.  A  good  deal  of  trouble  has  been 
caused  in  the  past  by  uncertainty  in  the  closure  of  the  night-alarm 
circuit  at  the  drop  contact.  Some  of  the  companies  have  employed 
the  form  of  circuit  shown  in  Fig. 
275  to  overcome  this.  Instead  of 
the  night-alarm  buzzer  being  placed 
directly  in  the  circuit  that  is  closed 
by  the  drop,  a  relay  5  and  a  high- 
voltage  battery  6  are  placed  in  this 
circuit.  The  buzzer  and  the  battery 
for  operating  it  are  placed  in  a  local 
circuit  controlled  by  this  relay.  It 
will  be  seen  by  reference  to  Fig. 
275  that  when  the  shutter  falls,  it 
will,  by  closing  the  contact  4,  com- 
plete the  circuit  from  the  battery  6 
through  the  relay  5 — assuming  switch  3  to  be  closed — and  thus 
cause  the  operation  of  the  relay.  The  relay,  in  turn,  by  pulling  up 
its  armature,  will  close  the  circuit  of  the  buzzer  2  through  the  battery 
7  and  cause  the  buzzer  to  sound. 

The  advantage  of  this  method  over  the  direct  method  of  oper- 
ating the  buzzer  is  that  any  imperfection  in  the  night-alarm  contact 
at  the  drop  is  much  less  likely  to  prevent  the  flow  of  current  of  the 
high-voltage  battery  6  than  of  the  low- voltage  battery  1,  shown  in 
connection  with  Fig.  274.  This  is  because  the  higher  voltage  is  much 
more  likely  to  break  down  any  very  thin  bit  of  insulation,  such  as 
might  be  caused  by  a  minute  particle  of  dust  or  oxide  between 
contacts  that  are  supposed  to  be  closed  by  the  falling  of  the  shut- 
ter. It  has  been  common  to  employ  for  battery  6  a  dry-cell  battery 


Fig.  275.     Night-Alarm  Circuit  with 
Relay 


350  TELEPHONY 

giving  about  20  or  24  volts,  and  for  the  operation  of  the  buzzer 
itself,  a  similar  battery  of  about  two  cells  giving  approximately  3 
volts. 

Night-Alarm  Contacts.  The  night-alarm  contact  4  °f  the  drop 
shown  diagrammatically  in  Figs.  274  and  275  would,  if  taken  literally, 
indicate  that  the  shutter  itself  actually  forms  one  terminal  of  the  cir- 
cuit and  the  contact  against  which  it  falls,  the  other.  This  has  not 
been  found  to  be  a  reliable  way  of  closing  the  night-alarm  contacts 
and  this  method  is  indicated  in  these  figures  and  in  other  figures  in 
this  work  merely  as  a  convenient  way  of  representing  the  matter 
diagrammatically.  As  a  matter  of  fact  the  night-alarm  contacts  are 
ordinarily  closed  by  having  the  shutter  fall  against  one  spring,  which 
is  thereby  pressed  into  engagement  with  another  spring  or  contact, 
as  shown  in  Fig.  249.  This  method  employs  the  shutter  only  as  a 
means  for  mechanically  causing  the  one  spring  to  press  against  the 
other,  the  shutter  itself  forming  no  part  of  the  circuit.  The  reason 
why  it  is  not  a  good  plan  to  have  the  shutter  itself  act  as  one  terminal 
of  the  circuit  is  that  this  necessitates  the  circuit  connections  being 
led  to  the  shutter  through  the  trunnions  on  which  the  shutter  is 
pivoted.  This  is  bad  because,  obviously,  the  shutter  must  be  loosely 
supported  on  its  trunnions  in  order  to  give  it  sufficiently  free  move- 
ment, and,  as  is  well  known,  loose  connections  are  not  conducive  to 
good  electrical  contacts. 

Grounded=  and  MetaIIic=Circuit  Lines.  When  grounded  cir- 
cuits were  the  rule  rather  than  the  exception,  many  of  the  switch- 
boards were  particularly  adapted  for  their  use  and  could  not  be  used 
with  metallic-circuit  lines.  These  grounded-circuit  switchboards 
provided  but  a  single  contact  in  the  jack  and  a  single  contact  on 
the  plug,  the  cords  having  but  a  single  strand  reaching  from  one 
plug  to  the  other.  The  ringing  keys  and  listening  keys  were  like- 
wise single-contact  keys  rather  than  double.  The  clearing-out  drop 
and  the  operator's  talking  circuit  and  the  ringing  generator  were 
connected  between  the  single  strand  of  the  cord  and  the  ground  as 
was  required. 

The  grounded-circuit  switchboard  has  practically  passed  out  of 
existence,  and  while  a  few  of  them  may  be  in  use,  they  are  not  manu- 
factured at  present.  The  reason  for  this  is  that  while  many  grounded 
circuits  are  still  in  use,  there  are  very  few  places  where  there  are 


SIMPLE  MAGNETO  SWITCHBOARD 


351 


not  some  metallic-circuit  lines,  and  while  the  grounded-circuit 
switchboard  will  not  serve  for  metallic-circuit  lines,  the  metallic- 
circuit  switchboard  will  serve  equally  well  for  either  metallic-cir- 
cuit or  grounded  lines,  and  will  interconnect  them  with  equal 
facility.  This  fact  will  be  made  clear  by  a  consideration  of  Figs. 
276,  277,  and  278. 

Connection  between  Two  Similar  Lines.     In  Fig.  276  a  common 
magneto  cord  circuit  is  shown  connecting  two  metallic-circuit  lines; 


Pig.  276.     Connection  Between  Metallic  Lines 

in  Fig.  277  the  same  cord  circuit  is  shown  connecting  two  grounded 
lines.  In  this  case  the  line  wire  1  of  the  left-hand  line  is,  when  the 
plugs  are  inserted,  continued  to  the  tip  of  the  answering  plug,  thence 
through  the  tip  strand  of  the  cord  circuit  to  the  tip  of  the  calling  plug, 
then  to  the  tip  spring  of  the  right-hand  jack  and  out  to  the  single  con- 


Fig.  277.     Connection  Between  Grounded  Lines 

ductor  of  that  line.  The  entire  sleeve  portion  of  the  cord  circuit 
becomes  grounded  as  soon  as  the  plugs  are  inserted  in  the  jacks  of 
such  a  line.  Hence,  we  see  that  the  sleeve  contacts  of  the  plug  and 
the  sleeve  conductor  of  the  cord  are  connected  to  ground  through 
the  permanent  ground  connection  of  the  sleeve  conductors  of  the  jack 
as  soon  as  the  plug  is  inserted  into  the  jack.  Thus,  when  the  cord 
circuit  of  a  metallic-circuit  switchboard  is  used  to  connect  two  ground- 
ed circuits  together,  the  tip  strand  of  the  cord  is  the  connecting  link 
between  the  two  conductors,  while  the  sleeve  strand  of  the  cord  merely 


352 


TELEPHONY 


serves  to  ground  one  side  of  the  clearing-out  drop  and  one  side  each 
of  the  operator's  telephone  set  and  the  ringing  generator  when  their 
respective  keys  are  operated. 

Connection  between  Dissimilar  Lines.  Fig.  278  shows  how  the 
same  cord  circuit  and  the  same  arrangement  of  line  equipment  may 
be  used  for  connecting  a  grounded  line  to  a  metallic-circuit  line. 
The  metallic  circuit  line  is  shown  on  the  left  and  the  grounded  line  on 
the  right.  When  the  two  plugs  are  inserted  into  the  respective  jacks 
of  this  figure,  the  right-hand  conductor  of  the  metallic  circuit  shown 
on  the  left  will  be  continued  through  the  tip  strand  of  the  cord  cir- 
cuit to  the  line  conductor  of  the  grounded  line  shown  on  the  right. 
The  left-hand  conductor  of  the  metallic-circuit  line  will  be  connected 
to  ground  because  it  will  be  continued  through  the  sleeve  strand  of 
the  cord  circuit  to  the  sleeve  contact  of  the  calling  plug  and  thence 
to  the  sleeve  contact  of  the  jack  of  the  grounded  line,  which  sleeve 
contact  is  shown  to  be  grounded.  The  talking  circuit  between  the 
two  connected  lines  in  this  case  may  be  traced  as  follows:  From 
the  subscriber's  station  at  the  left  through  the  right-hand  limb  of 
the  metallic-circuit  line,  through  the  tip  contact  and  tip  conductor 
of  the  cord  circuit,  to  the  single  limb  of  the  grounded-circuit  line, 
thence  to  the  sub-station  of  that  line  and  through  the  talking  ap- 
paratus there  to  ground.  The  return  path  from  the  right-hand 


TJ 


Fig.  278.     Connection  Between  DissimilarJLines 

station  is  by  way  of  ground  to  the  ground  connection  at  the  central 
office,  thence  to  the  sleeve  contact  of  the  grounded  line  jack,  through 
the  sleeve  conductor  of  the  cord  circuit,  to  the  sleeve  contact  of  the 
metallic-circuit  line  jack,  and  thence  by 'the  left-hand  limb  of  the 
metallic-circuit  line  to  the  subscriber's  station. 

A  better  way  of  connecting  a  metallic-circuit  line  to  a  grounded 
line  is  by  the  use  of  a  special  cord  circuit  involving  a  repeating 
coil,  such  a  connection  being  shown  in  Fig.  279.  The  cord  circuit 
in  this  case  differs  in  no  respect  from  those  already  shown  except 


SIMPLE  MAGNETO  SWITCHBOARD 


353 


that  a  repeating  coil  is  associated  with  it  in  such  a  way  as  to  conduc- 
tively  divide  the  answering  side  from  the  calling  side.  Obviously, 
whatever  currents  come  over  the  line  connected  with  the  answering 
plug  will  pass  through  the  windings  1  and  2  of  this  coil  and  will  induce 
corresponding  currents  in  the  windings  3  and  4>  which  latter  currents 


Fig.  279.    Connection  of  Dissimilar  Lines  through  Repeating  Coil 

will  pass  out  over  the  circuit  of  the  line  connected  with  the  calling 
plug.  When  a  grounded  circuit  is  connected  to  a  metallic  circuit 
in  this  manner,  no  ground  is  thrown  onto  the  metallic  circuit.  The 
balance  of  the  metallic  circuit  is,  therefore,  maintained. 

To  ground  one  side  of  a  metallic  circuit  frequently  so  unbal- 
ances it  as  to  cause  it  to  become  noisy,  that  is,  to  have  currents 
flowing  in  it,  by  induction  or  from  other  causes,  other  than  the 
currents  which  are  supposed  to  be  there  for  the  purpose  of  con- 
veying speech. 

Convertible  Cord  Circuits.  The  consideration  of  Fig.  279  brings 
us  to  the  subject  of  so-called  convertible  cord  circuits.  Some  switch- 
boards, serving  a  mixture  of  metallic  and  grounded  lines,  are  provided 
with  cord  circuits  which  may  be  converted  at  will  by  the  operator  from 
the  ordinary  type  shown  in  Fig.  276  to  the  type  shown  in  Fig.  279. 
The  advantage  of  this  will  be  obvious  from  the  following  consider- 
ation. When  a  call  originates  on  any  line,  either  grounded  or  me- 
tallic, the  operator  does  not  know  which  kind  of  a  line  is  to  be 
called  for.  She,  therefore,  plugs  into  this  line  with  any  one  of  her 
answering  plugs  and  completes  the  connection  in  the  usual  way. 
If  the  call  is  for  the  same  kind  of  a  circuit  as  that  over  which  the  call 
originated,  she  places  the  converting  key  in  such  a  position  as  will 
connect  the  conductors  of  the  cord  circuit  straight  through;  while  if 
the  connection  is  for  a  different  kind  of  a  line  than  that  on  which 
the  call  originated  she  throws  the  converting  key  into  such  a  posi- 


354 


TELEPHONY 


tion  as  to  include  the  repeating  coil.  A  study  of  Fig.  280  will  show 
that  when  the  converting  key,  which  is  commonly  referred  to  as  the 
repeating-coil  key,  is  in  one  position,  the  cord  conductors  will  be  cut 
straight  through,  the  repeating  coil  being  left  open  in  both  its  wind- 
ings; and  when  it  is  thrown  to  its  other  position,  the  connection  be- 


Fig.  280.     Convertible  Cord  Circuit 

tween  the  answering  and  calling  sides  of  the  cord  circuit  will  be 
severed  and  the  repeating  coil  inserted  so  as  to  bring  about  the  same 
effects  and  circuit  arrangements  as  are  shown  in  Fig.  279. 

Cord=Circuit  Considerations.  Simple  Bridging  Drop  Type. 
The  matter  of  cord  circuits  in  magneto  switchboards  is  deserving  of 
much  attention.  So  far  as  talking  requirements  are  concerned,  the 
ordinary  form  of  cord  circuit  with  a  clearing-out  drop  bridged  across 
the  two  strands  is  adequate  for  nearly  all  conditions  except  those  where 
a  grounded-  and  a  metallic-circuit  line  are  connected  together,  in 
which  case  the  inclusion  of  a  repeating  coil  has  some  advantages. 

From  the  standpoint  of  signaling,  however,  this  type  of  cord 
circuit  has  some  disadvantages  under  certain  conditions.  In  order 

to  simplify  the  discussion  of  this 


500 aj. 


f 


and  other  cord-circuit  matters, 
reference  will  be  made  to  some 
diagrams  from  which  the  ring- 
ing and  listening  keys  and 
talking  apparatus  have  been 
entirely  omitted.  In  Fig.  281 
the  regular  bridging  type  of 
clearing-out  drop-cord  circuit  is 

shown,  this  being  the  type  already  discussed  as  standard.  For 
ordinary  practice  it  is  all  right.  Certain  difficulties  are  expe- 
rienced with  it,  however,  where  lines  of  various  lengths  and  various 
types  of  sub-station  apparatus  are  connected.  For  instance,  if  a  long 


Fig.  281.     Bridging  Drop-Cord  Circuit 


SIMPLE  MAGNETO  SWITCHBOARD  355 

bridging  line  be  connected  with  one  end  of  this  cord  circuit  and  a 
short  line  having  a  low-resistance  series  ringer  be  connected  with  the 
other  end,  then  a  station  on  the  long  line  may  have  some  difficulty 
in  throwing  the  clearing-out  drop,  because  of  the  low-resistance  shunt 
that  is  placed  around  it  through  the  short  line  and  the  low-resistance 
ringer.  In  other  words,  the  clearing-out  drop  is  shunted  by  a  com- 
paratively low-resistance  line  and  ringer  and  the  feeble  currents  ar- 
riving from  a  distant  station  over  the  long  line  are  not  sufficient  to 
operate  the  drop  thus  handicapped.  The  advent  of  the  various  forms 
of  party-line  selective  signaling  and  the  use  of  such  systems  in  con- 
nection with  magneto  switchboards  has  brought  in  another  difficulty 
that  sometimes  manifests  itself  with  this  type  of  cord  circuit.  If  two 
ordinary  magneto  telephones  are  connected  to  the  two  ends  of  this 
cord  circuit,  it  is  obvious  that  when  one  of  the  subscribers  has  hung 
up  his  receiver  and  the  other  subscriber  rings  off,  the  bell  of  the 
other  subscriber  will  very  likely  be  rung  even  though  the  clearing- 
out  drop  operates  properly;  it  would  be  better  in  any  event  not 
to  have  this  other  subscriber's  bell  rung,  for  he  may  understand 
it  to  be  a  recall  to  his  telephone.  When,  however,  a  party  line  is 
connected  through  such  a  cord  circuit  to  an  ordinary  line  having 
bridging  instruments,  for  instance,  the  difficulty  due  to  ringing  off 
becomes  even  greater.  When  the  subscriber  on  the  magneto  line 
operates  his  generator  to  give  the  clearing-out  signal,  he  is  very  likely 
to  ring  some  of  the  bells  on  the  other  line  and  this,  of  course,  is 
an  undesirable  thing.  This  may  happen  even  in  the  case  of  har- 
monic bells  on  the  party  line,  since  it  is  possible  that  the  sub- 
scriber on  the  magneto  line  in  turning  his  generator  will,  at  some 
phase  of  the  operation,  strike  just  the  proper  frequency  to  ring  some 
one  of  the  bells  on  the  harmonic  party  line.  It  is  obvious,  there- 
fore, that  there  is  a  real  need  for  a  cord  circuit  that  will  prevent 
through  ringing. 

One  way  of  eliminating  the  through-ringing  difficulty  in  the  type 
of  cord  circuit  shown  in  Fig.  281  would  be  to  use  such  a  very  low- 
wound  clearing-out  drop  that  it  would  practically  short-circuit  the 
line  with  respect  to  ringing  currents  and  prevent  them  from  passing 
on  to  the  other  line.  This,  however,  is  not  a  good  thing  to  do,  since 
a  winding  sufficiently  low  to  shunt  the  effective  ringing  current  would 
also  be  too  low  for  good  telephone  transmission. 


356 


TELEPHONY 


ZM.f- 
/OOO  W. 

r 

raw 

Fig.  282.     Series  Drop-Cord  Circuit 


Series  Drop  Type.  Another  type  of  cord  circuit  that  was  largely 
used  by  the  Stromberg-Carlson  Telephone  Manufacturing  Company 
at  one  time  is  shown  in  Fig.  282.  In  this  the  clearing-out  drop  was 
not  bridged  but  was  placed  in  series  in  the  tip  side  of  the  line  and  was 

shunted  by  a  condenser.  The 
resistance  of  the  clearing-out  drop 
was  1,000  ohms  and  the  capacity 
of  the  condenser  was  2  micro- 
farads. It  is  obvious  that  this 
way  of  connecting  the  clearing- 
out  drop  was  subject  to  the 
ringing-through  difficulty,  since 
the  circuit  through  which  the 

clearing-out  current  necessarily  passed  included  the  telephone  in- 
strument of  the  line  that  was*  not  sending  the  clearing-out  signal. 
This  form  was  also  objectionable  because  it  was  necessary  for  the 
subscriber  to  ring  through  the  combined  resistance  of  two  lines, 
and  in  case  the  other  line  happened  to  be  open,  no  clearing- 
out  signal  would  be  received.  While  this  circuit,  therefore,  was 
perhaps  not  quite  so  likely  as  the  other  to  tie  up  the  subscriber, 
that  is,  to  leave  him  connected  without  the  ability  to  send  a  clearing- 
out  signal,  yet  it  was  sure  to  ring  through,  for  the  clearing-out  drop 
could  not  be  thrown  without  the  current  passing  through  the  other 
subscriber's  station. 

Non-Ring-Through  Type.  An  early  attempt  at  a  non-ring- 
through  cord  is  shown  in  Fig.  283,  this  having  once  been  standard 

with  the  Dean  Electric  Company. 
It  made  use  of  two  condensers  of  1 
microfarad  each,  one  in  each  side 
of  the  cord  circuit.  The  clearing- 
out  drop  was  of  500  ohms  resist- 
ance and  was  connected  from  the 
answering  side  of  the  tip  conduc- 
tor to  the  calling  side  of  the  sleeve 
conductor.  In  this  way  whatever 
clearing-out  current  reached  the 

central  office  passed  through  at  least  one  of  the  condensers  and  the 
clearing-out  drop,  In  order  for  the  clearing-out  current  to  pass  on 


23 

/  M.F. 

50OW 

ItoSjT 

I  > 

/H.F. 

-I— 

Pig.  283.     Dean  Non-Ring-Through 
Cord  Circuit 


357 


beyond  the  central  office  it  was  necessary  for  it  to  pass  through  the 
two  condensers  in  series.  This  arrangement  had  the  advantage  of 
giving  a  positive  ring-off,  regardless  of  the  condition  of  the  connected 
line.  Obviously,  even  if  the  line  was  short-circuited,  the  ringing 
currents  from  the  other  line  would  still  be  forced  through  the  clearing- 
out  drop  on  account  of  the  high  effective  resistance  of  the  1-micro- 
farad  condenser  connected  in  series  with  the  short-circuited  line. 
Also  the  clearing-out  signal  would  be  properly  received  if  the  con- 
nected line  were  open,  since  the  clearing-out  drop  would  still  be 
directly  across  the  cord  circuit.  This  arrangement  also  largely  pre- 
vented through  ringing,  since  the  currents  would  pass  through  the 
1-microfarad  condenser  and  the  500-ohm  drop  more  readily  than 
through  the  two  condensers  connected  in  series. 

In  Fig.  284  is  shown  the  non-ring-through  arrangement  of  cord 
circuit  adopted  by  the  Monarch  Company.  In  this  system  the  clear- 
ing-out drop  has  two  windings, 
either  of  which  will  operate  the  ar- 
mature. The  two  windings  are 
bridged  across  the  cord  circuit, 
with  a  ^-microfarad  condenser  in 
series  in  the  tip  strand  between  the 
two  winding  connections.  While 
the  low-capacity  condenser  will 
allow  the  high-frequency  talking 
current  to  pass  readily  without 

affecting  it  to  any  appreciable  extent,  it  offers  a  high  resistance  to  a 
low-frequency  ringing  current,  thus  preventing  it  from  passing  out  on 
a  connected  line  and  forcing  it  through  one  of  the  windings  of  the 
coil.  There  is  a  tendency  to  transformer  action  in  this  arrangement, 
one  of  the  windings  serving  as  a  primary  and  the  other  as  a  secondary, 
but  this  has  not  prevented  the  device  from  being  highly  successful. 

A  modification  of  this  arrangement  is  shown  in  Fig.  285,  where- 
in a  double-wound  clearing-out  drop  is  used,  and  a  ^-microfarad  con- 
denser is  placed  in  series  in  each  side  of  the  cord  circuit  between 
the  winding  connections  of  the  clearing-out  drop.  This  circuit  should 
give  a  positive  ring-off  under  all  conditions  and  should  prevent 
through  ringing  except  as  it  may  be  provided  by  the  transformer 
action  between  the  two  windings  on  the  same  core. 


Fig.  284.     Monarch  Non-Ring-Through 
Cord  Circuit 


358 


TELEPHONY 


Another  rather  ingenious  method  of  securing  a  positive  ring-off 
and  yet  of  preventing  in  a  certain  degree  the  undesirable  ringing- 
through  feature  is  shown  in  the  cord  circuit,  Fig.  286.  In  this  two 

non-inductive  coils  1  and  2  are 
shown  connected  in  series  in  the 
tip  and  sleeve  strands  of  the  coils, 
respectively.  Between  the  neu- 
tral point  of  these  two  non-induc- 
tive windings  is  connected  the 
clearing-out  drop  circuit.  Voice 
currents  find  ready  path  through 


C.O.D. 


500  W] 


f 


Fig.  285. 


Non-Ring-Through  Cord 
Circuit 


these  non-inductive  windings  be- 
cause of  the  fact  that,  being  non- 
inductive,  they  present  only  their  straight  ohmic  resistance.  The 
impedance  of  the  clearing-out  drop  prevents  the  windings  being 
shunted  across  the  two  sides  of  the  cord  circuit.  With  this  circuit  a 
positive  ring-off  is  assured  even  though  the  line  connected  with  the 
one  sending  the  clearing-out  signal  is  short-circuited  or  open.  If  it 
is  short-circuited,  the  shunt  around  the  clearing-out  drop  will  still 
have  the  resistance  of  two  of  the  non-inductive  windings  included 
in  it,  and  thus  the  drop  will  never  be  short-circuited  by  a  very  low- 
resistance  path.  Obviously,  an  open  circuit  in  the  line  will  not  pre- 
vent the  clearing-out  signal  being 
received.  While  this  is  an  ingeni- 
ous scheme,  it  is  not  one  to  be 
highly  recommended  since  the  non- 

"-?  {fOjCXjf  _[?        inductive  windings,  in  order  to  be 

effective  so  far  as  signaling  is  con- 
cerned, must  be  of  considerable 
resistance  and  this  resistance  is  in 
series  in  the  talking  circuit.  Even 
non-inductive  resistance  is  to  be 
avoided  in  the  talking  circuit  when  it  is  of  considerable  magnitude 
and  where  there  are  other  ways  of  solving  the  problem. 

Double  Clearing-out  Type.  Some  people  prefer  two  clearing- 
out  drops  in  each  cord  circuit,  so  arranged  that  the  one  will  be 
responsive  to  currents  sent  from  the  line  with  which  the  answering 
plug  is  connected  and  the  other  responsive  only  to  currents  sent  from 


Fig.  286. 


Cord  Circuit  with  Differential 
Windings 


SIMPLE  MAGNETO  SWITCHBOARD 


359 


IM.F. 


50OW. 


/M.F 


Fig.  287.     Double  Clearing-Out  Drops 


the  line  with  which  the  calling  plug  is  connected.  Such  a  scheme, 
shown  in  Fig.  287,  is  sometimes  employed  by  the  Dean,  the  Monarch, 
and  the  Kellogg  companies.  Two  500-ohm  clearing-out  drops  of 
ordinary  construction  are  bridged  across  the  cord  circuit  and  in-each 
side  of  the  cord  circuit  there  is  included  between  the  drop  connec- 
tions a  1 -microfarad  condenser. 

m^mm 

Ringing  currents  originating  on 
the  line  with  which  the  answer- 
ing plug  is  connected  will  pass 
through   the    clearing-out    drop, 
which  is  across  that  side  of  the 
cord  circuit,   without  having  to 
pass  through  any  condensers.   In 
order  to  reach  the  other  clearing- 
out   drop  the  ringing  current  must  pass  through  the  two  1-micro- 
farad  condensers  in  series,  this  making  in  effect  only  ^-microfarad. 
As  is  well  known,  a  ^-microfarad  condenser  not  only  transmits  voice 
currents  with  ease  but  also  offers  a  very  high  apparent  resistance 
to    ringing   currents.     With    the   double  clearing-out  drop    system 
the  operator  is  enabled  to  tell  which  subscriber  is  ringing  off.     If 
both  shutters  fall  she  knows  that  both  subscribers  have  sent  clear- 
ing-out   signals    and    she,    therefore,    pulls    down    the    connection 
without  the  usual  precaution    of   listening  to   see   whether  one  of 
the  subscribers  may  be  waiting  for  another  connection.     This  dou- 
ble clearing-out  system   is  analogous  to  the  complete  double-lamp 
supervision  that  will  be  referred  to  more  fully  in  connection  with 
common-battery  circuits.      There  is  not  the  need  for  double  super- 
vision in  magneto  work,  however,  that  there  is  in  common-battery 
work  because  of  the  fact  that  in  magneto  work  the  subscribers  fre- 
quently fail  to  remember  to  ring  off,  this  act  being  entirely  voluntary 
on  their  part,  while  in  common-battery  work,  the  clearing-out  signal 
is  given  automatically  by  the  subscriber  when  he  hangs  up  his 
receiver,  thus  accomplishing  the  desired  end  without  the  necessity  of 
thoughtfulness  on  his  part. 

Another  form  of  double  clearing-out  cord  circuit  is  shown  in 
Fig.  288.  In  this  the  calling  and  the  answering  plugs  are  separated 
by  repeating  coils,  a  condenser  of  1-microfarad  capacity  being  in- 
serted between  each  pair  of  windings  on  the  two  ends  of  the  circuit. 


360 


TELEPHONY 


The  clearing-out  drops  are  placed  across  the  calling  and  answering 
cords  in  the  usual  manner.  The  condenser  in  this  case  prevents  the 
drop  being  short-circuited  with  respect  to  ringing  currents  and  yet 


Fig.  288.     Double  Clearing-Out  Drops 

permits  the  voice  currents  to  flow  readily  through  it.  The  high 
impedance  of  the  drop  forces  the  voice  currents  to  take  the  path 
through  the  repeating  coil  rather  than  through  the  drop.  This  cir- 
cuit has  the  advantage  of  a  repeating-coil  cord  circuit  in  permitting 
the  connection  of  metallic  and  grounded  lines  without  causing  the 
unbalancing  of  the  metallic  circuits  by  the  connection  to  them  of  the 
grounded  circuits. 

Recently  there  has  been  a  growing  tendency  on  the  part  of  some 
manufacturers  to  control  their  clearing-out  signals  by  means  of 
relays  associated  with  cord  circuits,  these  signals  sometimes  being 
ordinary  clearing-out  drops  and  sometimes  incandescent  lamps. 

In  Fig.  289  is  shown  the  cord  circuit  sometimes  used  by  the 
L.  M.  Ericsson  Telephone  Manufacturing  Company.  A  high-wound 


.  C.O.D. 

HPJLJJ 


*^    Q< 

A^z~\x3  ^^^J 
r\LL  /     [^ 


Fig.  289.     Relay-Controlled  Clearing-Out  Drop 

relay  is  normally  placed  across  the  cord  and  this,  besides  having  a 
high-resistance  and  impedance  winding  has  a  low-resistance  locking 
winding  so  arranged  that  when  the  relay  pulls  up  its  armature  it 
will  close  a  local  circuit  including  this  locking  winding  and  local 
battery.  When  once  pulled  up  the  relay  will,  therefore,  stay  up 


SIMPLE  MAGNETO  SWITCHBOARD  .      361 

due  to  the  energizing  of  this  locking  coil.  Another  contact  oper- 
ated by  the  relay  closes  the  circuit  of  a  low-wound  clearing-out  drop 
placed  across  the  line,  thus  bridging  it  across  the  line.  The 
condition  of  high  impedance  is  maintained  across  the  cord  circuit 
normally  while  the  subscribers  are  talking;  but  when  either  of  them 
rings  off,  the  high-wound  relay  pulls  up  and  locks,  thus  com- 
pleting the  circuit  of  the  clearing-out'  drop  across  the  cords. 
The  subsequent  impulses  sent  from  the  subscribers'  generators 
operate  this  drop.  The  relay  is  restored  or  unlocked  and  the 
clearing-out  drop  disconnected  from  the  cord  circuit  by  means 
of  a  key  which  opens  the  locking  circuit  of  the  relay.  This  key 
is  really  a  part  of  the  listening  key  and  serves  to  open  this 
locking  circuit  whenever  the  listening  key  is  operated.  The  clear- 
ing-out drop  is  also  automatically  restored  by  the  action  of  the 
listening  key,  this  connection  being  mechanical  rather  than  elec- 
trical. 

Recall  Lamp: — The  Monarch  Company  sometimes  furnishes 
what  it  terms  a  recall  lamp  in  connection  with  the  clearing-out  drops 
on  its  magneto  switchboards. 
The  circuit  arrangement  is  shown 
in  Fig.  290,  wherein  the  drop  is 
the  regular  double- wound  clear- 
ing-out drop  like  that  of  Fig.  284. 
The  armature  carries  a  contact 
spring  adapted  to  close  the  local 

circuit  of  a  lamp  whenever  it  is 

L   j       T<U        u-  e  j-U'     •         Fig.  290.    Cord  Circuit  with  Recall  Lamp 

attracted.     The  object  ol  this  is 

to  give  the  subscriber,  whose  line  still  remains  connected  by  a  cord 
circuit,  opportunity  to  recall  the  central  office  if  the  operator  has  not 
restored  the  clearing-out  drop. 

Lamp-Signal  Type.  There  has  been  a  tendency  on  the  part  of 
some  manufacturing  companies  to  advocate,  instead  of  drop  signals, 
incandescent  lamp  signals  for  the  cord  circuits,  and  sometimes  for  the 
line  circuits  on  magneto  boards.  In  most  cases  this  may  be  looked 
upon  as  a  "frill."  Where  line  lamps  instead  of  drops  have  been  used 
on  magneto  switchboards,  it  has  been  the  practice  to  employ,  instead 
of  a  drop,  a  locking  relay  associated  with  each  lamp,  which  was  so 
arranged  that  when  the  relay  was  energized  by  the  magneto  current 


362 


TELEPHONY 


from  the  subscriber's  station,  it  would  pull  up  and  lock,  thus  closing 
the  lamp  circuit. 

The  local  circuit,  or  locking  circuit,  which  included  the  lamp 
was  carried  through  a  pair  of  contacts  in  the  corresponding  jacks 
so  arranged  that  when  the  plug  was  inserted  in  answer  to  the  call, 
this  locking  lamp  circuit  would  be  open,  thereby  extinguishing  the 
lamp  and  also  unlocking  the  relay.  There  seems  to  be  absolutely 
no  good  reason  why  lamp  signals  should  be  substituted  for  mechanical 
drops  in  magneto  switchboards.  There  is  no  need  for  the  economy 
in  space  which  the  lamp  signal  affords,  and  the  complications  brought 
in  by  the  locking  relays,  and  the  requirements  for  maintaining  a 
local  battery  suitable  for  energizing  the  lamps  are  not  warranted 
for  ordinary  cases. 

In  Fig.  291  is  shown  a  cord  circuit,  adaptable  to  magneto  switch- 
boards, provided  with  double  lamp  signals  instead  of  clearing-out 


Pig.  291.     Cord  Circuit  with  Double  Lamp  Signals 

drops.  Two  high-wound  locking  relays  are  bridged  across  the  line, 
the  cord  strands  being  divided  by  1-microfarad  condensers.  When 
the  high-wound  coil  of  either  relay  is  energized  by  the  magneto  cur- 
rent from  the  subscriber's  station,  the  relay  pulls  up  and  closes  a 
locking  circuit  including  a  battery  and  a  coil  2,  the  contact  3  of  the 
locking  relay,  and  also  the  contact  ^  of  a  restoring  key.  This  cir- 
cuit may  be  traced  from  the  ground  through  battery,  coil  2,  contact 
3  controlled  by  the  relay,  and  contact  4  controlled  by  the  restoring 
key,  and  back  to  ground.  In  multiple  with  the  locking  coil  2 
is  the  lamp,  which  is  illuminated,  therefore,  whenever  the  locking 
circuit  is  closed.  Pressure  on  the  restoring  key  breaks  the  lock- 
ing circuit  of  either  of  the  lamps,  thereby  putting  out  the  lamp 
and  at  the  same  time  restoring  the  locking  relay  to  its  normal  po- 
sition. 


SIMPLE  MAGNETO  SWITCHBOARD  363 

Lamps  vs.  Drops  in  Cord  Circuits.  So  much  has  been  said  and 
written  about  the  advantages  of  incandescent  lamps  as  signals  in  switch- 
boards and  about  the  merits  of  the  common-battery  method  of  sup- 
plying current  to  the  subscribers,  that  there  has  been  a  tendency 
for  people  in  charge  of  the  operation  of  small  exchanges  to  substi- 
tute the  lamp  for  the  drop  in  a  magneto  switchboard  in  order  to  give 
the  general  appearance  of  common-battery  operations.  There  has 
also  been  a  tendency  to  employ  the  common-battery  system  of  opera- 
tion in  many  places  where  magneto  service  should  have  been  used, 
a  mistake  which  has  now  been  realized  and  corrected.  In  places 
where  the  simple  magneto  switchboard  is  the  thing  to  use,  the 
simpler  it  is  the  better,  and  the  employment  of  locking  relays  and 
lamp  signals  and  the  complications  which  they  carry  with  them,  is  not 
warranted. 

Switchboard  Assembly.  The  assembly  of  all  the  parts  of  a 
simple  magneto  switchboard  into  a  complete  whole  deserves  final 
consideration.  The  structure  in  which  the  various  parts  are  mounted, 
referred  to  as  the  cabinet,  is  usually  of  wood. 

Functions  of  Cabinet.  The  purpose  of  the  cabinet  is  not  only 
to  form  a  support  for  the  various  pieces  of  apparatus  but  also  to  pro- 
tect them  from  dust  and  mechanical  injury,  and  to  hold  those  parts 
that  must  be  manipulated  by  the  operator  in  such  relation  that  they 
may  be  most  convenient  for  use,  and  thus  best  adapted  for  carrying 
out  their  various  functions.  Other  points  to  be  provided  for  in 
the  design  of  the  cabinet  and  the  arrangement  of  the  various  parts 
within  are:  that  all  the  apparatus  that  is  in  any  way  liable  to  get 
out  of  order  may  be  readily  accessible  for  inspection  and  repairs;  and 
that  provision  shall  be  made  whereby  the  wiring  of  these  various 
pieces  of  apparatus  may  be  done  in  a  systematic  and  simple  way 
so  as  to  minimize  the  danger  of  crossed,  grounded,  or  open  circuits, 
and  so  as  to  provide  for  ready  repair  in  case  any  of  these  injuries 
do  occur. 

Wall-Type  Switchboards.  The  simplest  form  of  switchboard  is 
that  for  serving  small  communities  in  rural  districts.  Ordinarily 
the  telephone  industry  in  such  a  community  begins  by  a  group  of 
farmers  along  a  certain  road  building  a  line  connecting  the  houses 
of  several  of  them  and  installing  their  own  instruments.  This  line 
is  liable  to  be  extended  to  some  store  at  the  village  or  settlement, 


364 


TELEPHONY 


thus  affording  communication  between  these  farmers  and  the  cen- 
ter of  their  community.  Later  on  those  residing  on  other  roads  do 
the  same  thing  and  connect  their  lines  to  the  same  store  or  central 
point.  Then  it  is  that  some  form  of  switchboard  is  established,  and 
perhaps  the  storekeeper's  daughter  or  wife  is  paid  a  small  fee  for 
attendance. 

A  switchboard  well-adapted  for  this  class  of  service  where  the 
number  of  lines  is  small,  is  shown  in  Fig.  292.     In  this  the  operator's 


Fig.  292.     Wall  Switchboard  with  Telephone 

talking  apparatus  and  her  calling  apparatus  are  embodied  in  an 
ordinary  magneto  wall  telephone.  The  switchboard  proper  is 
mounted  alongside  of  this,  and  the  two  line  binding  posts  of  the 
telephone  are  connected  by  a  pair  of  wires  to  terminals  of  the  oper- 
ator's plug,  which  plug  is  shown  hanging  from  the  left-hand  portion 
of  the  switchboard.  The  various  lines  centering  at  this  point  ter- 
minate in  the  combined  drops  and  jacks  on  the  switchboard,  of  which 
there  are  20  shown  in  this  illustration.  Beside  the  operator's  plug 
there  are  a  number  of  pairs  of  plugs  shown  hanging  from  the  switch- 
board cabinet.  These  are  connected  straight  through  in  pairs, 


SIMPLE  MAGNETO  SWITCHBOARD 


365 


there  being  no  clearing-out  drops  or  keys  associated  with  them  in 
the  arrangement.  Each  line  shown  is  provided  with  an  extra  jack, 
the  purpose  of  which  will  be  presently  understood. 

The  method  of  operation  is  as  follows:  When  a  subscriber  on 
a  certain  line  desires  to  get  connection  through  the  switchboard 
he  turns  his  generator  and  throws  the  drop.  The  operator  in  order 
to  communicate  with  him  inserts  the  plug  in  which  her  telephone 
terminates  into  the  jack,  and  removes  her  receiver  from  its  hook. 
Having  learned  that  it  is  for  a  certain  subscriber  on  another  line, 
she  withdraws  her  plug  from  the 
jack  of  the  calling  line  and  in- 
serts it  into  the  jack  of  the  called 
line,  then,  hanging  up  her  re- 
ceiver, she  turns  the  generator 
crank  in  accordance  with  the 
proper  code  to  call  that  sub- 
scriber. When  that  subscriber 
responds  she  connects  the  two 
lines  by  inserting  the  two  plugs 
of  a  pair  into  their  respective 
jacks,  and  the  subscribers  are 
thus  placed  in  communication. 
The  extra  jack  associated  with 
each  line  is  merely  an  open  jack 
having  its  terminals  connected  re- 
spectively with  the  two  sides  of 
the  line.  Whenever  an  operator 
desires  to  listen  in  on  two  con- 
nected lines  she  does  so  by  in- 
serting the  operator's  plug  into  one  of  these  extra  jacks  of  the  con- 
nected lines,  and  she  may  thus  find  out  whether  the  subscribers 
are  through  talking  or  whether  either  one  of  them  desires  another 
connection.  The  drops  in  such  switchboards  are  commonly  high 
wound  and  left  permanently  bridged  across  the  line  so  as  to  serve 
as  clearing-out  drops.  The  usual  night-alarm  attachment  is  pro- 
vided, the  buzzer  being  shown  at  the  upper  right-hand  portion  of 
the  cabinet. 

Another  type  of  switchboard  commonly  employed  for  this  kind 


Fig.  293.     Combined  Telephone  and 
Switchboard 


366 


TELEPHONY 


of  service  is  shown  in  Fig.  293,  in  which  the  telephone  and  the  switch- 
board cabinet  are  combined.  The  operation  of  this  board  is  practi- 
cally the  same  as  that  of  Fig.  292,  although  it  has  manually-restored 
drops  instead  of  self-restoring  drops;  the  difference  between  these  two 
types,  however,  is  not  material  for  this  class  of  service.  For  such 
work  the  operator  has  ample  time  to  attend  to  the  restoring  of  the 


Fig.  294.     Upright  Magneto 
Switchboard 


Fig.  295.    Upright  Magneto 
Switchboard — Rear  View 


drop  and  the  only  possible  advantage  in  the  combined  drop-and- 
jack  for  this  class  of  work  is  that  it  prevents  the  operator  from 
forgetting  to  restore  the  drops.  However,  she  is  not  likely  to  do 
this  with  the  night-alarm  circuit  in  operation,  since  the  buzzer  or  bell 
would  continue  to  ring  as  long  as  the  drop  was  down. 

Upright  Type  Switchboard.     By  far  the  most  common  type  of 
magneto  switchboard  is  the  so-called  upright  type,  wherein  the  drops 


SIMPLE  MAGNETO  SWITCHBOARD 


367 


and  jacks  are  mounted  on  the  face  of  upright  panels  rising  from  a 
horizontal  shelf,  which  shelf  contains  the  plugs,  the  keys,  and  any 
other  apparatus  which  the  operator  must  manipulate.  Front  and 
rear  views  of  such  a  switchboard,  as  manufactured  by  the  Kellogg 
Company,  are  shown  in  Figs.  294  and  295.  This  particular  board 
is  provided  with  fifty  combined  drops  and  jacks  and,  therefore, 

i 


Fig.  296.     Details  of  Drop,  Jack,  Plug,  and  Key  Arrangement 

equipped  for  fifty  subscribers'  lines.  The  drops  and  jacks  are 
mounted  in  strips  of  five,  and  arranged  in  two  panels.  The  clear- 
ing-out drops,  of  which  there  are  ten,  are  arranged  at  the  bottom 
of  the  two  panels  in  a  single  row  and  may  be  seen  immediately 
above  the  switchboard  plugs.  There  are  ten  pairs  of  cords  and 
plugs  with  their  associated  ringing  and  listening  keys,  the  plugs  being 


CONOC.NSE.KS   FOR   DOUBLE  CLEAR    OUT 
CORD    CIRCUITS    ONLY    USED    WITH 
HARMONIC     SYSTEM 


ANSWE.RIN' 
CALLING    PLO 
RING    BACK    KEY, 
LISTENING,   AND   RINQINQKE.Y 
HINQELD     KEY    SHE 


GENERATOR 
CRANK 


Fig.  297.    Cross-Section  of  Upright  Switchboard 


SIMPLE  MAGNETO  SWITCHBOARD 


369 


mounted  on  the  rear  portion  of  the  shelf,  while  the  ringing  and  listen- 
ing keys  are  mounted  on  the  hinged  portion  of  the  shelf  in  front  of 
the  plugs. 

A  better  idea  of  the  arrangement  of  drops,  jacks,  plugs,  and  keys 
may  be  had  from  an  illustration  of  a  Dean  magneto  switchboard  shown 
in  Fig.  296.  The  clearing-out  drops  and  the  arrangement  of  the 
plugs  and  keys  are  clearly  shown.  The  portion  of  the  switchboard 
on  which  the  plugs  are  mounted  is  always  immovable,  the  plugs 
being  provided  with  seats  through  which  holes  are  bored  of  suffi- 
cient size  to  permit  the  switchboard  cord  to  pass  beneath  the  shelf. 
When  one  of  these  plugs  is  raised,  the  cord  is  pulled  up  through  this 
hole  thus  allowing  the  plug  to  be  placed  in  any  of  the  jacks. 

The  key  arrangement  shown  in  this  particular  cut  is  instructive. 
It  will  be  noticed  that  the  right-hand  five  pairs  of  plugs  are  provided 
with  ordinary  ringing  and  listening  keys,  while  the  left-hand  five 
are  provided  with  party-line  ringing  keys  and  listening  keys.  The 
listening  key  in  each  case  is  the  one  in  the  rear  and  is  alike  for  all 
of  the  cord  pairs.  The  right-hand  five  ringing  keys  are  so  arranged 
that  pressing  the  lever  to  the  rear  will  ring  on  the  answering  cord, 
while  pressing  it  toward  the  front  will  cause  ring- 
ing current  to  flow  on  the  calling  plug.  In  the 
left-hand  five  pairs  of  cords  shown  in  this  cut,  the 
pressure  of  any  one  of  the  keys  causes  a  ringing 
current  of  a  certain  frequency  to  flow  on  the  call- 
ing cord,  this  frequency  depending  upon  which 
one  of  the  keys  is  pressed. 

An  excellent  idea  of  the  grouping  of  the  vari- 
ous  pieces   of  apparatus   in   a  complete  simple 
magneto  switchboard  may  be  had  from  Fig.  297. 
While  the  arrangement  here  shown  is  applicable 
particularly  to  the  apparatus  of  the  Dean  Electric 
Company,  the  structure  indicated  is  none-the-less 
generally  instructive,  since  it  represents  good  prac- 
tice in  this  respect.     In  this  drawing  the  stationary 
plug  shelf  with  the  plug  seat  is  clearly  shown  and  also  the  hinged 
key  shelf.     The  hinge  of  the  key  shelf  is  an  important  feature  and 
is  universally  found  in  all  switchboards  of  this  general  type.     The 
key  shelf  may  be  raised  and  thus  expose  all  of  the  wiring  leading  to 


Fig.  298.     Cord 
Weight 


370  TELEPHONY 

the  keys,  as  well  as  the  various  contacts  of  the  keys  themselves,  to 
inspection. 

As  will  be  seen,  the  switchboard  cords  leading  from  the  plugs 
extend  down  to  a  point  near  the  bottom  of  the  cabinet  where  they 
pass  through  pulley  weights  and  then  up  to  a  stationary  cord  rack. 
On  this  cord  rack  are  provided  terminals  for  the  various  conduc- 


Fig.  299.     Magneto  Switchboard,  Target  Signals 


tors  in  the  cord,  and  it  is  at  this  point  that  the  cord  conductors  join 
the  other  wires  leading  to  the  other  portions  of  the  apparatus  as 
required.  A  good  form  of  cord  weight  is  shown  in  Fig.  298;  and 
obviously  the  function  of  these  weights  is  to  keep  the  cords  taut  at 
all  times  and  to  prevent  their  tangling. 


SIMPLE  MAGNETO  SWITCHBOARD 


371 


The  drawing,  Fig.  297,  also  gives  a  good  idea  of  the  method  of 
mounting  the  hand  generator  that  is  ordinarily  employed  with  such 
magneto  switchboards.  The  shaft  of  the  generator  is  merely  con- 
tinued out  to  the  front  of  the  key  shelf  where  the  usual  crank  is 
provided,  by  means  of  which  the  operator  is  able  to  generate  the 


Fig.  300.     Rear  View  of  Target  Signal,  Magneto  Switchboard 

necessary  ringing  current.  Beside  the  hand  generator  at  each  oper- 
ator's position,  it  is  quite  common  in  magneto  boards,  of  other  than  the 
smallest  sizes,  to  employ  some  form  of  ringing  generator,  either  a 
power-driven  generator  or  a  pole  changer  driven  by  battery  current  for 
furnishing  ringing  current  without  effort  on  the  part  of  the  operator 


472 


TELEPHONY 


Switchboards  as  shown  in  Figs.  294  and  295,  are  called  single- 
position  switchboards  because  they  afford  room  for  a  single  operator. 
Ordinarily  for  this  class  of  work  a  single  operator  may  handle  from 
one  to  two  hundred  lines,  although  of  course  this  depends  on  the 


Fig.  301.     Dean  Two-Position  Switchboard 

amount  of  traffic  on  the  line,  and  this,  in  turn,  depends  on  the  char- 
acter of  the  subscribers  served,  and  also  on  the  average  number 
of  stations  on  a  line.  Another  single-position  switchboard  is  shown 
in  Figs.  299  and  300,  being  a  front  and  rear  view  of  the  simple 
magneto  switchboard  of  the  Western  Electric  Company,  which  is 


SIMPLE  MAGNETO  SWITCHBOARD 


373 


provided  with  the  target  signals  of  that  company   rather   than   the 
usual  form  of  drop. 

Where  a  switchboard  must  accommodate  more  lines  than  can  be 
handled  by  a  single  operator,  the  cabinet  is  made  wider  so  as  to  afford 


Fig.  302.     Rear  View  of  Dean  Two-Position  Switchboard 

room  for  more  than  one  operator  to  be  seated  before  it.  Sometimes 
this  is  accomplished  by  building  the  cabinet  wider,  or  by  putting  two 
such  switchboard  sections  as  are  shown  in  Figs.  294  or  299  side  by 
side.  A  two-position  switchboard  section  is  shown  in  front  and 
rear  views  in  Figs.  301  and  302. 


374 


TELEPHONY 


Sectional  Switchboards.  The  problem  of  providing  for  growth  in 
a  switchboard  is  very  much  the  same  as  that  which  confronts  one  in 
buying  a  bookcase  for  his  library.  The  Western  Electric  Company 
has  met  this  problem,  for  very  small  rural  exchanges,  in  much  the 
same  way  that  the  sectional  bookcase  manufacturers  have  provided 
for  the  possible  increase  in  bookcase  capacity.  Like  the  sectional 
bookcase,  this  sectional  switchboard  may  start  with  the  smallest  of 
equipment — a  single  sectional  unit — and  may  be  added  to  vertically 


Pig.  303.     Sectional  Switchboard- Wall  Type 

as  the  requirements  increase,  the  original  equipment  being  usable  in 
its  more  extended  surroundings. 

This  line  of  switchboards  is  illustrated  in  Figs.  303  to  306.  The 
beginning  may  be  made  with  either  a  wall  type  or  an  upright  type  of 
switchboard,  the  former  being  mounted  on  brackets  secured  to  the 
wall,  and  the  latter  on  a  table.  A  good  idea  of  the  wall  type  is  shown 
in  Fig.  303.  Three  different  kinds  of  sectional  units  are  involved 
in  this:  first,  the  unit  which  includes  the  cords,  plugs,  clearing- 
out  drops,  listening  jacks,  operator's  telephone  set  and  generator; 
second,  the  unit  containing  the  line  equipment,  including  a  strip  of 
ten  magneto  line  signals  and  their  corresponding  jacks;  third,  the 


SIMPLE  MAGNETO  SWITCHBOARD 


375 


finishing  top,  which  includes  no  equipment  except  the  support  for 
the  operator's  talking  apparatus. 

The  first  of  the  units  in  Fig.  303  forms  the  foundation  on  which 
the  others  are  built.  Two  of  the  line-equipment  units  are  shown; 
these  provide  for  a  total  of  twenty  lines.  The  top  rests  on  the 
upper  line-equipment  unit,  and  when  it  becomes  necessary  to  add 
one  or  more  line-equipment  units  as  the  switchboard  grows,  this 


Fig.  304.     Sectional  Switchboard— Wall  Type 

top  is  merely  taken  off,  the  other  line-equipment  units  put  in  place 
on  top  of  those  already  existing,  and  the  top  replaced.  The  wall 
type  of  sectional  switchboard  is  so  arranged  that  the  entire  structure 
may  be  swung  out  from  the  wall,  as  indicated  in  Fig.  304,  exposing 
all  of  the  apparatus  and  wiring  for  inspection.  Each  of  the  sectional 
units  is  provided  with  a  separate  door,  as  indicated,  so  that  the  rear 
door  equipment  is  added  to  automatically  as  the  sections  are  added. 
In  the  embodiment  of  the  sectional  switchboard  idea  shown  in  these 
two  figures  just  referred  to.  ne  ringing  and  listening  keys  are  pro- 


376 


TELEPHONY 


vided,  but  the  operator's  telephone  and  generator  terminate  in  a 
special  plug — the  left-hand  one  shown  in  Fig.  303 — and  when  the 
operator  desires  to  converse  with  the  connected  subscribers,  she  does 
so  by  inserting  the  operator's  plug  into  one  of  the  jacks  immediately 
below  the  clearing-out  drop  corresponding  to  the  pair  of  plugs  used 
in  making  the  connection.  The  arrangement  in  this  case  is  exactly 
the  same  in  principle  as  that  described  in  Fig.  292.  The  operator's 
generator  is  so  arranged  in  connection  with  this  left-hand  operator's 

plug  that  the  turning  of  the  gener- 
ator crank  automatically  switches 
the  operator's  telephone  set  off  and 
switches  the  generator  on,  just  the 
same  as  a  switch  hook  may  do  in 
a  subscriber's  series  telephone. 


Fig.  305. 


Sectional  Switchboard- 
Table  Type 


Fig.  306. 


Sectional  Switchboard- 
Table  Type 


The  upright  type  of  sectional  switchboard  is  shown  in  Figs. 
305  and  306,  which  need  no  explanation  in  view  of  the  foregoing, 
except  to  say  that,  in  the  particular  instrument  illustrated,  ringing 
and  listening  keys  are  provided  instead  of  the  jack-and-plug  arrange- 
ment of  the  wall  type.  In  this  case  also,  the  top  section  carries  an 
arm  for  supporting  a  swinging  transmitter  instead  of  the  hook  sup- 
port for  the  combined  transmitter  and  receiver. 


CHAPTER  XXII 
THE  SIMPLE  COMMON=BATTERY  SWITCHBOARD 

Advantages  of  Common=Battery  Operation.  The  advantages  of 
the  common-battery  system  of  operation,  alluded  to  in  Chapter  XIII, 
may  be  briefly  summarized  here.  The  main  gain  in  the  common- 
battery  system  of  supply  is  the  simplification  of  the  subscribers' 
instruments,  doing  away  with  the  local  batteries  and  the  magneto 
generators,  and  the  concentration  of  all  these  many  sources  of  cur- 
rent into  one  single  source  at  the  central  office.  A  considerable 
saving  is  thus  effected  from  the  standpoint  of  maintenance,  since 
the  simpler  common-battery  instrument  is  not  so  likely  to  get  out 
of  order  and,  therefore,  does  not  have  to  be  visited  so  often  for  repairs, 
and  the  absence  of  local  batteries,  of  course,  makes  the  renewal  of 
the  battery  parts  by  members  of  the  maintenance  department, 
unnecessary.  Another  decided  advantage  in  the  common-battery 
system  is  the  fact  that  the  centralized  battery  stands  ready  always 
to  send  current  over  the  line  when  the  subscriber  completes  the 
circuit  of  the  line  at  his  station  by  removing  his  receiver  from  its 
hook.  The  common-battery  system,  therefore,  lends  itself  naturally 
to  the  purposes  of  automatic  signaling,  since  it  is  only  necessary  to 
place  at  the  central  office  a  device  in  the  circuit  of  each  line  that  will 
be  responsive  to  the  current  which  flows  from  the  central  battery 
when  the  subscriber  removes  his  receiver  from  its  hook.  It  is  thus 
that  the  subscriber  is  enabled  automatically  to  signal  the  central 
office  when  he  desires  a  connection;  and  as  will  be  shown,  it  is  by 
the  same  sort  of  means,  associated  with  the  cord  circuits  used  in  con- 
necting his  line  with  some  other  line,  that  the  operator  is  automatically 
notified  when  a  disconnection  is  desired,  the  cessation  of  current 
through  the  subscriber's  line  when  he  hangs  up  his  receiver  being 
made  to  actuate  certain  responsive  devices  which  are  associated  with 
the  cord  at  that  time  connected  with  his  line,  and  which  convey 
the  proper  disconnect  signal  to  the  operator. 


378  TELEPHONY 

Concentration  of  sources  of  energy  into  a  single  large  unit,  the 
simplification  of  the  subscriber's  station  equipment,  and  the  ready 
adaptability  to  automatic  signaling  from  the  subscriber  to  the  central 
office  are,  therefore,  the  reasons  for  the  existence  of  the  common- 
battery  system. 

Common  Battery  vs.  Magneto.  It  must  not  be  supposed,  how- 
ever, that  the  common-battery  system  always  has  advantages  over 
the  magneto  system,  and  that  it  is  superior  to  the  magneto  or  local- 
battery  system  for  all  purposes.  It  is  the  outward  attractiveness 
of  the  common-battery  system  and  the  arguments  in  its  favor,  so 
readily  made  by  over-zealous  salesmen,  that  has  led,  in  many 
cases,  to  the  adoption  of  this  system  when  the  magneto  system 
would  better  have  served  the  purpose  of  utility  and  economy. 

To  say  the  least,  the  telephone  transmission  to  be  had  from 
common-battery  systems  is  no  better  than  that  to  be  had  from  local- 
battery  systems,  and  as  a  rule,  assuming  equality  in  other  respects, 
it  is  not  as  good.  It  is  perhaps  true,  however,  that  under  average 
conditions  common-battery  transmission  is  somewhat  better,  be- 
cause whereas  the  local  batteries  at  the  subscribers'  stations  in 
the  local-battery  system  are  not  likely  to  be  in  uniformly  first-class 
condition,  the  battery  in  a  common-battery  system  will  be  kept  up 
to  its  full  voltage  except  under  the  grossest  neglect. 

The  places  in  which  the  magneto,  or  local-battery,  system  is  to 
be  preferred  to  the  common-battery  system,  in  the  opinion  of  the 
writers,  are  to  be  found  in  the  small  rural  communities  where  the 
lines  have  a  rather  great  average  length;  where  a  good  many  sub- 
scribers are  likely  to  be  found  on  some  of  the  lines;  where  the  sources 
of  electrical  power  available  for  charging  storage  batteries  are  likely 
either  not  to  exist,  or  to  be  of  a  very  uncertain  nature;  and  where  it 
is  not  commercially  feasible  to  employ  a  high-grade  class  of  attend- 
ants, or,  in  fact,  any  attendant  at  all  other  than  the  operator  at  the 
central  office. 

In  large  or  medium-sized  exchanges  it  is  always  possible  to  pro- 
cure suitable  current  for  charging  the  storage  batteries  required  in 
common-battery  systems,  and  it  is  frequently  economical,  on  ac- 
count of  the  considerable  quantity  of  energy  that  is  thus  used,  to 
establish  a  generating  plant  in  connection  with  the  central  office  for 
developing  the  necessary  electrical  energy.  In  very*  small  rural 


SIMPLE  COMMON-BATTERY  SWITCHBOARD       379 

places  there  are  frequently  no  available  sources  of  electrical  energy, 
and  the  expense  of  establishing  a  power  plant  for  the  purpose  can- 
not be  justified.  But  even  if  there  is  an  electric  light  or  railway 
system  in  the  small  town,  so  that  the  problem  of  available  current 
supply  does  not  exist,  the  establishment  of  a  common-battery  system 
with  its  storage  battery  and  the  necessary  charging  machinery  requires 
the  daily  attendance  at  the  central  office  of  some  one  to  watch  and 
care  for  this  battery,  and  this,  on  account  of  the  small  gross  revenue 
that  may  be  derived  from  a  small  telephone  system,  often  involves 
a  serious  financial  burden. 

There  is  no  royal  road  to  a  proper  decision  in  the  matter,  and  no 
sharp  line  of  demarcation  may  be  drawn  between  the  places  where 
common-battery  systems  are  superior  to  magneto  and  vice  versa.  It 
may  be  said,  however,  that  in  the  building  of  all  new  telephone 
plants  having  over  about  500  local  subscribers,  the  common-battery 
system  is  undoubtedly  superior  to  the  magneto.  If  the  plant  is  an 
old  one,  however,  and  is  to  be  re-equipped,  the  continuance  of  mag- 
neto apparatus  might  be  justified  for  considerably  larger  exchanges 
than  those  having  500  subscribers. 

Telephone  operating  companies  who  have  changed  over  the 
equipment  of  old  plants  from  magneto  to  common  battery  have 
sometimes  been  led  into  rather  serious  difficulty,  owing  to  the  fact 
that  their  lines,  while  serving  tolerably  well  for  magneto  work,  were 
found  inadequate  to  meet  the  more  exacting  demands  of  common- 
battery  work.  Again  in  an  old  plant  the  change  from  magneto  to 
common-battery  equipment  involves  not  only  the  change  of  switch- 
boards, but  also  the  change  of  subscribers'  instruments  that  are 
otherwise  good,  and  this  consideration  alone  often,  in  our  opinion, 
justifies  the  replacing  of  an  old  magneto  board  with  a  new  magneto 
board,  even  if  the  exchange  is  of  such  size  as  to  demand  a  small 
multiple  board. 

Where  the  plant  to  be  established  is  of  such  size  as  to  leave  doubt 
as  to  whether  a  magneto  or  a  common-battery  switchboard  should 
be  employed,  the  questions  of  availability  of  the  proper  kind  of  power 
for  charging  the  batteries,  the  proper  kind  of  help  for  maintaining 
the  batteries  and  the  more  elaborate  central-office  equipment,  the 
demands  and  previous  education  of  the  public  to  be  served,  all  are 
factors  which  must  be  considered  in  reaching  the  decision. 


380  TELEPHONY 

It  is  not  proper  to  say  that  anything  like  all  exchanges  having 
fewer  than  500  local  lines,  should  be  equipped  with  magneto  service. 
Where  all  the  lines  are  short,  where  suitable  power  is  available,  and 
where  a  good  grade  of  attendants  is  available — as,  for  instance,  in 
the  case  of  private  telephone  exchanges  that  serve  some  business 
establishment  or  other  institution  located  in  one  building  or  a  group 
of  buildings — the  common-battery  system  is  to  be  recommended  and 
is  largely  used,  even  though  it  may  have  but  a  dozen  or  so  subscrib- 
ers' lines.  It  is  for  such  uses,  and  for  use  in  those  regular  public- 
service  exchange  systems  where  the  conditions  are  such  as  to  war- 
rant the  common-battery  system,  and  yet  where  the  number  of  lines 
and  the  traffic  are  small  enough  to  be  handled  by  such  a  small  group 
of  operators  that  any  one  of  them  may  reach  over  the  entire  face  of 
the  board,  that  the  simple  non-multiple  common-battery  system 
finds  its  proper  field  of  usefulness. 

Line  Signals.  The  principles  and  means  by  which  the  sub- 
scriber is  enabled  to  call  the  central-office  operator  in  a  common- 
battery  system  have  been  referred  to  briefly  in  Chapter  III.  We  will 
review  these  at  this  point  and  also  consider  briefly  the  way  in  which 
the  line  signals  are  associated  with  the  connective  devices  in  the 
subscribers'  lines. 

Direct-Line  Lamp.  The  simplest  possible  way  is  to  put  the 
line  signal  directly  in  the  circuit  of  the  line  in  series  with  the  central- 
office  battery,  and  so  to  arrange  the  jack  of  the  corresponding  line 
that  the  circuit  through  the  line  signal  will  be  open  when  the  oper- 
ator inserts  a  plug  into  that  jack.  This  arrangement  is  shown  in 
Fig.  307  where  the  subscriber's  station  at  the  left  is  indicated  in  the 
simplest  of  its  forms.  It  is  well  to  repeat  here  that  in  all  common- 
battery  manual  systems,  the  subscriber's  station  equipment,  regard- 
less of  the  arrangement  or  type  of  its  talking  and  signaling  appara- 
tus, must  have  these  features:  First,  that  the  line  shall  be  nor- 
mally open  to  direct  currents  at  the  subscriber's  station;  second, 
that  the  line  shall  be  closed  to  direct  currents  when  the  subscriber 
removes  his  receiver  from  its  hook  in  making  or  in  answering  a  call ; 
third,  that  the  line  normally,  although  open  to  direct  currents,  shall 
afford  a  proper  path  for  alternating  or  varying  currents  through  the 
signal  receiving  device  at  the  sub-station.  The  subscriber's  station 
arrangement  shown  in  Fig.  307,  and  those  immediately  following, 


SIMPLE  COMMON-BATTERY  SWITCHBOARD        381 

is    the  simplest    arrangement  that  possesses  these    three    necessary 
features  for  common-battery  service. 

Considering  the  arrangement  at  the  central  office,  Fig.  307, 
the  two  limbs  of  the  line  are  permanently  connected  to  the  tip  and 
sleeve  contacts  of  the  jack.  These  two  main  contacts  of  the  jack 
normally  engage  two  anvils  so  connected  that  the  tip  of  the  jack  is 
ordinarily  connected  through  its  anvil  to  ground,  while  the  sleeve 
of  the  jack  is  normally  connected  through  its  anvil  to  a  circuit 
leading  through  the  line  signal — in  this  case  a  lamp — and  the 


Fig.  307.     Direct-Line  Lamp 

common  battery,  and  thence  to  ground.  The  operation  is  obvious. 
Normally  no  current  may  flow  from  the  common  battery  through 
the  signal  because  the  line  is  open  at  the  subscriber's  station.  The 
removal  of  the  subscriber's  receiver  from  its  hook  closes  the  circuit 
of  the  line  and  allows  the  current  to  flow  through  the  lamp,  causing 
it  to  glow.  When  the  operator  inserts  the  plug  into  the  jack,  in  re- 
sponse to  the  call,  the  circuit  through  the  lamp  is  cut  off  at  the  jack 
and  the  lamp  goes  out. 

This  arrangement,  termed  the  direct-line  lamp  arrangement, 
is  largely  used  in  small  common-battery  telephone  systems  where  the 
lines  are  very  short,  such  as  those  found  in  factories  or  other  places 
where  the  confines  of  the  exchange  are  those  of  a  building  or  a 
group  of  neighboring  buildings.  Many  of  the  so-called  private- 
branch  exchanges,  which  will  be  considered  more  in  detail  in  a  later 
chapter,  employ  this  direct-line  lamp  arrangement. 

Direct-Line  Lamp  with  Ballast.  Obviously,  however,  this 
direct-line  lamp  arrangement  is  not  a  good  one  where  the  lines  vary 
widely  in  length  and  resistance.  An  incandescent  lamp,  as  is  well 
known,  must  not  be  subjected  to  too  great  a  variation  in  current.  If 


382  TELEPHONY 

the  current  that  is  just  right  in  amount  to  bring  it  to  its  intended 
degree  of  illumination  is  increased  by  a  comparatively  small  amount, 
the  life  of  the  lamp  will  be  greatly  shortened,  and  too  great  an  in- 
crease will  result  in  the  lamp's  burning  out  immediately.  On  the 
other  hand,  a  current  that  is  too  small  will  not  result  in  the  proper 
illumination  of  the  lamp,  and  a  current  of  one-half  the  proper  nor- 
mal value  will  just  suffice  to  bring  the  lamp  to  a  dull  red  glow.  With 
lines  that  are  not  approximately  uniform  in  length  and  resistance  the 
shorter  lines  would  afford  too  great  a  flow  of  current  to  the  lamps  and 
the  longer  lines  too  little,  and  there  is  always  the  danger  present,  unless 
means  are  taken  to  prevent  it,  that  if  a  line  becomes  short-circuited 
or  grounded  near  the  central  office,  the  lamp  will  be  subjected  to 
practically  the  full  battery  potential  and,  therefore,  to  such  a  current 
as  will  burn  it  out.  One  of  the  very  ingenious  and,  we  believe,  prom- 
ising methods  that  has  been  proposed  to  overcome  this  difficulty  is 
that  of  the  iron-wire  ballast,  alluded  to  in  Chapter  III.  This,  it 
will  be  remembered,  consists  of  an  iron-wire  resistance  enclosed  in  a 
vacuum  chamber  and  so  proportioned  with  respect  to  the  flow  of 
current  that  it  will  be  subjected  to  a  considerable  heating  effect  by 
the  amount  of  current  that  is  proper  to  illuminate  the  lamp.  As  has 
already  been  pointed  out,  carbon  has  a  negative  temperature  coeffi- 
cient, that  is,  its  resistance  decreases  when  heated.  Iron,  on  the 
other  hand,  has  a  positive  temperature  coefficient,  its  resistance  in- 


Fig    308      Direct-Line  Lamp  with  Ballast 

creasing  when  heated.  When  such  an  iron-wire  ballast  is  put  in 
series  with  the  incandescent  lamp  forming  the  line  signal,  as  shown 
in  Fig.  308,  it  is  seen  that  the  resistance  of  the  carbon  in  the  lamp 
filament  and  of  the  iron  in  the  ballast  will  act  in  opposite  ways  when 
the  current  increases  or  decreases.  An  increase  of  current  will  tend 
to  heat  up  the  iron  wire  of  the  ballast  and,  therefore,  increase  its  re- 


SIMPLE  COMMON-BATTERY  SWITCHBOARD       383 


sistance,  and  the  ballast  is  so  proportioned  that  it  will  hold  the  cur- 
rent that  may  flow  through  the  lamp  within  the  proper  maximum  and 
minimum  limits,  regardless  of  the  resistance  of  the  line  in  which  the 
lamp  is  used.  This  arrangement  has  not  gone  into  wide  use  up  to 
the  present  time. 

Line  Lamp  with  Relay.     By  far  the  most  common  method  of 
associating  the  line  lamp  with   the  line   is  to  employ  a  relay,  of 


Fig.  309.     Line  Lamp  with  Relay 

which  the  actuating  coil  is  in  the  line  circuit,  this  relay  serving  to 
control  a  local  circuit  containing  the  battery  and  the  lamp.  This 
arrangement  and  the  way  in  which  these  parts  are  associated  with 
the  jack  are  clearly  indicated  in  Fig.  309.  Here  the  relay  may  re- 
ceive any  amount  of  current,  from  the  smallest  which  will  cause  it 
to  pull  up  its  armature,  to  the  largest  which  will  not  injure  its  wind- 
ing by  overheat.  Relays  may  be  made  which  will  attract  their  arm- 
atures at  a  certain  minimum  current  and  which  will  not  burn  out 
when  energized  by  currents  about  ten  times  as  large,  and  it  is  thus 
seen  that  a  very  large  range  of  current  through  the  relay  wind- 
ing is  permissible,  and  that,  therefore,  a  very  great  latitude  as  to  line 
resistance  is  secured.  On  the  other  hand,  it  is  obvious  that  the  lamp 
circuit,  being  entirely  local,  is  of  uniform  resistance,  the  lamp  always 
being  subjected,  in  the  arrangement  shown,  to  practically  the  full 
battery  potential,  the  lamp  being  selected  to  operate  on  that  poten- 
tial. 

Pilot  Signals.  In  the  circuits  of  Figs.  307,  308,  and  309,  but  a 
single  line  and  its  associated  apparatus  is  shown,  and  it  may  not  be 
altogether  clear  to  the  uninitiated  how  it  is  that  the  battery  shown  in 
those  figures  may  serve,  without  interference  of  any  function,  a  larger 
number  of  lines  than  one.  It  is  to  be  remembered  that  this  battery 


384  TELEPHONY 

is  the  one  which  serves  not  only  to  operate  the  line  signals,  but  also 
to  supply  talking  current  to  the  subscribers  and  to  supply  current 
for  the  operation  of  the  cord-circuit  signals  after  the  cord  circuits  are 
connected  with  the  lines. 

In  Fig.  310  this  matter  is  made  clear  with  respect  to  the  asso- 
ciation of  this  common  battery  with  the  lines  for  operating  the  line 
signals,  and  also  another  important  feature  of  common-battery  work  is 
brought  out,  viz,  the  pilot  lamp  and  its  association  with  a  group  of 
line  lamps.  Three  subscribers'  lines  only  are  shown,  but  this  serves 
clearly  to  illustrate  the  association  of  any  larger  number  of  lines 
with  the  common  battery.  Ignoring  at  first  the  pilot  relay  and  the 
pilot  lamp,  it  will  be  seen  that  each  of  the  tip-spring  anvils  of  the 
jacks  is  connected  to  a  common  wire  1  which  is  grounded.  Each 
of  the  sleeve-contact  anvils  is  connected  through  the  coil  of  the  line 
relay  to  another  common  wire  2,  which  connects  with  the  live  side 
of  the  common  battery.  Obviously,  therefore,  this  arrangement 
corresponds  with  that  of  Fig.  309,  since  the  battery  may  furnish  cur- 
rent to  energize  any  one  of  the  line  relays  upon  the  closure  of  the 
circuit  of  the  corresponding  line.  Each  of  the  relay  armatures  in 
Fig.  310  is  connected  to  ground. 

Here  we  wish  to  bring  out  an  important  thing  about  telephone 
circuit  diagrams  which  is  sometimes  confusing  to  the  beginner,  but 
which  really,  when  understood,  tends  to  prevent  confusion.  The 
showing  of  a  separate  ground  for  each  of  the  line-relay  armatures 
does  not  mean  that  literally  each  one  of  these  armatures  is  connected 
by  a  separate  wire  to  earth,  and  it  is  to  be  understood  that  the  three 
separate  grounds  shown  in  connection  with  these  relay  armatures  is 
meant  to  indicate  just  such  a  set  of  affairs  as  is  shown  in  connection 
with  the  tip-spring  anvils  of  the  jacks,  all  of  which  are  connected  to 
a  common  wire  which,  in  turn,  is  grounded.  Obviously,  the  result 
is  the  same,  but  in  the  case  of  this  particular  diagram  it  is  seen  that 
a  great  deal  of  crossing  of  lines  is  prevented  by  showing  a  separate 
ground  at  each  one  of  the  relay  armatures.  The  same  practice  is 
followed  in  connection  with  the  common  battery.  Sometimes  it  is 
very  inconvenient  in  a  complicated  diagram  to  run  all  of  the  wires 
that  are  supposed  to  connect  with  one  terminal  of  the  battery  across 
the  diagram  to  represent  this  connection.  It  is  permissible,  there- 
fore, and  in  fact  desirable,  that  separate  battery  symbols  be  shown 


SIMPLE  COMMON-BATTERY  SWITCHBOARD       385 


Z- 


L//YE 
RELAY 


L/fiE  LAMP 


z- 


—  L/ME  LAMP 


wherever  by  so  doing  the  diagram  will  be  simplified,  the  understand- 
ing being,  in  the  absence  of  other  information  or  of  other  indications, 
that  the  same  battery  is  referred  to,  just  as  the  same  ground  is  re- 
ferred to  in  connection  with  the  relay  armatures  in  the  figure  under 
discussion. 

Each  line  lamp  in  Fig.  310  is  shown  connected  on  one  hand  to 
its  corresponding  line  relay  contact  and  on  the  other  hand  to  a  com- 
mon wire  which  leads  through  the  winding  of  the  pilot  relay  to  the 

live  side   of   the   battery.     It   is     

obvious  here  that  whenever  any 
one  of  the  line  relays  attracts  its 
armature  the  local  circuit  con- 
taining the  corresponding  lamp 
and  the  common  battery  will  be 
closed  and  the  lamp  illuminated. 

Whenever  any  line  relay  op- 
erates, the  current,  which  is  sup- 
plied to  its  lamp,  must  come 
through  the  pilot-relay  winding, 
and  if  a  number  of  line  relays  are 
energized,  then  the  current  flow 
of  the  corresponding  lamps  must 
flow  through  this  relay  winding. 
Therefore,  this  relay  winding 
must  be  of  low  resistance,  so  that 
the  drop  through  its  winding  may 
not  be  sufficient  to  interfere  with 
the  proper  burning  of  the  lamps, 
even  though  a  large  number  of 
lamps  be  fed  simultaneously 
through  it.  The  pilot  relay  must 

be  so  sensitive  that  the  current,  even  through  one  lamp,  will  cause 
it  to  attract  its  armature.  When  it  does  attract  its  armature  it  causes 
illumination  of  the  pilot  lamp  in  the  same  way  that  the  line  relays 
cause  the  illumination  of  the  line  lamps. 

The  pilot  lamp,  which  is  commonly  associated  with  a  group  of  line 
lamps  that  are  placed  on  anyone  operator's  position  of  the  switchboard, 
is  located  in  a  conspicuous  place  in  the  switchboard  cabinet  and  is 


L//YE 
RELAY 


L//YE 
RELAY 


-  L/ME  LAMP 


)P/LOT 


P/LOT} 
RELAY 


L       LAMP 


Fig.  310.     Pilot-Lamp  Operation 


386 


TELEPHONY 


Fig.   311.     Battery  Supply  Through 
Impedance  Coils 


provided  with  a  larger  lens  so  as  to  make  a  more  striking  signal.     As 
a  result,  whenever  any  line  lamp  on  a  given  position  lights,  the  pilot 
lamp  does  also  and  serves  to  attract  the  attention,  even  of  those  lo- 
cated  in   distant  portions  of  the 
room,  to  the  fact  that  a  call  exists 
on  that  position  of  the  board,  the 
line  lamp  itself,  which  is  simul- 
taneously   lighted,    pointing    out 
the  particular  line  on  which  the 
call  exists. 

Pilot  lamps,  in  effect,  perform 
similar  service  to  the  night  alarm 
in  magneto  boards,  but,  of  course,  they  are  silent  and  do  not  attract 
attention  unless  within  the  range  of  vision  of  the  operator.     They 
are  used  not  only  in  connection  with  line  lamps,  but  also  in  con- 
nection with  the  cord-circuit  lamps 
or  signals,  as  will  be  pointed  out. 
Cord  Circuit.     Battery  Supply. 
Were  it  not  for  the  necessity  of 
providing  for  cord-circuit  signals 
in  common-battery  switchboards, 
the  common-battery  cord  circuit 

FiS'  312'  RepeSScoSy  thTOUeh         would  be  scarcely  more  complex 

than  that  for  magneto  working. 

Stripped  of  all  details,  such  as  signals,  ringing  and  listening  keys, 
and  operator's  equipment,  cord  circuits  of  three  different  types  are 
shown  in  Figs.  311,  312,  and  313.  These  merely  illustrate  the  way 

in  which  the  battery  is  associated 
with  the  cord  circuits  and  through 
them  with  the  line  circuits  for  sup- 
plying current  for  talking  purposes 
to  the  subscribers.  It  is  thought 
that  .this  matter  will  be  clear  in 
view  of  the  discussion  of  the  meth- 
ods by  which  current  is  supplied 
to  the  subscribers'  transmitters  in 
common-battery  systems  as  discussed  in  Chapter  XIII.  While 
the  arrangements  in  this  respect  of  Figs.  311,  312,  and  313  illustrate 


Fig.    313.      Battery  Supply  with 
Impedance  Coils  and  Condensers 


SIMPLE  COMMON-BATTERY  SWITCHBOARD        387 

only  three  of  the  methods,  these  three  are  the  ones  that  have  been 
most  widely  and  successfully  used. 

Supervisory  Signals.  The  signals  that  are  associated  with  the 
cord  circuits  are  termed  supervisory  signals  because  of  the  fact  that 
by  their  means  the  operator  is  enabled  to  supervise  the  condition  of 
the  lines  during  times  when  they  are  connected  for  conversation. 
The  operation  of  these  supervisory  signals  may  be  best  understood 
by  considering  the  complete  circuits  of  a  simple  switchboard  and 
must  be  studied  in  conjunction  with  the  circuits  of  the  lines  as  well  as 
those  of  the  cords. 

Complete  Circuit.  Such  complete  circuits  are  shown  in  Fig.  314. 
The  particular  arrangement  indicated  is  that  employed  by  the  Kel- 


STAT/ON  -XI- 


STAT/OM  -B- 


Fig.  314.     Simple  Common-Battery  Switchboard 

logg  Company,  and  except  for  minor  details  may  be  considered  as 
typical  of  other  makes  also.  Two  subscribers'  lines  are  shown  ex- 
tending from  Station  A  and  Station  B,  respectively,  to  the  central 
office.  The  line  wires  are  shown  terminating  in  jacks  in  the  same 
manner  as  indicated  in  Figs.  307,  308,  and  309,  and  their  circuits 
are  normally  continued  from  these  jacks  to  the  ground  on  one  side 
and  to  the  line  relay  and  battery  on  the  other.  The  jack  in  this 
case  has  three  contacts  adapted  to  register  with  three  corresponding 


388  TELEPHONY 

contacts  in  each  of  the  plugs.  The  thimble  of  the  jack  in  this  case 
forms  no  part  of  the  talking  circuit  and  is  distinct  from  the  two  jack 
springs  which  form  the  line  terminals.  It  and  the  auxiliary  contact 
1  in  each  of  the  plugs  with  which  it  registers,  are  solely  for  the  purpose 
of  co-operating  in  the  control  of  the  supervisory  signals. 

The  tip  and  sleeve  strands  of  the  cord  are  continuous  from  one 
plug  to  the  other  except  for  the  condensers.  The  two  batteries  in- 
dicated in  connection  with  the  cord  circuit  are  separate  batteries, 
a  characteristic  of  the  Kellogg  system.  One  of  these  batteries  serves 
to  supply  current  to  the  tip  and  sleeve  strand  of  the  cord  circuit 
through  the  two  windings  3  and  4>  respectively,  of  the  supervisory 
relay  connected  with  the  answering  side  of  the  cord  circuit,  while  the 
other  battery  similarly  supplies  current  through  the  windings  5  and  6 
of  the  supervisory  relay  associated  with  the  calling  side  of  the  cord 
circuit.  The  windings  of  these  relays,  therefore,  act  as  impedance 
coils  and  the  arrangement  by  which  battery  current  is  supplied  to 
the  cord  circuits  and,  therefore,  to  the  lines  of  the  connected  subscrib- 
ers, is  seen  to  be  the  combined  impedance  coil  and  condenser  ar- 
rangement discussed  in  Chapter  XIII. 

As  soon  as  a  plug  is  inserted  into  the  jack  of  a  line,  the  line 
relay  will  be  removed  from  the  control  of  the  line,  and  since  the 
two  strands  of  the  cord  circuit  now  form  continuations  of  the  two 
line  conductors,  the  supervisory  relay  will  be  substituted  for  the  line 
relay  and  will  be  under  control  of  the  line.  Since  all  of  the  current 
which  passes  to  the  line  after  a  plug  is  inserted  must  pass  through 
the  cord-circuit  connection  and  through  the  relay  windings,  and  since 
current  can  only  flow  through  the  line  when  the  subscriber's  re- 
ceiver is  off  its  hook,  it  follows  that  the  supervisory  relays  will  only 
be  energized  after  the  corresponding  plug  has  been  inserted  into  a 
jack  of  the  line  and  after  the  subscriber  has  removed  his  receiver. 
Unlike  the  line  relays,  the  supervisory  relays  open  their  contacts 
to  break  the  local  circuits  of  the  supervisory  lamps  7  and  8  when 
the  relay  coils  are  energized,  and  to  close  them  when  de-energized ;  but 
the  armatures  of  the  supervisory  relays  do  alone  control  the  circuits 
of  the  supervisory  lamps.  These  circuits  are  normally  held  open  in 
another  place,  that  is,  between  the  plug  contacts  1  and  the  jack 
thimbles.  It  is  only,  therefore,  when  a  plug  is  inserted  into  a  jack 
and  when  the  supervisory  relay  is  de-energized,  that  the  supervisory 


SIMPLE  COMMON-BATTERY  SWITCHBOARD        389 

lamp  may  be  lighted.  When  a  plug  is  inserted  into  a  jack  and 
when  the  corresponding  supervisory  relay  is  de-energized,  the  circuit 
may  be  traced  from  ground  at  the  cord-circuit  batteries  through  the 
left-hand  battery,  for  instance,  through  lamp  7,  thence  through  the 
contacts  of  the  supervisory  relay  to  the  contact  1  of  the  plug,  thence 
through  the  thimble  of  the  jack  to  ground.  When  a  plug  is  inserted 
into  the  jack,  therefore,  the  necessary  arrangements  are  completed  for 
the  supervisory  lamp  to  be  under  the  control  of  the  subscriber.  Under 
this  condition,  whenever  the  subscriber's  receiver  is  on  its  hook,  the 
circuit  of  the  line  will  be  broken,  the  supervisory  relay  will  be  de- 
energized,  and  the  supervisory  lamp  will  be  lighted.  When,  on  the 
other  hand,  the  subscriber's  receiver  is  off  its  hook,  the  circuit  of 
the  line  will  be  complete,  the  supervisory  relay  will  be  energized, 
and  the  supervisory  lamp  will  be  extinguished. 

Salient  Features  of  Supervisory  Operation.  It  will  facilitate 
the  student's  understanding  of  the  requirements  and  mode  of  oper- 
ation of  common -battery  supervisory  signals  in  manual  systems, 
whether  simple  or  multiple,  if  he  will  firmly  fix  the  following  facts 
in  his  mind.  In  order  that  the  supervisory  signal  may  become  opera- 
tive at  all,  some  act  must  be  performed  by  the  operator — this  being 
usually  the  act  of  plugging  into  a  jack — and  then,  until  the  connection 
is  taken  down,  the  supervisory  signal  is  under  the  control  of  the 
subscriber,  and  it  is  displayed  only  when  the  subscriber's  receiver 
is  placed  on  its  hook. 

Cycle  of  Operations.  We  may  now  trace  through  the  complete 
cycle  of  operations  of  the  simple  common-battery  switchboard,  the 
circuits  of  which  are  shown  in  Fig.  314.  Assume  all  apparatus  in 
its  normal  condition,  and  then  assume  that  the  subscriber  at  Station 
A  removes  his  receiver  from  its  hook.  This  pulls  up  the  line  relay 
and  lights  the  line  lamp,  the  pilot  relay  also  pulling  up  and  lighting 
the  common  pilot  lamp  which  is  not  shown.  In  response  to  this 
call,  the  operator  inserts  the  answering  plug  and  throws  her  listening 
key  L.K.  The  operator's  talking  set  is  thus  bridged  across  the  cord 
circuit  and  she  is  enabled  to  converse  with  the  calling  subscriber. 
The  answering  supervisory  lamp  7  did  not  light  when  the  operator 
inserted  the  answering  plug  into  the  jack,  because,  although  the 
contacts  in  the  lamp  circuit  were  closed  by  the  plug  contact  1  engag- 
ing the  thimble  of  the  jack,  the  lamp  circuit  was  held  open  by  the 


390  TELEPHONY 

attraction  of  the  supervisory  relay  armature,  the  subscriber's  receiver 
being  off  its  hook.  Learning  that  the  called-for  subscriber  is  the  one 
at  Station  B,  the  operator  inserts  the  calling  plug  into  the  jack  at 
that  station  and  presses  the  ringing  key  R.K.,  in  order  to  ring  the 
bell.  The  act  of  plugging  in,  it  will  be  remembered,  cuts  off  the 
line-signaling  apparatus  from  connection  with  that  line.  As  the 
subscriber  at  Station  B  was  not  at  his  telephone  when  called  and  his 
receiver  was,  therefore,  on  its  hook,  the  insertion  of  the  calling  plug 
did  not  energize  the  supervisory  relay  coils  5  and  6,  and,  therefore, 
that  relay  did  not  attract  its  armature.  The  supervisory  lamp  8 
was  thus  lighted,  the  circuit  being  from  ground  through  the  right- 
hand  cord-circuit  battery,  lamp  8,  back  contacts  of  the  supervisory 
relay,  third  strand  of  the  cord  to  contact  1  of  the  calling  plug,  and 
thence  to  ground  through  the  thimble  of  the  jack.  The  lighting  of 
this  lamp  is  continued  until  the  party  at  Station  B  responds  by  re- 
moving his  receiver  from  its  hook,  which  completes  the  line  circuit, 
energizes  relay  windings  5  and  6,  causes  that  relay  to  attract  its  arm- 
ature, and  thus  break  the  circuit  of  the  lamp  8.  Both  supervisory 
lamps  remain  out  as  long  as  the  two  subscribers  are  conversing, 
but  when  either  one  of  them  hangs  up  his  receiver  the  corresponding 
supervisory  relay  becomes  de-energized  and  the  corresponding  lamp 
lights.  When  both  of  the  lamps  become  illuminated,  the  operator 
knows  that  both  subscribers  are  through  talking  and  she  takes  down 
the  connection. 

Countless  variations  have  been  worked  in  the  arrangement 
of  the  line  and  cord  circuits,  but  the  general  mode  of  operation  of 
this  particular  circuit  chosen  for  illustration  is  standard  and  should 
be  thoroughly  mastered.  The  operation  of  other  arrangements 
will  be  readily  understood  from  an  inspection  of  the  circuits,  once 
the  fundamental  mode  of  operation  that  is  common  to  all  of  them 
is  well  in  mind. 

Lamps.  The  incandescent  lamps  used  in  connection  with  line 
and  supervisory  signals  are  specially  manufactured,  but  differ  in  no 
sense  from  the  larger  lamps  employed  for  general  lighting  purposes, 
save  in  the  details  of  size,  form,  and  method  of  mounting.  Usually 
these  lamps  are  rated  at  about  one-third  candle-power,  although 
they  have  a  somewhat  larger  candle-power  as  a  rule.  They  are 
manufactured  to  operate  on  various  voltages,  the  most  usual  operating 


SIMPLE  COMMON-BATTERY  SWITCHBOARD 


391 


Fig.  315.     Switchboard  Lamp 


pressures  being  12,  24,  and  48  volts.  The  24- volt  lamp  consumes 
about  one-tenth  of  an  ampere  when  fully  illuminated,  the  lamp  thus 
consuming  about  2.4  watts.  The  12-  and  48-volt  lamps  consume 
about  the  same  amount  of  energy  and  corresponding  amounts  of 
current. 

Lamp  Mounting.  The  usual 
form  of  screw-threaded  mounting 
employed  in  lamps  for  commercial 
lighting  was  at  first  applied  to  the 
miniature  lamps  used  for  switch- 
board work,  but  this  was  found  unsatisfactory  and  these  lamps  are 
now  practically  always  provided  with  two  contact  strips,  one  on 
each  side  of  the  glass  bulb,  these  strips  forming  respectively  the 
terminals  for  the  two  ends  of  the  filament  within.  Such  a  construc- 
tion of  a  common  form  of  lamp  is  shown  in  Fig.  315,  where  these 
terminals  are  indicated  by  the  numerals  1  and  2,  3  being  a  dry 
wooden  block  arranged  between  the  terminals  at  one  end  for  secur- 
ing greater  rigidity  between  them. 

The  method  of  mounting  these  lamps  is  subject  to  a  good  deal 
of  variation  in  detail,  but  the  arrangement  is  always  such  that  the 
lamp  is  slid  in  between  two  metallic  contacts  forming  terminals  of 
the  circuit  in  which  the  lamp  is  to  operate.  Such  an  arrangement 


11 1 1 1 1 1 ULU 


Pig.  316.     Line  Lamp  Mounting 


of  springs  and  the  co-operating  mounting  forming  a  sort  of  socket 
for  the  reception  of  switchboard  lamps  is  referred  to  as  a  lamp  jack. 
These  are  sometimes  individually  mounted  and  sometimes  mounted 
in  strips  in  much  the  same  way  that  jacks  are  mounted  in  strips. 
A  strip  of  lamp  jacks  as  manufactured  by  the  Kellogg  Company  is 


392 


TELEPHONY 


shown  in  Fig.  316.  The  opalescent  lens  is  adapted  to  be  fitted  in 
front  of  the  lamp  after  it  has  been  inserted  into  the  jack.  Fig.  317 
gives  an  excellent  view  of  an  individually-mounted  lamp  jack  with 
its  lamp  and  lens,  this  also  being  of  Kellogg 
manufacture.  This  figure  shows  a  section  of 
the  plug  shelf  which  is  bored  to  receive  a 
lamp.  In  order  to  protect  the  lamps  and 
lenses  from  breakage,  due  to  the  striking  of 
the  plugs  against  them,  a  metal  shield  is 
placed  over  the  lens,  as  shown  in  this  figure, 
this  being  so  cut  away  as  to  allow  sufficient 
openings  for  the  light  to  shine  through.  Some- 
times instead  of  employing  lenses  in  front  of 
the  lamps,  a  flat  piece  of  translucent  material  is 
used  to  cover  the  openings  of  the  lamp,  this 
being  protected  by  suitable  perforated  strips 
of  metal.  A  strip  of  lamp  jacks  employing 
this  feature  is  shown  in  Fig.  318,  this  being  of 
Dean  manufacture.  An  advantage  of  this  for 
certain  types  of  work  is  that  the  flat  translucent 
plate  in  front  of  the  lamp  may  readily  carry  designating  marks, 
such  as  the  number  of  the  line  or  something  to  indicate  the  char- 
acter of  the  line,  which  marks  may  be  readily  changed  as  required. 


Fig.  317.     Supervisory 
Lamp  Mounting 


Fig.  318.     Line  Lamp  Mounting 

In  the  types  made  by  some  manufacturers  the  only  difference 
between  the  pilot' lamp  and  the  line  lamp  is  in  the  size  of  the  lens  in 
front  of  it,  the  jack  and  the  lamp  itself  being  the  same  for  each,  while 
others  use  a  larger  lamp  for  the  pilot.  In  Fig.  319  are  shown  two  in- 
dividual lamp  jacks,  the  one  at  the  top  being  for  supervisory  lamps 
and  the  one  at  the  bottom  being  provided  with  a  large  lens  for  serving 
as  a  pilot  lamp. 


SIMPLE  COMMON-BATTERY  SWITCHBOARD        393 

Mechanical  Signals.  As  has  been  stated  the  so-called  mechan- 
ical signals  are  sometimes  used  in  small  common-battery  switch- 
boards instead  of  lamps.  Where  this  is  done  the  coil  of  the  signal, 


Fig.  319.     Individual  Lamp  Jacks 

if  it  is  a  line  signal,  is  substituted  in  the  line  circuit  in  place  of  the  re- 
lay coil.  If  the  signals  are  used  in  connection  with  cord  circuits 
for  supervisory  signals,  their  coils  are  put  in  the  circuit  in  place  of  the 
supervisory  relay  coils.  (These  signals  are  referred  to  in  Chapter 
III  in  connection  with  Fig.  23.)  They  are  so  arranged  that  the 
attraction  of  the  armature  lifts  a  target  on  the  end  of  a  lever,  and  this 
causes  a  display  of  color  or  form.  The  release  of  the  armature  allows 
this  target  to  drop  back,  thus  obliterating  the  display.  Such  signals, 
often  called  visual  signals  and  electromagnet  signals,  should  be  dis- 
tinguished from  the  drops  considered  in  connection  with  magneto 
switchboards  in  which  the  attraction  of  the  armature  causes  the 
display  of  the  signal  by  the  falling  of  a  drop,  the  signal  remaining 
displayed  until  restored  by  some  other  means,  the  restoration  de- 
pending in  no  wise  on  when  the  armature  is  released. 

Western  Electric.     The  mechanical  signal  of  the  Western  Elec- 
tric Company,  shown  in  Fig.   320,  has    a  target    similar  to    that 


394  TELEPHONY 

shown  in  Fig.  254  but  without  a  latch.  It  is  turned  to  show  a  dif- 
ferent color  by  the  attraction  of  the  armature  and  allowed  to  resume 
its  normal  position  when  the  armature  is  released. 


Fig.    320. 


Kellogg.  Fig.  321  gives  a  good  idea  of  a  strip  of  mechanical 
signals  as  manufactured  by  the  Kellogg  Company.  This  is  known 
as  the  gridiron  signal  on  account  of  the  cross-bar  striping  of  its 
target.  The  white  bars  on  the  target  normally  lie  just  behind  the 
cross-bars  on  the  shield  in  front,  but  a  slight  raising  of  the  target — 


Pig.  321.     Strip  of  Gridiron  Signals 

about  one-eighth  of  an  inch — exposes  these  white  bars  to  view,  op- 
posite the  rectangular  openings  in  the  front  shield. 

Monarch.  In  Fig  322  is  shown  the  visual  signal  manufac- 
tured by  the  Monarch  Telephone  Company. 

Relays.  The  line  relays  for  common-battery  switchboards 
likewise  assume  a  great  variety  of  forms.  The  well-known  type  of 
relay  employed  in  telegraphy  would  answer  the  purpose  well  but 


SIMPLE  COMMON-BATTERY  SWITCHBOARD         395 

for  the  amount  of  room  that  it  occupies,  as  it  is  sometimes  necessary 
to  group  a  large  number  of  relays  in  a  very  small  space.  Nearly  all 
present-day  relays  are  of  the  single-coil  type,  and  in  nearly  all  cases 
the  movement  of  the  armature  causes  the  movement  of  one  or  more 
switching  springs,  which  are  thus  made  to  engage  or  disengage  their 


Fig.  322.     Mechanical  Signal 


associated  spring  or  springs.  One 'of  the  most  widely  used  forms 
of  relays  has  an  L-shaped  armature  hung  across  the  front  of  a  for- 
wardly  projecting  arm  of  iron,  on  the  knife-edge  corner  of  which  it 
rocks  as  moved  by  the  attraction  of  the  magnet.  The  general  form 
of  this  relay  was  illustrated  in  Fig.  95.  Sometimes  this  relay  is 
made  up  in  single  units  and  frequently  a  large  number  of  such  single 
units  are  mounted  on  a  single  mounting  plate.  This  matter  will 
be  dealt  with  more  in  detail  in  the  discussion  of  common-battery 
multiple  switchboards.  In  other  cases  these  relays  are  built  en 
bloc,  a  rectangular  strip  of  soft  iron  long  enough  to  afford  space  for 
ten  relays  side  by  side  being  bored  out  with  ten  cylindrical  holes  to 


Fig.   323.     Strip  of  Relays 


receive  the  electromagnets.  The  iron  of  the  block  affords  a  return 
path  for  the  lines  of  force.  The  L-shaped  armatures  are  hung  over 
the  front  edge  of  this  block,  so  that  their  free  ends  lie  opposite  the 
magnet  cores  within  the  block.  This  arrangement  as  employed  by 
the  Kellogg  Company  is  shown  in  two  views  in  Figs.  323  and  324. 


396 


TELEPHONY 


A  bank  of  line  relays  especially  adapted  for  small  common- 
battery  switchboards  as  made  by  the  Dean  Company,  is  shown  in 
Fig.  325. 


Fig.  324.     Strip  of  Relays 


Jacks.  The  jacks  in  common-battery  switchboards  are  almost 
always  mounted  in  groups  of  ten  or  twenty,  the  arrangement  being 
similar  to  that  discussed  in  connection  with  lamp  strips.  Ordinarily 


.-  „  -  (. '.  ^  --V'-V  •->'•%''>' ''V'''* 

"""|,-|.  '-  ,,,-  t.  ••V'-V-V-V'-'1 

'-'''•'''-''•-''•->''-^-V-V<-V>v. 


Fig.   325.     Bank  of  Relays 


in  common-battery  work  the  jack  is  provided  with  two  inner  contacts 
so  as  to  cut  off  both  sides  of  the  signaling  circuit  when  the  operator 
plugs  in.  A  strip  of  such  jacks  is  shown  in  Fig.  326. 


Fig.  326.     Strip  of  Cut-Off  Jacks 

Ringing  and  listening  keys  for  simple  common-battery  switch- 
boards differ  in  no  essential  respect  from  those  employed  in  magneto 
boards. 


SIMPLE  COMMON-BATTERY  SWITCHBOARD         397 

Switchboard  Assembly.  The  general  assembly  of  the  parts  of 
a  simple  common-battery  switchboard  deserves  some  attention.  The 
form  of  the  switchboard  need  not  differ  essentially  from  that  em- 
ployed in  magneto  work,  but  ordinarily  the  cabinet  is  somewhat 
smaller  on  account  of  the  smaller  amount  of  room  required  by  its 
lamps  and  jacks.  An  excellent  idea  of  the  line  jacks  and  lamps, 
plugs,  keys,  and  supervisory  signals  may  be  obtained  from  Fig.  327, 


Fig.  327.     Details  of  Lamp,  Plug,  and  Key  Mounting 

which  is  a  detail  view  taken  from  a  Kellogg  board.  In  the  vertical 
panel  of  the  board  above  the  plug  shelf  are  arranged  the  line  jacks 
and  the  lamps  in  rows  of  twenty  each,  each  lamp  being  immediately 
beneath  its  corresponding  jack.  Such  jacks  are  ordinarily  mounted 
on  |-inch  centers  both  vertically  and  horizontally,  so  that  a  group 
of  one  hundred  lamps  and  line  jacks  will  occupy  a  space  only  slightly 
over  10  by  5  inches.  Such  economy  of  space  is  not  required  in  the 
simple  magneto  board,  because  the  space  might  easily  be  made  larger 
without  in  any  way  taxing  the  reach  of  the  operator.  The  reason  for 


398 


TELEPHONY 


this  comparatively  close  mounting  is  a  result,  not  of  the  require- 
ments of  the  simple  non-multiple  common-battery  board  itself,  but 
of  the  fact  that  the  jack  strips  and  lamp  strips,  which  are  required 
in  very  large  numbers  in  multiple  boards,  have  to  be  mounted  ex- 
tremely close  together,  and  as  the  same  lamp  strips  and  jack  strips 
are  often  available  for  simple  switchboards,  an  economy  in  manu- 
facture is  effected  by  adherence  to  the  same  general  dimensions. 


Fig.  328.     Simple  Common-Battery  Switchboard 
with  Removable  Relay  Panel 

A  rear  view  of  a  common  form  of  switchboard  cabinet,  known 
as  the  upright  type  and  manufactured  by  the  Dean  Company,  is  shown 
in  Fig.  328.  In  this  all  the  relays  are  mounted  on  a  hinged  rack,  which, 
when  opened  out  as  indicated,  exposes  the  wiring  to  view  for  in- 
spection or  repairs.  Access  to  both  sides  of  the  relays  is  thus  given 
to  the  repairman  who  may  do  all  his  work  from  the  rear  of  the  board 
without  disturbing  the  operator. 


SIMPLE  COMMON-BATTERY  SWITCHBOARD 


399 


Fig.  329  shows  a  three-position  cabinet  of  Kellogg  manufacture, 
this  being  about  the  limit  in  size  of  boards  that  could  properly  be 
called  simple.  Obviously,  where  a  switchboard  cabinet  must  be 
made  of  greater  length  than  this,  i.e.,  than  is  required  to  accom- 
modate three  operators,  it  becomes  too  long  for  the  operators  to 
reach  all  over  it  without  undue  effort  or  without  moving  from  their 


Pig.  329      Three-Position  Lamp  Board 


seats.  The  so-called  transfer  board  and  the  multiple  board  (to  be 
considered  in  subsequent  chapters),  constitute  methods  of  relief 
from  such  a  condition  in  larger  exchanges. 


CHAPTER   XXIII 
TRANSFER  SWITCHBOARD 

When  the  traffic  originating  in  a  switchboard  becomes  so  great 
as  to  require  so  many  operators  that  the  board  must  be  made  so  long 
that  any  one  of  the  operators  cannot  reach  over  its  entire  face,  the 
simple  switchboard  does  not  suffice.  Either  some  form  of  trans- 
fer switchboard  or  of  multiple  switchboard  must  be  used.  In  this 
chapter  the  transfer  switchboard  will  be  briefly  discussed. 

The  transfer  switchboard  is  so  named  because  its  arrangement 
is  such  that  some  of  the  connections  through  ii  are  handled  by 
means  of  two  operators,  the  operator  who  answers  the  call  trans- 
ferring it  to  another  operator  who  completes  the  connection  desired. 

Limitations  of  Simple  Switchboard.  Conceive  a  number  of 
simple  magneto  Switchboards,  or  a  number  of  common-battery 
switchboards,  arranged  side  by  side,  their  number  being  so  great  as 
to  form,  by  their  combination,  a  board  too  long  for  the  ordinary  cords 
and  plugs  to  reach  between  its  extremities.  On  each  of  these  simple 
switchboards,  which  we  will  say  are  each  of  the  one-position  type, 
there  terminates  a  group  of  subscribers'  lines  so  great  in  number, 
considering  the  traffic  on  them,  that  the  efforts  of  one  operator  will 
just  about  be  taxed  to  properly  attend  to  their  calls  during  the  busiest 
hours  of  the  day.  If,  now,  these  subscribers  would  be  sufficiently 
accommodating  to  call  for  no  other  subscribers  than  those  whose 
lines  terminate  on  the  same  switchboard  section  or  on  one  of  the 
immediately  adjacent  switchboard  sections,  all  would  be  well,  but 
subscribers  will  not  be  so  restricted.  They  demand  universal  serv- 
ice; that  is,  they  demand  the  privilege  of  having  their  own  lines  con- 
nected with  the  line  of  any  other  person  in  the  exchange.  Obviously, 
in  the  arrangement  just  conceived,  any  operator  may  answer  any 
call  originating  at  her  own  board  and  complete  the  connection  with 
the  desired  subscriber  if  that  subscriber's  jack  terminates  on  her  own 
section  or  on  one  of  the  adjacent  ones.  Beyond  that  she  is  power- 
less unless  other  means  are  provided. 


TRANSFER  SWITCHBOARD  401 

Transfer  Lines.  In  the  transfer  board  these  other  means  con- 
sist in  the  provision  of  groups  of  local  trunk  lines  or  transfer  lines 
extending  from  each  switchboard  position  to  each  other  non-adjacent 
switchboard  position.  When  an  operator  receives  a  call  for  some  line 
on  a  non-adjacent  position,  having  answered  this  call  with  her  an- 
swering plug,  she  inserts  the  calling  plug  into  the  jack  of  one  of  these 
transfer  lines  that  leads  to  the  proper  other  section.  The  operator 
at  that  section  is  notified  either  verbally  or  by  signal,  and  she  com- 
pletes the  connection  between  the  other  end  of  the  transfer  line  and 
the  line  of  the  called  subscriber;  the  connection  between  the  two  sub- 
scribers thus  being  effected  through  the  cords  of  the  two  operators 
in  question  linked  together  by  the  transfer  line.  Such  a  transfer 
line  as  just  described,  requiring  the  connection  at  each  of  its  ends 
by  one  of  the  plugs  of  the  operator's  cord  pair,  is  termed  a  jack- 
ended  trunk  or  a  jack-ended  transfer  line  because  each  of  its  ends 
terminates  in  a  jack. 

There  is  another  method  of  accomplishing  the  same  general 
result  by  the  employment  of  the  so-called  plug-ended  trunk  or  plug- 
ended  transfer  line.  In  this  the  trunk  or  transfer  line  terminates  at 
one  end,  the  answering  end,  in  a  jack  as  before,  and  the  connection 


3 
'/ 


Fig.  330.     Jack-Ended  Transfer  Circuit 

is  made  with  it  by  the  answering  operator  by  means  of  the  calling 
plug  of  the  pair  with  which  she  answered  the  originating  call.  The 
other  end  of  this  trunk,  instead  of  terminating  in  a  jack,  ends  in  a  plug 
and  the  second  operator  involved  in  the  connection,  after  being  no- 
tified, picks  up  this  plug  and  inserts  it  in  the  jack  of  the  called  sub- 
scriber, thus  completing  the  connection  without  employing  one  of 
her  regular  cord  pairs. 

Jack-Ended  Trunk.  In  Fig.  330  are  shown  the  circuits  of  a 
commonly  employed  jack-ended  trunk  for  transfer  boards.  The 
talking  circuit,  as  usual,  is  shown  in  heavy  lines  and  terminates  in 
the  tip  and  sleeve  of  the  transfer  jacks  at  each  end.  The  auxiliary 


402  TELEPHONY 

contacts  in  these  jacks  and  the  circuits  connecting  them  are  absolutely 
independent  of  the  talking  circuit  and  are  for  the  purpose  of  signaling 
only,  the  arrangement  of  the  jacks  being  such  that  when  a  plug  is 
inserted,  the  spring  1  will  break  from  spring  2  and  make  with  spring 
3.  Obviously,  the  insertion  of  a  plug  in  either  of  the  jacks  will 
establish  such  connections  as  to  light  both  lamps,  since  the  engage- 
ment of  spring  1  with  spring  3  in  either  of  the  jacks  will  connect  both 
of  the  lamps  in  multiple  across  the  battery,  this  connection  includ- 
ing always  the  contacts  1  and  2  of  the  other  jack.  From  this  it  fol- 
lows that  the  insertion  of  a  plug  in  the  other  end  of  the  trunk  will, 
by  breaking  contact  between  springs  1  and  2,  put  out  both  the 
lamps.  One  plug  inserted  will,  therefore,  light  both  lamps;  two 
plugs  inserted  or  two  plugs  withdrawn  will  extinguish  both  lamps. 
If  an  operator  located  at  one  end  of  this  trunk  answers  a  call 
and  finds  that  the  called-for  subscriber's  line  terminates  within 
reach  of  the  operator  near  the  other  end  of  this  trunk,  she  will 
insert  a  calling  plug,  corresponding  to  the  answering  plug  used  in 
answering  a  call,  into  the  jack  of  this  trunk  and  thus  light  the  lamp 
at  both  its  ends.  The  operator  at  the  other  end  upon  seeing  this 
transfer  lamp  illuminated  inserts  one  of  her  answering  plugs  into 
the  jack,  and  by  means  of  her  listening  key  ascertains  the  number 


^^m 


Fig.  331.     Jack-Ended  Transfer  Circuit 

of  the  subscriber  desired,  and  immediately  inserts  her  calling  plug 
into  the  jack  of  the  subscriber  wanted  and  rings  him  in  the  usual 
manner.  The  act  of  this  second  operator  in  inserting  her  answer- 
ing plug  into  the  jack  extinguishes  the  lamp  at  her  own  end  and 
also  at  the  end  where  the  call  originated,  thus  notifying  the  answer- 
ing operator  that  the  call  has  been  attended  to.  As  long  as  the 
lamps  remain  lighted,  the  operators  know  that  there  is  an  unattended 
connection  on  that  transfer  line.  Such  a  transfer  line  is  called  a 
two-way  line  or  a  single-track  line,  because  traffic  over  it  may  be  in 
either  direction.  In  Fig.  331  is  shown  a  trunk  that  operates  in  a 


TRANSFER  SWITCHBOARD  403 

similar  way  except  that  the  two  lamps,  instead  of  being  arranged 
in  multiple,  are  arranged  in  series. 

Plug-Ended  Trunk.  In  Fig.  332  is  shown  a  plug-ended  trunk, 
this  particular  arrangement  of  circuits  being  employed  by  the  Mon- 
arch Company  in  its  transfer  boards.  This  is  essentially  a  one- 
way trunk,  and  traffic  over  it  can  pass  only  in  the  direction  of  the 
arrow.  Traffic  in  the  opposite  direction  between  any  two  opera- 


T 


Fig.  332.     Jack-  and  Plug-Ended  Transfer  Circuit 

tors  is  handled  by  another  trunk  or  group  of  trunks  similar  to  this 
but  "pointed"  in  the  other  direction.  For  this  reason  such  a  system 
is  referred  to  as  a  double-track  system.  The  operation  of  signals  is 
the  same  in  this  case  as  in  Fig.  330,  except  that  the  switching  device 
at  the  left-hand  end  of  the  trunk  instead  of  being  associated  with 
the  jack  is  associated  with  the  plug  seat,  which  is  a  switch  closely 
associated  with  the  seat  of  a  plug  so  as  to  be  operated  whenever 
the  plug  is  withdrawn  from  or  replaced  in  its  seat.  The  operation 
of  this  arrangement  is  as  follows :  Whenever  an  operator  at  the  right- 
hand  end  of  this  trunk  receives  a  call  for  a  subscriber  whose  line 
terminates  within  the  reach  of  the  operator  at  the  left-hand  end  of 
the  trunk,  she  inserts  the  calling  plug  of  the  pair  used  in  answering 
the  calling  subscriber  into  the  jack  of  the  trunk,  and  thus  lights  both 
of  the  trunk  lamps.  The  operator  at  the  other  end  of  the  trunk, 
seeing  the  trunk  lamp  lighted,  raises  the  plug  from  its  seat  and,  having 
learned  the  wishes  of  the  calling  subscriber,  inserts  this  plug  into 
the  jack  of  the  called  subscriber  without  using  one  of  her  regular 
pairs.  When  she  raised  the  trunk  plug  from  its  seat,  she  permitted 
the  long  spring  1  of  the  plug  seat  switch  to  rise,  thus  extinguishing 
both  lamps  and  giving  the  signal  to  the  originating  operator  that 
the  trunk  connection  has  received  attention.  On  taking  down  the 
connection,  the  withdrawal  of  the  plug  from  the  right  hand  of  the 
trunk  lights  both  lamps,  and  the  restoring  of  the  trunk  plug  to  its 
normal  seat  again  extinguishes  both  lamps. 


404 


TELEPHONY 


Plug=Seat  Switch.  The  plug-seat  switch  is  a  device  that  has 
received  a  good  deal  of  attention  not  only  for  use  with  transfer  sys- 
tems, but  also  for  use  in  a  great  variety  of  ways  with  other  kinds  of 
manual  switching  systems.  The  placing  of  a  plug  in  its  seat  or 
withdrawing  it  therefrom  offers  a  ready  means  of  accomplishing 
some  switching  or  signaling  operation  automatically.  The  plug- 
seat  switch  has,  however,  in  spite  of  its  possibilities,  never  come  into 
wide  use,  and  so  far  as  we  are  aware  the  Monarch  Telephone  Manu- 
facturing Company  is  the  only  company  of  prominence  which  incor- 
porates it  in  its  regular  output.  The  Monarch  plug-switch  mechan- 


Fig.  333.     Plug-Seat  Switch 

ism  is  shown  in  Fig.  333,  and  its  operation  is  obvious.  It  may  be 
stated  at  this  point  that  one  of  the  reasons  why  the  plug-seat  switch 
has  not  been  more  widely  adopted  for  use,  is  the  difficulty  that  has 
been  experienced  due  to  lint  from  the  switchboard  cords  collecting 
on  or  about  the  contact  points.  In  the  construction  given  in  the 
detailed  cut,  upper  .part,  Fig.  333,  is  shown  the  means  adopted  by 
the  Monarch  Company  for  obviating  this  difficulty.  The  contact 
points  are  carried  in  the  upper  portion  of  an  inverted  cup  mounted 
on  the  under  side  of  the  switchboard  shelf,  and  are  thus  protected, 
in  large  measure,  from  the  damaging  influence  of  dust  and  lint. 

Methods  of  Handling  Transfers.     One  way  of  giving  the  num- 
ber of  the  called  subscriber  to  the  second  operator  in  a  transfer  system 


TRANSFER  SWITCHBOARD 


405 


is  to  have  that  operator  listen  in  on  the  circuit  after  it  is  continued 
to  her  position  and  receive  the  number  either  from  the  first  operator 
or  from  the  subscriber.  Receiving  it  from  the  first  operator  has  the 
disadvantage  of  compelling  the  first  operator  to  wait  on  the  circuit 
until  the  second  operator  responds;  receiving  it  from  the  subscriber 
has  the  disadvantage  of  sometimes  being  annoying  to  him.  This, 
however,  is  to  be  preferred  to  the  loss  of  time  on  the  part  of  the 
originating  operator  that  is  entailed  by  the  first  method.  A  better 
way  than  either  of  these  is  to  provide  between  the  various  operators 
working  in  a  transfer  system,  a  so-called  order-wire  system.  An 


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-f>A 

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Fig.  334.     Order- Wire  Arrangement 

order  wire,  as  ordinarily  arranged,  is  a  circuit  terminating  at  one 
end  permanently  in  the  head  receiver  of  an  operator,  and  terminating 
at  the  other  end  in  a  push  button  which,  when  depressed,  will  con- 
nect the  telephone  set  of  the  operator  at  that  end  with  the  order  wire. 
The  operator  at  the  push-button  end  of  the  order  wire  may,  there- 
fore, at  will,  communicate  with  the  other  operator  in  spite  of  any- 
thing that  the  other  operator  may  do.  An  order-wire  system  suit- 
able for  transfer  switchboards  consists  in  an  order  wire  leading 
from  each  operator's  receiver  to  a  push  button  at  each  of  the  other 
operator's  positions,  so  that  every  operator  has  it  within  her  power  to 
depress  a  key  or  button  and  establish  communication  with  a  cor- 


406  TELEPHONY 

responding  operator.  When,  therefore,  an  operator  in  a  transfer 
system  answers  a  call  that  must  be  completed  through  a  transfer 
circuit,  she  establishes  connection  with  that  transfer  circuit  and 
then  informs  the  operator  at  the  other  end  of  that  circuit  by  order 
wire  of  the  number  of  the  trunk  and  the  number  of  the  subscriber 
with  which  that  trunk  is  to  be  connected.  Fig.  334  shows  a  system 
of  order-wire  buttons  by  means  of  which  each  operator  may  connect 
her  telephone  set  with  that  of  every  other  operator  in  the  room,  the 
number  in  this  case  being  confined  to  three.  Assuming  that  each 
pair  of  wires  leading  from  the  lower  portion  of  this  figure  terminates 
respectively  in  the  operator's  talking  apparatus  of  the  three  re- 
spective operators,  then  it  is  obvious  that  operator  No.  1,  by  de- 
pressing button  No.  2,  will  connect  her  telephone  set  with  that  of 
operator  No.  2;  likewise  that  any  operator  may  communicate  with 
any  other  operator  by  depressing  the  key  bearing  the  corresponding 
number. 

Limitations  of  Transfer  System.  It  may  be  stated  that  the 
transfer  system  at  present  has  a  limited  place  in  the  art  of  telephony. 
The  multiple  switchboard  has  outstripped  it  in  the  race  for  popular 
approval  and  has  demonstrated  its  superiority  in  practically  all 
large  manual  exchange  work  This  is  not  because  of  lack  of  effort 
on  the  part  of  telephone  engineers  to  make  the  transfer  system  a 
success  in  a  broad  way.  A  great  variety  of  different  schemes,  all 
embodying  the  fundamental  idea  of  having  one  operator  answer  the 
call  and  another  operator  complete  it  through  a  trunk  line,  have 
been  tried.  In  San  Francisco,  the  Sabin-Hampton  system  was  in 
fairly  successful  service  and  served  many  thousands  of  lines  for  a 
number  of  years.  It  was,  however,  afterwards  replaced  by  modern 
multiple  switchboards. 

Examples  of  Obsolete  Systems.  The  Sabin-Hampton  system 
was  unique  in  many  respects  and  involved  three  operators  in  each 
connection.  It  was  one  of  the  very  first  systems  which  employed 
automatic  signaling  throughout  and  did  away  with  the  subscribers' 
generators.  It  did  not,  however,  dispense  with  the  subscribers' 
local  batteries. 

Another  large  transfer  system,  used  for  years  in  an  exchange 
serving  at  a  time  as  many  as  5,000,  was  employed  at  Grand  Rapids, 
Michigan  This  was  later  replaced  by  an  automatic  switchboard. 


407 

Field  of  Usefulness.  The  real  field  of  utility  for  the  transfer 
system  today  is  to  provide  for  the  growth  of  simple  switchboards 
that  have  extended  beyond  their  originally  intended  limits.  By  the 
adding  of  additional  sections  to  the  simple  switchboard  and  the  estab- 
lishment of  a  comparatively  cheap  transfer  system,  the  simple  boards 


Pig.  335.    Three- Position  Transfer  Switchboard         v 

may  be  made  to  do  continued  service  without  wasting  the  investment 
in  them  by  discarding  them  and  establishing  a  completely  new  sys- 
tem. However,  switchboards  are  sometimes  manufactured  in  which 
the  transfer  system  is  included  as  a  part  of  the  original  equipment. 
In  Fig.  335  is  shown  a  three-position  transfer  switchboard,  manu- 
factured by  the  Monarch  Telephone  Company.  At  first  glance  the 
switchboard  appears  to  be  exactly  like  those  described  in  Chapter 
XXI,  but  on  close  observation,  the  transfer  jacks  and  signals  may 


408  TELEPHONY 

be  seen  in  the  first  and  third  positions,  just  below  the  line  jacks  and 
signals.  There  is  no  transfer  equipment  in  the  second  position  of 
this  switchboard  because  the  operator  at  that  position  is  able  to  reach 
the  jacks  of  all  the  lines  and,  therefore,  is  able  to  complete  all  calls 
originating  on  her  position  without  the  use  of  any  transfer  equip- 
ment. Referring  to  Fig.  301,  which  illustrates  a  two-position  simple 
switchboard,  it  may  readily  be  seen  that  if  the  demands  for  tele- 
phone service  in  the  locality  in  which  this  switchboard  is  installed 
should  increase  so  as  to  require  the  addition  of  more  switchboard 
positions,  this  switchboard  could  readily  be  converted  to  a  transfer 
switchboard  by  placing  the  necessary  transfer  jacks  and  signals  in 
the  vacant  space  between  the  line  jacks  and  clearing-out  drops. 


CHAPTER  XXIV 
PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD 

Field  of  Utility.  The  multiple  switchboard,  unlike  the  trans- 
fer board,  provides  means  for  each  operator  to  complete,  without 
assistance,  a  connection  with  any  subscriber's  line  terminating  in 
the  switchboard  no  matter  how  great  the  number  of  lines  may  be. 
It  is  used  only  where  the  simple  switchboard  will  not  suffice;  that  is, 
where  the  number  of  lines  and  the  consequent  traffic  is  so  great  as 
to  require  so  many  operators  and,  therefore,  so  great  a  length  of 
board  as  to  make  it  impossible  for  any  one  operator  to  reach  all  over 
the  face  of  the  board  without  moving  from  her  position. 

The  Multiple  Feature.  The  fundamental  feature  of  the  multi- 
ple switchboard  is  the  placing  of  a  jack  for  every  line  served  by  the 
switchboard  within  the  reach  of  every  operator.  This  idea  under- 
lying the  multiple  switchboard  may  be  best  grasped  by  merely 
considering  the  mechanical  arrangement  and  grouping  of  parts 
without  regard  to  their  details  of  operation.  The  idea  is  sometimes 
elusive,  but  it  is  really  very  simple.  If  the  student  at  the  outset  will  not 
be  frightened  by  the  very  large  number  of  parts  that  are  sometimes 
involved  in  multiple  switchboards,  and  by  the  great  complexity 
which  is  apparent  in  the  wiring  and  in  the  action  of  these  parts ;  and 
will  remember  that  this  apparent  complexity  results  from  the  great 
number  of  repetitions  of  the  same  comparatively  simple  group  of 
apparatus  and  circuits,  much  will  be  done  toward  a  mastery  of  the 
subject. 

The  multiple  switchboard  is  divided  into  sections,  each  section 
being  about  the  width  and  height  that  will  permit  an  ordinary  oper- 
ator to  reach  conveniently  all  over  its  face.  The  usual  width  of  a 
section  brought  about  by  this  limitation  is  from  five  and  one-half  to 
six  feet.  Such  a  section  affords  room  for  three  operators  to  sit  side 
by  side  before  it.  Now  each  line,  instead  of  having  a  single  jack 
as  in  the  simple  switchboard,  is  provided  with  a  number  of  jacks 


410  TELEPHONY 

and  one  of  these  is  placed  on  each  of  the  sections,  so  that  each  one 
of  the  operators  may  have  within  her  reach  a  jack  for  each  line. 
It  is  from  the  fact  that  each  line  has  a  multiplicity  of  jacks,  that  the 
term  multiple  switchboard  arises. 

Number  of  Sections.  Since  there  is  a  jack  for  each  line  on  each 
section  of  the  switchboard,  it  follows  that  on  each  section  there  are  as 
many  jacks  as  there  are  lines ;  that  is,  if  the  board  were  serving  5,000 
lines  there  would  be  5,000  jacks.  Let  us  see  now  what  it  is  that 
determines  the  number  of  sections  in  a  multiple  switchboard.  In 
the  final  analysis,  it  is  the  amount  of  traffic  that  arises  in  the  busiest 
period  of  the  day.  Assume  that  in  a  particular  office  serving  5,000 
lines,  the  subscribers  call  at  such  a  very  low  rate  that  even  at  the 
busiest  time  of  the  day  only  enough  calls  are  made  to  keep,  say,  three 
operators  busy.  In  this  case  there  would  be  no  need  for  the  mul- 
tiple switchboard,  for  a  single  section  would  suffice.  The  three 
operators  seated  before  that  section  would  be  able  to  answer  and 
complete  the  connections  for  all  of  the  calls  that  arose.  But  sub- 
scribers do  not  call  at  this  exceedingly  low  rate.  A  great  many  more 
calls  would  arise  on  5,000  lines  during  the  busiest  hour  than  could 
be  handled  by  three  operators  and,  therefore,  a  great  many  more 
operators  would  be  required.  Space  has  to  be  provided  for  these 
operators  to  work  in,  and  as  each  section  accommodates  three  oper- 
ators the  total  number  of  sections  must  be  at  least  equal  to  the  total 
number  of  required  operators  divided  by  three. 

Let  us  assume,  for  instance,  that  each  operator  can  handle  200 
calls  during  the  busy  hour.  Assume  further  that  during  the  busy 
hour  the  average  number  of  calls  made  by  each  subscriber  is  two. 
One  hundred  subscribers  would,  therefore,  originate  200  calls  within 
this  busy  hour  and  this  wrould  be  just  sufficient  to  keep  one  operator 
busy.  Since  one  operator  can  handle  only  the  calls  of  one  hundred 
subscribers  during  the  busy  hour,  it  follows  that  as  many  operators 
must  be  employed  as  there  are  hundreds  of  subscribers  whose  lines 
are  served  in  a  switchboard,  and  this  means  that  in  an  exchange  of 
5,000  subscribers,  50  operators'  positions  would  be  required,  or  16§ 
sections.  Each  of  these  sections  would  be  equipped  with  the  full 
5,000  jacks,  so  that  each  operator  could  have  a  connection  terminal 
for  each  line. 

The  Multiple.     These  groups  of  5,000  jacks,  repeated  on  each 


PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD    411 

of  the  sections  are  termed  multiple  jacks,  and  the  entire  equipment 
of  these  multiple  jacks  and  their  wiring  is  referred  to  as  the  multiple. 
It  will  be  shown  presently  that  the  multiple  jacks  are  only  used  for 
enabling  the  operator  to  connect  with  the  called  subscriber.  In 
other  words  these  jacks  are  for  the  purpose  of  enabling  each  oper- 
ator to  have  within  her  reach  any  line  that  may  be  called  for  regard- 
less of  what  line  originates  the  call.  We  will  now  consider  what 
arrangements  are  provided  for  enabling  the  operator  to  receive  the 
signal  indicating  a  call  and  what  provisions  are  made  for  her  to  an- 
swer the  call  in  response  to  such  a  signal. 

Line  Signals.  Obviously  it  is  not  necessary  to  have  the  line 
signals  repeated  on  each  section  of  the  board  as  are  the  multiple  jacks. 
If  a  line  has  one  definite  place  on  the  switchboard  where  its  signal 
may  be  received  and  its  call  may  be  answered,  that  suffices.  Each 
line,  therefore,  in  addition  to  having  its  multiple  jacks  distributed 
one  on  each  section  of  the  switchboard,  has  a  line  signal  and  an 
individual  jack  immediately  associated  with  it,  located  on  one  only 
of  the  sections.  This  signal  usually  is  in  the  form  of  a  lamp  and 
is  termed  the  line  signal,  and  this  jack  is  termed  the  answering  jack 
since  it  is  by  means  of  it  that  the  operator  always  answers  a  call  in 
response  to  the  line  signal. 

Distribution  of  Line  Signals.  It  is  evident  that  it  would  not 
do  to  have  all  of  these  line  signals  and  answering  jacks  located  at 
one  section  of  the  board  for  then  they  would  not  be  available  to  all 
of  the  operators.  They  are,  therefore,  distributed  along  the  board 
in  such  a  way  that  one  group  of  them  will  be  available  to  one 
operator,  another  group  to  another  operator,  and  so  on;  the  num- 
ber of  answering  jacks  and  signals  in  any  one  group  being  so  pro- 
portioned with  respect  to  the  number  of  calls  that  come  in  over 
them  during  the  busy  hour  that  it  will  afford  just  about  enough 
calls  to  keep  the  operator  at  that  position  busy. 

We  may  summarize  these  conditions  with  respect  to  the  jack  and 
line-signal  equipment  of  the  multiple  switchboard  by  saying  that 
each  line  has  a  multiple  jack  on  each  section  of  the  board  and  in 
addition  to  this  has  on  one  section  of  the  board  an  answering  jack 
and  a  line  signal.  These  answering  jacks  and  line  signals  are  dis- 
tributed in  groups  along  the  face  of  the  board  so  that  each  operator 
will  receive  her  proper  quota  of  the  originating  calls  which  she  will 


412  TELEPHONY 

answer  and,  by  virtue  of  the  multiple  jack,  be  able  to  complete  the 
connections  with  the  desired  subscribers  without  moving  from  her 
position. 

Cord  Circuits.  Each  operator  is  also  provided  with  a  number 
of  pairs  of  cords  and  plugs  with  proper  supervisory  or  clearing-out 
signals  and  ringing  and  listening  keys,  the  arrangement  in  this  re- 
spect being  similar  to  that  already  described  in  connection  with  the 
simple  switchboard. 

Guarding  against  Double  Connections.  From  what  has  been 
said  it  is  seen  that  a  call  originating  on  a  given  line  may  be  answered 
at  one  place  only,  but  an  outgoing  connection  with  that  line  may 
be  made  at  any  position.  This  fact  that  a  line  may  be  connected 
with  when  called  for  at  any  one  of  the  sections  of  the  switchboard 
makes  necessary  the  provision  that  two  or  more  connections  will  not 
be  made  with  the  same  line  at  the  same  time.  For  instance,  if  a  call 
came  in  over  a  line  whose  signal  was  located  on  the  first  position  of 
the  switchboard  for  a  connection  with  line  No.  1,000,  the  operator 
at  the  first  position  would  connect  this  calling  line  with  No.  1,000 
through  the  multiple  jack  on  the  first  section  of  the  switchboard 
Assume  now  that  some  line,  whose  signal  was  located  on  the  39th 
position  of  the  switchboard,  should  call  also  for  line  No.  1,000  while 
that  line  was  still  connected  with  the  first  calling  subscriber.  Ob- 
viously confusion  would  result  if  the  operator  at  the  39th  position, 
not  knowing  that  line  No.  1,000  was  already  busy,  should  connect  this 
second  line  with  it,  thereby  leaving  both  of  the  calling  subscribers 
connected  with  line  No.  1,000,  and  as  a  result  all  of  these  three  sub- 
scribers connected  together. 

The  provisions  for  suitable  means  for  preventing  the  making 
of  a  connection  with  a  line  that  is  already  switched  at  some  other 
section  of  the  switchboard,  has  offered  one  of  the  most  fertile  fields 
for  invention  in  the  whole  telephone  art.  The  ways  that  have  been 
proposed  for  accomplishing  this  are  legion.  Fortunately  common 
practice  has  settled  on  one  general  plan  of  action  and  that  is  to  so 
arrange  the  circuits  that  whenever  a  line  is  switched  at  one  section, 
such, an  electrical  condition  will  be  established  on  the  forward  con- 
tacts of  all  of  its  multiple  jacks  that  any  operator  at  any  other  section 
in  attempting  to  make  a  connection  with  that  line  will  be  notified 
of  the  fact  that  it  is  already  switched  by  an  audible  signal,  which  she 


PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD    413 


will  receive  in  her  head  receiver.  On  the  other  hand  the  arrange- 
ment is  such  that  when  a  line  is  not  busy,  i.  e.,  it  is  not  switched  at 
any  of  the  positions  of  the  switchboard,  the  operator  on  attempting 
to  make  a  connection  with  such  a  line  will  receive  no  such  guarding 
signal  and  will,  therefore,  proceed  with  the  connection. 

We  may  liken  a  line  in  a  multiple  switchboard  to  a  lane  having 
a  number  of  gates  giving  access  to  it.  One  of  these  gates — the  an- 
swering jack — is  for  the  exclusive  use  of  the  proprietor  of  that  lane. 
All  of  the  other  gates  to  the  lane — the  multiple  jacks — are  for  afford- 
ing means  for  the  public  to  enter.  But  whenever  any  person  en- 
ters one  of  these  gates,  a  signal  is  automatically  put  up  at  all  of  the 
other  gates  forbidding  any  other  person  to  enter  the  lane  as  long  as 
the  first  person  is  still  within. 


szcr/ow  no.  / 


SECT/OM  /YO.2          SECT/OH  N0.3        -S£C77O/Y  HO.4 


J 

MULT:&JACKS 


33     J 

MULT/PLE 
JACKS 


>  3 

Ml/LT/PLE. 
JACKS 


r 


PLUGS AM) COP.OS    \  PLUGS  A/W  CORDS      PLUGS  AW  CORDS     PLUGS  AND  CORDS  \ 

Fig.  336.     Principle  of  Multiple  Switchboard 

Diagram  Showing  Multiple  Board  Principle.  For  those  to  whom 
the  foregoing  description  of  the  multiple  board  is  not  altogether  clear, 
the  diagram  of  Fig.  336  may  offer  some  assistance.  Five  subscrib- 
ers' lines  are  shown  running  through  four  sections  of  a  switchboard. 
Each  of  these  lines  is  provided  with  a  multiple  jack  on  each  section 
of  the  board.  Each  line  is  also  provided  with  an  answering  jack  and 
a  line  signal  on  one  of  the  sections  of  the  board.  Thus  the  answer- 


414  TELEPHONY 

ing  jacks  and  the  line  signals  of  lines  1  and  2  are  shown  in  Section 
I,  that  of  line  4  is  shown  in  Section  II,  that  of  line  3  in  Section  III, 
and  that  of  line  5  in  Section  IV.  At  Section  I,  line  1  is  shown  in  the 
condition  of  having  made  a  call  and  having  had  this  call  answered 
by  the  operator  inserting  one  of  her  plugs  into  its  answering  jack. 
In  response  to  the  instructions  given  by  the  subscriber,  the  operator 
has  inserted  the  other  plug  of  this  same  pair  in  the  multiple  jack 
of  line  2,  thus  connecting  these  two  lines  for  conversation.  At  Sec- 
tion III,  line  3  is  shown  as  having  made  a  call,  and  the  operator  as 
having  answered  by  inserting  one  of  her  plugs  into  the  answering 
jack.  It  happens  that  the  subscriber  on  line  3  requests  a  connec- 
tion with  line  1,  and  the  condition  at  Section  III  is  that  where  the 
operator  is  about  to  apply  the  tip  of  the  calling  plug  to  the  jack  of 
line  1  to  ascertain  whether  or  not  that  line  is  busy.  As  before  stated, 
when  the  contact  is  made  between  the  tip  of  the  calling  plug  and  the 
forward  contact  of  the  multiple  jack,  the  operator  will  receive  a  click 
in  the  ear  (by  means  that  will  be  more  fully  discussed  in  later  chap- 
ters), this  click  indicating  to  her  that  line  1  is  not  available  for  con- 
nection because  it  is  already  switched  at  some  other  section  of  the 
switchboard. 

Busy  Test.  The  busy  signal,  by  which  an  operator  in  attempt- 
ing to  make  a  connection  is  informed  that  the  line  is  already  busy, 
has  assumed  a  great  variety  of  forms  and  has  brought  forth  many 
inventions.  It  has  been  proposed  by  some  that  the  insertion  of  a 
plug  into  any  one  of  the  jacks  of  a  line  would  automatically  close  a 
little  door  in  front  of  each  of  the  other  jacks  of  the  line,  therefore 
making  it  impossible  for  any  other  operator  to  insert  a  plug  as  long 
as  the  line  is  in  use.  It  has  been  proposed  by  others  to  ring  bells 
or  to  operate  buzzers  whenever  the  attempt  was  made  by  an  operator 
to  plug  into  a  line  that  was  already  in  use.  Still  others  have  proposed 
to  so  arrange  the  circuits  that  the  operator  would  get  an  electric 
shock  whenever  she  attempted  to  plug  into  a  busy  line.  The  scheme 
that  has  met  with  universal  adoption,  however,  is  that  the  operator 
shall,  when  the  tip  of  her  calling  plug  touches  the  forward  contact  of 
the  jack  of  a  line  that  is  already  switched,  receive  a  click  in  her  tele- 
phone which  will  forbid  her  to  insert  the  plug.  The  absence  of  this 
click,  or  silence  in  her  telephone,  informs  her  that  she  may  safely 
make  the  connection. 


PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD    415 

Principle.  The  means  by  which  the  operator  receives  or  fails 
to  receive  this  click,  according  to  whether  the  line  is  busy  or  idle, 
vary  widely,  but  so  far  as  the  writers  are  aware  they  all  have  one  fun- 
damental feature  in  common.  The  tip  of  the  calling  plug  and  the 
test  contact  of  all  of  the  multiple  jacks  of  an  idle  line  must  be  abso- 
lutely at  the  same  potential  before  the  test,  so  that  no  current  will 
flow  through  the  test  circuit  when  the  test  is  actually  made.  The 
test  thimbles  of  all  the  jacks  of  a  busy  line  must  be  at  a  different 
potential  from  the  tip  of  the  test  plug  so  that  a  current  will  flow  and 
a  click  result  when  the  test  is  made. 

Potential  of  Test  Thimbles.  It  has  been  found  an  easy  matter 
to  so  arrange  the  contacts  in  the  jacks  of  a  multiple  switchboard  that 
whenever  the  line  is  idle  the  test  thimbles  of  that  line  will  be  a  cer- 
tain potential,  the  same  as  that  of  all  the  unused  calling  plug  tips. 
It  has  also  been  easy  to  so  arrange  these  contacts  that  the  insertion  of 
a  plug  into  any  one  of  the  jacks  will,  by  virtue  of  the  contacts  estab- 
lished, change  the  potential  of  all  the  test  thimbles  of  that  line  so 
that  they  will  be  at  a  different  potential  from  that  of  the  tips  of  the 
calling  plugs.  It  has  not  been  so  easy,  however,  to  provide  that  these 
conditions  shall  exist  under  all  conditions  of  practice.  A  great  many 
busy  tests  that  looked  well  on  paper  have  been  found  faulty  in  prac- 
tice. As  is  always  the  case  in  such  instances,  this  has  been  true 
because  the  people  who  considered  the  scheme  on  paper  did  not 
foresee  all  of  the  conditions  that  would  arise  in  practice.  Many 
busy-test  systems  will  operate  properly  while  everything  connected 
with  the  switchboard  and  the  lines  served  by  it  remains  in  proper 
order.  But  no  such  condition  as  this  can  be  depended  on  in  prac- 
tice. Switchboards,  no  matter  how  perfectly  made  and  no  matter 
with  how  great  care  they  may  be  installed  and  maintained,  will  get 
out  of  order.  Telephone  lines  will  become  grounded  or  short-cir- 
cuited or  crossed  or  opened.  Such  conditions,  in  a  faulty  busy- 
test  system,  may  result  in  a  line  that  is  really  idle  presenting  a  busy 
test,  and  thus  barring  the  subscriber  on  that  line  from  receiving  calls 
from  other  lines  just  as  completely  as  if  his  line  were  broken.  Oh 
the  other  hand,  faulty  conditions  either  in  the  switchboard  or  in 
the  line  may  make  a  line  that  is  really  busy,  test  idle,  and  thus  result 
in  the  confusion  of  having  two  or  more  subscribers  connected  to  the 
same  line  at  the  same  time. 


416  TELEPHONY 

Busy-Test  Faults.  To  show  how  elusive  some  of  the  faults  of 
a  busy  test  may  be,  when  considered  on  paper,  it  has  come  within 
the  observation  of  the  writers  that  a  new  busy-test  system  was  thought 
well  enough  of  by  a  group  of  experienced  engineers  to  warrant  its 
installation  in  a  group  of  very  large  multiple  switchboards,  the  cost 
of  which  amounted  to  hundreds  of  thousands  of  dollars,  and  yet  when 
so  installed  it  developed  that  a  single  short-circuited  cord  in  a  posi- 
tion would  make  the  test  inoperative  on  all  the  cords  of  that  position 
— obviously  an  intolerable  condition.  Luckily  the  remedy  was  sim- 
ple and  easily  applied. 

In  a  well-designed  busy-test  system  there  should  be  complete 
silence  when  the  test  is  made  of  an  idle  line,  and  always  a  well-de- 
fined click  when  the  test  is  made  of  a  busy  line.  The  test  on  busy 
lines  should  result  in  a  uniform  click  regardless  of  length  of  lines 
or  the  condition  of  the  apparatus.  It  does  not  suffice  to  have  a  little 
click  for  an  idle  line  and  a  big  click  for  a  busy  line,  as  practice  has 
shown  that  this  results  in  frequent  errors  on  the  part  of  the  operators. 

Good  operating  requires  that  the  tip  of  the  calling  plug  be 
tapped  against  the  test  thimble  several  times  in  order  to  make  sure 
of  the  state  of  the  called  line. 

In  some  multiple  switchboards  the  arrangement  has  been  such 
that  the  jacks  of  a  line  would  test  busy  as  soon  as  the  subscriber 
on  that  line  removed  his  receiver  from  its  hook  to  make  a  call,  as 
well  as  while  any  plug  was  in  any  jack  of  that  line.  The  advocates 
of  this  added  feature,  in  connection  with  the  busy  test,  have  claimed 
that  the  receiver,  when  removed  from  its  hook  in  making  a  call, 
should  make  the  line  test  busy  and  that  a  line  should  not  be  connected 
with  when  the  subscriber's  receiver  was  off  its  hook  any  more  than 
it  should  be  when  it  was  already  connected  with  at  some  other  sec- 
tion of  the  switchboard.  While  it  is  true  that  a  line  may  be  properly 
termed  busy  when  the  subscriber  has  removed  his  receiver  in  order 
to  make  a  call,  it  is  not  true  that  there  is  any  real  necessity  for  guard- 
ing against  a  connection  with  it  while  he  is  waiting  for  the  operator 
to  answer.  Leaving  the  line  unguarded  for  this  brief  period  may 
result  in  the  subscriber,  who  intended  to  make  the  call,  having  to 
defer  his  call  until  he  has  conversed  with  the  party  who  is  trying  to 
reach  him.  This  cannot  be  said  to  be  a  detriment  to  the  service, 
however,  since  the  second  party  gets  the  connection  he  desires  much 


PRINCIPLES  OF  THE  MULTIPLE  SWITCHBOARD    417 

sooner  than  he  otherwise  would,  and  the  first  party  may  still  make 
his  first  intended  call  as  soon  as  he  has  disposed  of  the  party  who 
reached  him  while  he  was  waiting  for  his  own  operator  to  answer.  It 
may  be  said,  therefore,  in  connection  with  this  matter  of  making  the 
line  test  busy  as  soon  as  a  subscriber  has  removed  his  receiver  from 
the  hook,  that  it  is  not  considered  an  essential,  and  in  case  of  those 
switchboard  systems  which  naturally  work  out  that  way  it  is  not 
considered  a  disadvantage. 

Field  of  Each  Operator.  It  was  stated  earlier  in  this  chapter 
that  as  each  section  accommodated  three  operators,  the  total  number 
of  sections  in  a  switchboard  will  be  at  least  one-third  the  total  num- 
ber of  required  operators.  This  thought  needs  further  development, 
for  to  stop  at  that  statement  is  to  arrive  somewhat  short  of  the  truth. 
In  order  to  do  this  it  is  necessary  to  consider  the  field  in  the  multi- 
ple, reached  by  each  operator.  The  section  is  of  such  size,  or  should 
be,  that  an  operator  seated  in  the  center  position  of  it  may,  without 
undue  effort,  reach  all  over  the  multiple.  But  the  operator  at  the 
right-hand  position  cannot  reach  the  extreme  left  portion  of  the  mul- 
tiple of  that  section,  nor  can  the  operator  at  the  left  reach  the  ex- 
treme right.  How  then  may  each  operator  reach  a  jack  for  every 
line?  Remembering  that  the  multiple  jacks  are  arranged  exactly 
the  same  in  each  section>  each  jack  always  occupying  the  same  rela- 
tive position,  it  is  easy  to  see  that  while  the  operator  at  a  right-hand 
position  of  a  section  cannot  reach  the  left-hand  third  of  the  multi- 
ple in  her  own  section,  she  may  reach  the  left-hand  third  of  the  multi- 
ple in  the  section  at  her  right,  and  this,  together  with  the  center  and 
right-hand  thirds  of  her  own  section,  represents  the  entire  number 
of  lines.  So  it  is  with  the  left-hand  operator  at  any  section,  she 
reaches  two-thirds  of  all  the  lines  in  the  multiple  of  her  own  section 
and  one-third  in  that  of  the  section  at  her  left. 

End  Positions.  This  makes  it  necessary  to  inquire  about  the 
operators  at  the  end  positions  of  the  entire  board.  To  provide  for 
these  the  multiple  is  extended  one-third  of  a  section  beyond  them, 
so  as  to  supply  at  the  ends  of  the  switchboard  jacks  for  those  lines 
which  the  end  operators  cannot  reach  on  their  own  sections.  Some- 
times instead  of  adding  these  end  sections  to  the  multiple  for  the 
end  operators,  the  same  result  is  accomplished  by  using  only  the  full 
and  regular  sections  of  the  multiple,  and  leaving  the  end  positions 


418  TELEPHONY 

without  operators'  equipment,  as  well  as  without  answering  jacks. 
line  signals,  and  cords  and  plugs,  so  that  in  reality  the  end  operator 
is  at  the  middle  position  of  the  end  section.  This,  in  our  opinion, 
is  the  better  practice,  since  it  leaves  the  sections  standard,  and  makes 
it  easier  to  extend  the  switchboard  in  length,  as  it  grows,  by  the 
mere  addition  of  new  sections  without  disturbing  any  of  the  old 
multiple. 

Influence  of  Traffic.  We  wish  again  to  emphasize  the  fact  that 
it  is  the  traffic  during  the  busiest  time  of  day  and  not  the  number 
of  lines  that  determine  the  size  of  a  multiple  switchboard  so  far  as  its 
length  is  concerned.  The  number  of  lines  determines  the  size  of 
the  multiple  in  any  one  section,  but  it  is  the  amount  of  traffic,  the 
number  of  calls  that  are  made  in  the  busiest  period,  that  determines 
the  number  of  operators  required,  and  thus  the  number  of  positions. 
Had  this  now  very  obvious  fact  been  more  fully  realized  in  the  past, 
some  companies  would  be  operating  at  less  expense",  and  some  manu- 
facturers would  have  sold  less  expensive  switchboards. 

The  whole  question  as  to  the  number  of  positions  boils  down 
to  how  many  answering  jacks  and  line  signals  may  be  placed 
at  each  operator's  position  without  overburdening  the  operator  with 
incoming  traffic  at  the  busy  time  of  day.  Obviously,  some  lines  will 
call  more  frequently  than  others,  and  hence  the  proper  number  of 
answering  jacks  at  the  different  positions  will  vary.  Obviously,  also, 
due  to  changes  in  the  personnel  of  the  subscribers,  the  rates  of  calling 
of  different  groups  of  lines  will  change  from  time  to  time,  and  this 
may  necessitate  a  regrouping  of  the  line  signals  and  answering  jacks 
on  the  positions;  and  changes  in  the  personnel  of  the  operators  or 
in  their  skill  also  demand  such  regrouping. 

Intermediate  Frame.  The  intermediate  distributing  frame  is 
provided  for  this  purpose,  and  will  be  more  fully  discussed  in  sub- 
sequent chapters.  Suffice  it  to  say  here  that  the  intermediate  dis- 
tributing frame  permits  the  answering  jacks  and  line  signals  to  be 
shifted  about  among  the  operators'  positions,  so  that  each  position 
will  have  just  enough  originating  traffic  to  keep  each  of  the  oper- 
ators economically  busy  during  the  busiest  time  of  the  day. 


CHAPTER  XXV 
THE  MAGNETO  MULTIPLE  SWITCHBOARD 

Field  of  Utility.  The  principles  of  the  multiple  switchboard 
set  forth  in  the  last  chapter  were  all  developed  long  before  the  com- 
mon-battery system  came  into  existence,  and  consequently  all  of  the 
first  multiple  switchboards  were  of  the  magneto  type.  Although 
once  very  widely  used,  the  magneto  multiple  switchboard  has  almost 
passed  out  of  existence,  since  it  has  become  almost  universal  prac- 
tice to  equip  exchanges  large  enough  to  employ  multiple  boards 
with  common-battery  systems.  Nevertheless  there  is  a  field  for 
magneto  multiple  switchboards,  and  in  this  field  it  has  recently  been 
coming  into  increasing  favor.  In  those  towns  equipped  with  mag- 
neto systems  employing  simple  switchboards  or  transfer  switch- 
boards, and  which  require  new  switchboards  by  virtue  of  having 
outgrown  or  worn  out  their  old  ones,  the  magneto  multiple  switch- 
board is  frequently  found  to  best  fit  the  requirements  of  economy 
and  good  practice.  The  reason  for  this  is  that  by  its  use  the  mag- 
neto telephones  already  in  service  may  be  continued,  no  change 
being  required  outside  of  the  central  office.  Furthermore,  with 
the  magneto  multiple  switchboard  no  provision  need  be  made  for  a 
power  plant,  which,  in  towns  of  small  size,  is  often  an  important 
consideration.  Again,  many  companies  operate  over  a  considerable 
area,  involving  a  collection  of  towns  and  hamlets.  It  may  be  that  all 
of  these  towns  except  one  are  clearly  of  a  size  to  demand  magneto 
equipment  and  that  magneto  equipment  is  the  standard  throughout 
the  entire  territory  of  the  company.  If,  however,  one  of  the  towns, 
by  virtue  of  growth,  demands  a  multiple  switchboard,  this  condition 
affords  an  additional  argument  for  the  employment  of  the  magneto 
multiple  switchboard,  since  the  same  standards  of  equipment  and 
construction  may  be  maintained  throughout  the  entire  territory  of 
the  operating  company,  a  manifest  advantage.  On  the  other  hand, 
it  may  be  said  that  the  magneto  multiple  switchboard  has  no  proper 


420  TELEPHONY 

place  in  modern  exchanges  of  considerable  size — say,  having  upward 
of  one  thousand  subscribers — at  least  under  conditions  found  in  the 
United  States. 

Notwithstanding  the  obsolescence  of  the  magneto  multiple 
switchboard  for  large  exchanges,  a  brief  discussion  of  some  of  the 
early  magneto  multiple  switchboards,  and  particularly  of  one  of  the 
large  ones,  is  worth  while,  in  that  a  consideration  of  the  defects  of 
those  early  efforts  will  give  one  a  better  understanding  and  apprecia- 
tion of  the  modern  multiple  switchboard,  and  particularly  of  the 
modern  multiple  common-battery  switchboard,  the  most  highly 
organized  of  all  the  manual  switching  systems.  Brief  reference  will, 
therefore,  be  made  to  the  so-called  series  multiple  switchboard,  and 
then  to  the  branch  terminal  multiple  switchboard,  which  latter  was 
the  highest  type  of  switchboard  development  at  the  time  of  the 
advent  of  common-battery  working. 

Series=Multiple  Board.  In  Fig.  337  are  shown  the  circuits  of  a 
series  magneto  multiple  switchboard  as  developed  by  the  engineers 
of  the  Western  Electric  Company  during  the  eighties.  As  is  usual, 
two  subscribers'  lines  and  a  single  cord  circuit  are  shown.  One 
side  of  each  line  passes  directly  from  the  subscriber's  station  to  one 
side  of  the  drop,  and  also  branches  off  to  the  sleeve  contact  of  each 
of  the  jacks.  The  other  side  of  the  line  passes  first  to  the  tip  spring 
of  the  first  jack,  thence  to  the  anvil  of  that  jack  and  to  the  tip  spring 
of  the  next  jack,  and  so  on  in  series  through  all  of-  the  jacks  belonging 
in  that  line  to  the  other  terminal  of  the  drop  coil.  Normally,  there- 
fore, the  drop  is  connected  across  the  line  ready  to  be  responsive  to 
the  signal  sent  from  the  subscriber's  generator.  The  cord  circuit  is 
of  the  two-conductor  type,  the  plugs  being  provided  with  tip  and 
sleeve  contacts,  the  tips  being  connected  by  one  of  the  flexible  con- 
ductors through  the  proper  ringing  and  listening  key  springs,  and 
the  sleeve  being  likewise  connected  through  the  other  flexible  con- 
ductor and  the  other  springs  of  the  ringing  and  listening  keys.  It  is 
obvious  that  when  any  plug  is  inserted  into  a  jack,  the  circuit  of  the 
line  will  be  continued  to  the  cord  circuit  and  at  the  same  time  the 
line  drop  will  be  cut  out  of  the  circuit,  because  of  the  lifting  of  the 
tip  spring  of  the  jack  from  its  anvil.  Permanently  connected  between 
the  sleeve-  side  of  the  cord  circuit  and  ground  is  a  retardation  coil  1 
and  a  battery.  Another  retardation  coil  2  is  connected  between  the 


THE  MAGNETO  MULTIPLE  SWITCHBOARD 


421 


ground  and  a  point  on  the  operator's  telephone  circuit  between  the 
operator's  head  receiver  and  the  secondary  of  her  induction  coil. 
These  two  retardation  coils  have  to  do  with  the  busy  test,  the  action 
of  which  is  as  follows:  normally,  or  when  a  line  is  not  switched  at 
the  central  office,  the  test  thimbles  will  all  be  at  substantially  ground 
potential,  i.  e.,  they  are  supposed  to  be.  The  point  on  the  oper- 
ator's receiver  circuit  which  is  grounded  through  the  retardation 
coil  2  will  also  be  of  ground  potential  because  of  that  connection  to 
ground.  In  order  to  test,  the  operator  always  has  to  throw  her 


STAT/OM    A 


5 TAT/ OH  B 


OPERATORS 
TALK/MO    S£7~ 


Fig.  337.     Series  Magneto  Multiple  Switchboard 

listening  key  L.K.  into  the  listening  position.  She  also  has  to  touch 
the  tip  of  the  calling  plug  Pc  to  a  sleeve  or  jack  of  the  line  that  is  being 
tested.  If,  therefore,  a  test  is  made  of  an  idle  or  non-busy  line,  the 
touching  of  the  tip  of  the  calling  plug  with  the  test  thimble  of  that 
line  will  result  in  no  flow  of  current  through  the  operator's  receiver, 
because  there  will  be  no  difference  of  potential  anywhere  in  the  test 
circuit,  which  test  circuit  may  be  traced  from  the  test  thimble  of  the 
line  under  test  to  the  tip  of  the  calling  plug,  thence  through  the  tip 
strand  of  the  cord  to  the  listening  key,  thence  to  the  outer  anvil  of 


422  TELEPHONY 

the  listening  key  on  that  side,  through  the  operator's  receiver  to 
ground  through  the  impedance  coil  2.  If,  however,  the  line  had 
already  been  switched  at  some  other  section  by  the  insertion  of  either 
a  calling  or  answering  plug,  all  of  the  test  thimbles  of  that  line  would 
have  been  raised  to  a  potential  above  that  of  the  ground,  by  virtue 
of  the  battery  connected  with  the  sleeve  side  of  the  cord  circuit  through 
the  retardation  coil  1.  If  the  operator  had  made  a  test  of  such  a  line, 
the  tip  of  her  testing  plug  would  have  found  the  thimble  raised  to  the 
potential  of  the  battery  and,  therefore,  a  flow  of  current  would  oc- 
cur which  would  give  her  the  busy  click.  The  complete  test  circuit 
thus  formed  in  testing  a  busy  line  would  be  from  the  ungrounded 
pole  of  the  battery  through  the  impedance  coil  1  associated  with  the 
cord  that  was  already  in  connection  with  the  line,  thence  to  the 
sleeve  strand  of  that  cord  to  the  sleeve  of  the  jack  at  which  the  line 
was  already  switched,  thence  through  that  portion  of  the  line  cir- 
cuit to  which  all  of  the  sleeve  contacts  were  connected,  and  there- 
fore to  the  sleeve  or  test  thimble  of  the  jack  at  which  the  test  is  made, 
thence  through  the  tip  of  the  calling  plug  employed  in  making  the 
test  through  the  tip  side  of  that  cord  circuit  to  the  outer  listening 
key  contact  of  the  operator  making  the  test,  and  thence  to  ground 
through  the  operator's  receiver  and  the  impedance  coil  2.  The 
resultant  click  would  be  an  indication  to  the  operator  that  the  line 
was  already  in  use  and  that,  therefore,  she  must  not  make  the  con- 
nection. 

The  condenser  3  is  associated  with  the  operator's  talking  set 
and  with  the  extra  spring  in  the  listening  key  L,K»  in  such  a  manner 
that  when  the  listening  key  is  thrown,  the  tip  strand  of  the  cord  cir- 
cuit is  divided  and  the  condenser  included  between  them.  This  is 
for  the  purpose  of  preventing  any  potentials,  which  might  exist  on  the 
line  with  which  the  answering  plug  Pa  was  connected,  from  affecting 
the  busy-test  conditions. 

Operation.  The  operation  of  the  system  aside  from  the  busy- 
test  feature  is  just  like  that  described  in  connection  with  the  simple 
magneto  switchboard.  Assuming  that  the  subscriber  at  Station  A 
makes  the  call,  he  turns  his  hand  generator,  which  throws  the  drop 
on  his  line  at  the  central  office.  The  operator,  seeing  the  signal, 
inserts  the  answering  plug  of  one  of  her  idle  pairs  of  cords  into  the 
answering  jack  and  throws  her  listening  key  L.K.  This  enables 


THE  MAGNETO  MULTIPLE  SWITCHBOARD          423 

the  operator  to  talk  with  the  calling  subscriber,  and  having  found 
that  he  desires  a  connection  with  the  line  extending  to  Station  B. 
she  touches  the  tip  of  her  calling  plug  to  the  multiple  jack  of  that 
line  that  is  within  her  reach,  it  being  remembered  that  each  one  of 
the  multiple  jacks  shown  is  on  a  different  section.  She  leaves  the 
listening  key  in  the  listening  position  when  she  does  this.  If  the  line 
is  busy,  the  click  will  notify  her  that  she  must  not  make  the  connec- 
tion, in  which  case  she  informs  the  calling  subscriber  that  the  line  is 
busy  and  requests  him  to  call  again.  If,  however,  she  received  no  click, 
she  would  insert  the  calling  plug  into  the  jack,  thus  completing  the 
connection  between  the  two  lines.  She  would  then  press  the  ring- 
ing key  associated  with  the  calling  plug  and  that  momentarily  dis- 
connects the  calling  plug  from  the  answering  plug  and  at  the  same 
time  establishes  connection  between  the  ringing  generator  and  the 
called  line.  The  release  of  the  ringing  key  again  connects  the  call- 
ing and  answering  plugs  and,  therefore,  connects  the  two  subscribers' 
lines  ready  for  conversation.  All  that  is  then  necessary  is  that  the 
called  subscriber  shall  respond  and  remove  his  receiver  from  its 
hook,  the  calling  subscriber  already  having  done  this.  When  the 
conversation  is  finished,  both  of  the  subscribers  (if  they  remember  it) 
will  operate  their  ringing  generators,  which  will  throw  the  clearing- 
out  drop  as  a  signal  to  the  operator  for  disconnection.  If  it  should 
become  necessary  for  the  operator  to  ring  back  on  the  line  of  the 
calling  subscriber,  she  may  do  so  by  pressing  the  ringing  key  asso- 
ciated with  the  calling  plug. 

Frequently  this  multiple  switchboard  arrangement  was  used 
with  grounded  lines,  in  which  case  the  single  line  wire  extending 
from  the  subscriber's  station  to  the  switchboard  was  connected 
with  the  tip  spring  of  the  first  jack,  the  circuit  being  continued  in 
series  through  the  jack  to  the  drop  and  thence  to  ground  through 
a  high  non-inductive  resistance. 

Defects.  This  series  multiple  magneto  system  was  used  with 
a  great  many  variations,  and  it  had  a  good  many  defects.  One  of 
these  defects  was  due  to  the  necessaiy  extending  of  one  limb  of  the 
line  through  a  large  number  of  series  contacts  in  the  jacks.  This 
is  not  to  be  desired  in  any  case,  but  it  was  particularly  objectionable 
in  the  early  days  before  jacks  had  been  developed  to  their  present 
high  state  of  perfection.  A  particle  of  dust  or  other  insulating 


424  TELEPHONY 

matter,  lodging  between  the  tip  spring  and  its  anvil  in  any  one  of  the 
jacks,  would  leave  the  line  open,  thus  disabling  the  line  to  incoming 
signals,  and  also  for  conversation  in  case  the  break  happened  to 
occur  between  the  subscriber  and  the  jack  that  was  used  in  con- 
necting with  the  line.  Another  defect  due  to  the  same  cause  was 
that  the  line  through  the  switchboard  was  always  unbalanced  by 
the  insertion  of  a  plug,  one  limb  of  the  line  always  extending  clear 
through  the  switchboard  to  the  drop  and  the  other,  when  the  plug 
was  inserted,  extending  only  part  way  through  the  switchboard  and 
being  cut  off  at  the  jack  where  the  connection  was  made.  The  ob- 
jection will  be  apparent  when  it  is  remembered  that  the  wires  in  the 
line  circuit  connecting  the  multiple  jacks  are  necessarily  very  closely 
bunched  together  and,  therefore,  there  is  very  likely  to  be  cross- 
talk between  two  adjacent  lines  unless  the  two  limbs  of  each  line  are 
exactly  balanced  throughout  their  entire  length. 

Again  the  busy-test  conditions  of  this  circuit  were  not  ideal. 
The  fact  that  the  test  rings  of  the  line  were  connected  permanently 
with  the  outside  line  circuit  subjected  these  test  rings  to  whatever 
potentials  might  exist  on  the  outside  lines,  due  to  any  causes  what- 
ever, such  as  a  cross  with  some  other  wire;  thus  the  test  rings  of  an 
idle  line  might  by  some  exterior  cause  be  raised  to  such  a  potential 
that  the  line  would  test  busy.  It  may  be  laid  down  as  a  fundamental 
principle  in  good  multiple  switchboard  practice  that  the  busy-test 
condition  should  be  made  independent  of  any  conditions  on  the  line 
circuit  outside  of  the  central  office,  and  such  is  not  the  case  in  this 
circuit  just  described. 

Branch=Terminal  Multiple  Board.  The  next  important  step 
in  the  development  of  the  magneto  multiple  switchboard  was  that 
which  produced  the  so-called  branch-terminal  board.  This  came 
into  wide  use  in  the  various  Bell  operating  companies  before  the 
advent  of  the  common-battery  systems.  Its  circuits  and  the  princi- 
ples of  operation  may  be  understood  in  connection  with  Fig.  338. 
In  the  branch-terminal  system  there  are  no  series  contacts  in  the 
jacks  and  no  unbalancing  of  the  line  due  to  a  cutting  off  of  a  portion 
of  the  line  circuit  when  a  connection  was  made  with  it.  Further- 
more, the  test  circuits  were  entirely  local  to  the  central  office  and 
were  not  likely  to  be  affected  by  outside  conditions  on  the  line. 
This  switchboard  also  added  the  feature  of  the  automatic  restora- 


THE  MAGNETO  MULTIPLE  SWITCHBOARD 


425 


tion  of  the  drops,  thus  relieving  the  operator  of  the  burden  of  doing 
that  manually,  and  also  permitting  the  drops  to  be  mounted  on  a 
portion  of  the  switchboard  that  was  not  available  for  the  mounting 
of  jacks,  and  thus  permitting  a  greater  capacity  in  jack  equipment. 
Each  jack  has  five  contacts,  and  the  answering  and  multiple 
jacks  are  alike,  both  in  respect  to  their  construction  and  their  con- 
nection with  the  line.  The  drops  are  the  electrically  self-restoring 
type  shown  m  Fig.  263.  The  line  circuits  extended  permanently 
from  the  subscriber's  station  to  the  line  winding  of  the  drop  and  the 

STAT/Ort -A- 


STAT/Ofi  -B- 


Fig.  338.     Branch-Terminal  Magneto  Multiple  Switchboard 

two  limbs  of  the  line  branched  off  to  the  tip  and  sleeve  contacts  1 
and  2  respectively  of  each  jack.  Another  pair  of  wires  extended 
through  the  multiple  parallel  to  the  line  wires  and  these  branched  off 
respectively  to  the  contact  springs  3  and  4  of  each  of  the  jacks. 
This  pair  of  wires  formed  portions  of  the  drop-restoring  circuit, 
including  the  restoring  coil  6  and  the  battery  7,  as  indicated.  The 
test  thimble  5  of  each  of  the  jacks  is  connected  permanently  with  the 
spring  3  of  the  corresponding  jack  and,  therefore,  with  the  wire 
which  connects  through  the  restoring  coil  6  of  the  corresponding 
drop  to  ground  through  the  battery  7. 


426  TELEPHONY 

The  plugs  were  each  provided  with  three  contacts.  Two  of 
these  were  the  usual  tip  and  sleeve  contacts  connected  with  the 
two  strands  of  the  cord  circuit.  The  third  contact  8  was  not  connected 
with  any  portion  of  the  cord  circuit,  being  merely  an  insulated 
contact  on  the  plug  adapted,  when  the  plug  was  fully  inserted,  to 
connect  together  the  springs  3  and  4-  The  cord  circuit  itself  is  readily 
understood  from  the  drawing,  having  two  features,  however,  which 
merit  attention.  One  is  the  establishing  of  a  grounded  battery 
connection  to  the  center  portion  of  the  winding  of  the  receiver  for 
the  purposes  of  the  busy  test,  and  the  other  is  the  provision  of  a  re- 
storing coil  and  restoring  circuit  for  the  clearing-out  drop,  this  cir- 
cuit being  closed  by  an  additional  contact  on  the  listening  key  so  as 
to  restore  the  clearing-out  drop  whenever  the  listening  key  was  oper- 
ated. 

Operation.  An  understanding  of  the  operation  of  this  system 
is  easy.  The  turning  of  the  subscriber's  generator,  when  the  line 
was  in  its  normal  condition,  caused  the  display  of  the  line  signal. 
The  insertion  of  the  answering  plug,  in  response  to  this  call,  did 
three  things:  (1)  It  extended  the  line  circuit  to  the  tip  and  sleeve 
strand  of  the  cord  circuit.  (2)  It  energized  the  restoring  coil  6 
of  the  drop  by  establishing  the  circuit  from  the  contact  spring  3 
through  the  plug  contact  8  to  the  other  contact  spring  4,  thus  com- 
pleting the  circuit  between  the  two  normally  open  auxiliary  wires. 
(3)  The  connecting  of  the  springs  3  and  4  established  a  connection 
from  ground  to  the  test  thimbles  of  all  the  jacks  on  a  line,  the  spring 
4  being  always  grounded  and  the  spring  3  being  always  connected 
to  the  test  thimble  5. 

It  is  to  be  noted  that  on  idle  lines  the  test  rings  are  always  at 
the  same  potential  as  the  ungrounded  pole  of  the  battery  7,  being 
connected  thereto  through  the  winding  6  of  the  restoring  coil.  On 
all  busy  lines,  however,  the  test  rings  are  dead  grounded  through  the 
contact  8  of  the  plug  that  is  connected  with  the  line. 

The  tip  of  the  testing  plug  at  the  time  of  making  a  test  will  also 
be  at  the  same  potential  as  that  of  the  ungrounded  pole  of  the  bat- 
tery 7,  since  this  pole  of  the  battery  7  is  always  connected  to  the  cen- 
ter portion  of  the  operator's  receiver  winding,  and  when  the  listen- 
ing key  is  thrown  the  tip  of  the  calling  plug  is  connected  therewith 
and  is  at  the  same  potential.  When,  therefore,  the  operator  touches 


THE  MAGNETO  MULTIPLE  SWITCHBOARD         427 

the  tip  of  the  calling  plug  to  the  test  thimble  of  an  idle  line,  she  will 
get  no  click,  since  the  tip  of  the  plug  and  the  test  thimble  will  be  at 
the  same  potential.  If,  however,  the  line  has  already  been  switched 
at  another  section  of  the  board,  there  will  be  a  difference  of  potential, 
because  the  test  thimble  will  be  grounded,  and  the  circuit,  through 
which  the  current  which  causes  the  click  flows,  may  be  traced  from 
the  ungrounded  pole  of  the  battery  7  to  the  center  portion  of  the 
operator's  receiver,  thence  through  one-half  of  the  winding  to  the 
tip  of  the  calling  plug,  thence  to  the  test  thimble  of  the  jack  under 
test,  thence  to  the  spring  3  of  the  jack  on  another  section  at  which  the 
connection  exists,  through  the  contact  8  on  the  plug  of  that  jack  to 
the  spring  4,  and  thence  to  ground  and  back  to  the  other  terminal  of 
the  battery  7. 

Magnet  Windings.  Coils  of  the  line  and  clearing-out  drops 
by  which  these  drops  are  thrown,  are  wound  to  such  high  resistance 
and  impedance  as  to  make  it  proper  to  leave  them  permanently 
bridged  across  the  talking  circuit.  The  necessity  for  cutting  them 
out  is,  therefore,  done  away  with,  with  a  consequent  avoidance,  in 
the  case  of  the  line  drops,  of  the  provision  of  series  contacts  in  the 
jacks. 

Arrangement  of  Apparatus.  In  boards  of  this  type  the  line 
and  clearing-out  drops  were  mounted  in  the  extreme  upper  portion 
of  the  switchboard  face  so  as  to  be  within  the  range  of  vision  of  the 
operator,  but  yet  out  of  her  reach.  Therefore,  the  whole  face  of  the 
board  that  was  within  the  limit  of  the  operator's  reach  was  available 
for  the  answering  and  multiple  jacks.  A  front  view  of  a  little  over 
one  of  the  sections  of  the  switchboard,  involving  three  complete 
operator's  positions,  is  shown  in  Fig.  339,  which  is  a  portion  of  the 
switchboard  installed  by  the  Western  Electric  Company  in  one  of 
the  large  exchanges  in  Paris,  France.  (This  has  recently  been 
replaced  by  a  common-battery  multiple  board.)  In  this  the  line 
drops  may  be  seen  at  the  extreme  top  of  the  face  of  the  switchboard, 
and  immediately  beneath  these  the  clearing-out  drops.  Beneath 
these  are  the  multiple  jacks  arranged  in  banks  of  one  hundred,  each 
hundred  consisting  of  five  strips  of  twenty.  At  the  extreme  lower 
portion  of  the  jack  space  are  shown  the  answering  jacks  and  be- 
neath these  on  the  horizontal  shelf,  the  plugs  and  keys.  These  jacks 
were  mounted  on  ^-inch  centers,  both  vertically  and  horizontally, 


428 


TELEPHONY 


and  each  section  had  in  multiple  90  banks  of  100  each,  making  9,000 
in  all.  Subsequent  practice  has  shown  that  this  involves  too  large 
a  reach  for  the  operators  and  that,  therefore,  9,000  is  too  large  a  num- 


Fig.  339.     Face  of  Magneto  Multiple  Switchboard 

her  of  jacks  to  place  on  one  section  if  the  jacks  are  not  spaced  closer 
than  on  |-inch  centers.  With  the  jack  involving  as  many  parts  as 
that  required  by  this  branch  terminal  system,  it  was  hardly  feasible 
to  make  them  smaller  than  this  without  sacrificing  their  durability, 


THE  MAGNETO  MULTIPLE  SWITCHBOARD 


429 


and  one  of  the  important  features  of  the  common-battery  multiple 
system  which  has  supplanted  this  branch-terminal  magneto  system 
is  that  the  jacks  are  of  such  a  simple  nature  as  to  lend  themselves  to 
mounting  on  f-inch  centers,  and  in  some  cases  on  T3B-inch  centers. 

Modern  Magneto  Multiple  Board.  Coming  now  to  a  considera- 
tion of  modern  magneto  multiple  switchboards,  and  bearing  in  mind 
that  such  boards  are  to  be  found  in  modern  practice  only  in  compara- 
tively small  installations  and  then  only  under  rather  peculiar  condi- 
tions, as  already  set  forth,  we  will  consider  the  switchboard  of  the 
Monarch  Telephone  Manufacturing  Company  as  typical  of  good 
practice  in  this  respect. 

Line  Circuit.     The  line  and  cord  circuits  of  the  Monarch  sys- 


AM5WER/M6   JACK 

Fig.  340.     Monarch  Magneto  Multiple  Switchboard  Circuits 

tern  are  shown  in  Fig.  340.  It  will  be  seen  that  each  jack  has  in 
all  five  contacts,  numbered  from  1  to  5  respectively,  of  which  1  and 
4  are  the  springs  which  register  with  the  tip  and  ring  contacts  of  the 
plug  and  through  which  the  talking  circuit  is  continued,  while  2  and 
8  are  series  contacts  for  cutting  off  the  line  drop  when  a  plug  is  inserted, 


430  TELEPHONY 

and  5  is  the  test  contact  or  thimble  adapted  to  register  with  the  sleeve 
contact  on  the  plug  when  the  plug  is  fully  inserted.  The  line  cir- 
cuit through  the  drop  may  be  traced  normally  from  one  side  of  the 
line  through  the  drop  coil,  thence  through  all  of  the  pairs  of  springs 
2  and  3  in  the  jacks  of  that  line,  and  thence  to  spring  1  of  the  last  jack, 
this  spring  always  being  strapped  to  the  spring  2  in  the  last  jack,  and 
thence  to  the  other  side  of  the  line.  All  the  ring  springs  1  are  per- 
manently tapped  on  to  one  side  of  the  line,  and  all  of  the  tip  springs 
4  are  permanently  tapped  td  the  other  side  of  the  line.  This  system 
may,  therefore,  properly  be  called  a  branch-terminal  system.  It 
is  seen  that  as  soon  as  a  plug  is  inserted  into  any  of  the  jacks,  the 
circuit  through  the  drop  will  be  broken  by  the  opening  of  the  springs 
2  and  3  in  that  jack.  The  drop  shown  immediately  above  the 
answering  jack  is  so  associated  mechanically  with  that  jack  as  to  be 
mechanically  self-restored  when  the  answering  plug  is  inserted  into  the 
answering  jack  in  response  to  a  call.  The  arrangement  in  this  re- 
spect is  the  same  as  that  shown  in  Fig.  259,  illustrating  the  Monarch 
combined  drop  and  jack. 

Cord  Circuit.  The  cord  circuit  needs  little  explanation.  The 
tip  and  ring  strands  are  the  ones  which  carry  the  talking  current 
and  across  these  is  bridged  the  double-wound  clearing-out  drop,  a 
condenser  being  included  in  series  in  the  tip  strand  between  the 
two  drop  windings  in  the  manner  already  explained  in  connection 
with  Fig.  284.  The  third  or  sleeve  strand  of  the  cord  is  continuous 
from  plug  to  plug,  and  between  it  and  the  ground  there  is  perma- 
nently connected  a  retardation  coil. 

Test.  The  test  is  dependent  on  the  presence  or  absence  of 
a  path  to  ground  from  the  test  thimbles  through  some  retardation 
coil  associated  with  a  cord  circuit.  Obviously,  in  the  case  of  an 
idle  line  there  will  be  no  path  to  ground  from  the  test  thimbles,  since 
normally  they  are  merely  connected  to  each  other- and  are  insulated 
from  everything  else.  When,  however,  a  plug  is  inserted  into  a 
multiple  or  answering  jack,  the  test  thimbles  of  that  line  are  connected 
to  ground  through  the  retardation  coil  associated  with  the  third 
strand  of  the  plug  used  in  making  the  connection.  When  the  oper- 
ator applies  the  tip  of  the  calling  plug  to  a  test  contact  of  a  multiple 
jack  there  will  be  no  path  to  ground  afforded  if  the  line  is  idle,  while 
if  it  is  busy  the  potential  of  the  tip  of  the  test  plug  will  cause  a  cur- 


THE  MAGNETO  MULTIPLE  SWITCHBOARD         431 

rent  to  flow  to  ground  through  the  impedance  coil  associated  with 
the  plug  used  in  making  the  connection.  This  will  be  made  clearer 
by  tracing  the  test  circuit.  With  the  listening  key  thrown  this  may 
be  traced  from  the  live  side  of  the  battery  through  the  retardation 
coil  6,  which  is  common  to  an  operator's  position,  thence  through 
the  tip  side  of  the  listening  key  to  the  tip  conductor  of  the  calling  cord, 
and  thence  to  the  tip  of  the  calling  plug  and  the  thimble  of  the  jack 
under  test.  If  the  line  is  idle  there  will  be  no  path  to  ground  from 


Fig.  341.     Magneto  Multiple  Switchboard 

this  point  and  no  click  will  result,  but  if  the  line  is  busy,  current  will 
flow  from  the  tip  of  the  test  plug  to  the  thimble  of  the  jack  tested, 
thence  by  the  test  wire  in  the  multiple  to  the  thimble  of  the  jack  at 
which  a  connection  already  exists,  and  thence  to  ground  through  the 
third  strand  of  the  cord  used  in  making  that  connection  and  the 
impedance  coil  associated  therewith.  The  current  which  flows  in 
this  test  circuit  changes  momentarily  the  potential  of  the  tip  side 
of  the  operator's  telephone  circuit,  thus  unbalancing  her  talking 
circuit  and  causing  a  click. 

If  this  test  system  were  used  in  a  very  large  board  where  the 


432  TELEPHONY" 

multiple  would  extend  through  a  great  many  sections,  there  would  be 
some  liability  of  a  false  test  due  to  the  static  capacity  of  the  test 
contacts  and  the  test  wire  running  through  the  multiple.  For  small 
boards,  however,  where  the  multiple  is  short,  this  system  has  proven 
reliable.  A  multiple  magneto  switchboard  employing  the  form  of 
circuits  just  described  is  shown  in  Fig.  341.  This  switchboard  con- 
sists of  three  sections  of  two  positions  each.  The  combined  answer- 
ing jacks  and  drops  may  be  seen  at  the  lower  part  of  the  face  of  the 
switchboard  and  occupying  somewhat  over  one-half  of  the  jack  and 
drop  space.  The  multiple  jacks  are  above  the  answering  jacks  and 
drops  and  it  may  be  noted  that  the  same  arrangement  and  number 
of  these  jacks  is  repeated  in  each  section.  This  switchboard  may 
be  extended  by  adding  more  sections  and  increasing  the  multiple  in 
those  already  installed  to  serve  1,600  lines. 

Assembly.     In  connection  with  the  assembly  of  these  magneto 
multiple  switchboards,  as  installed  by  the  Monarch  Company,  Fig. 

342  shows  the  details  of  the  cord  rack  at 
the  back  of  the  board.  It  shows  how  the 
ends  of  the  switchboard  cords  opposite  to 
the  ends  that  are  fastened  to  the  plugs  are 
connected  permanently  to  terminals  on  the 
cord  rack,  at  which  point  the  flexible  con- 
ductors are  brought  out  to  terminal  clips  or 
binding  posts,  to  which  the  wires  leading 
from  the  other  portions  of  the  cord  circuit 
are  led.  In  order  to  relieve  the  conductors 
in  the  cords  from  strain,  the  outer  braiding 
of  the  cord  at  the  rack  end  is  usually  ex- 

Fig'  342'  neCc°torsRack  °on"     tended  to  form  what  is  called  a  strain  cord, 
and   this   attached   to  an  eyelet  under  the 

cord  rack,  so  that  the  weight  of  the  cord  and  the  cord  weights  will 
be  borne  by  the  braiding  rather  than  by  the  conductors.  This 
leaves  the  insulated  conductors  extending  from  the  ends  of  the 
cords  free  to  hang  loose  without  strain  and  be  connected  to  the 
terminals  as  shown.  This  method  of  connecting  cords,  with  vari- 
ations in  form  and  detail,  is  practically  universal  in  all  types  of 
switchboards. 

A  detail  of  the  assembly  of  the  drops  and  jacks  in  such  a  switch- 


THK  MAGNETO  MULTIPLE  SWITCHBOARD 


433 


board  is  shown  in  Fig.  343.  The  single  pair  of  clearing-out  drops 
is  mounted  in  the  lower  part  of  the  vertical  face  of  the  switchboard 
just  above  the  space  occupied  by  the  plug  shelf.  Vertical  stile  strips 
extend  above  the  clearing-out  drop  space  for  supporting  the  drops 


Fig.   343.     Drop  and  Jack  Mounting 


and  jacks.  A  single  row  of  10  answering  jacks  and  the  correspond- 
ing line  drops  are  shown  in  place.  Above  these  there  would  be 
placed,  in  the  completely  assembled  board,  the  other  answering  jacks 
and  line  signals  that  were  to  occupy  this  panel,  and  above  these  the 


Fig.  344.     Keyboard  Wiring 


strips  ot  multiple  jacks.  The  rearwardly  projecting  pins  from  the 
stile  strips  are  for  the  support  of  the  multiple  jack  strips,  these  pins 
supporting  the  strips  horizontally  by  suitable  multiple  clips  at  the 
ends  of  the  jack  strips ;  the  jack  strips  being  fastened  from  the  rear 


434  TELEPHONY 

by  means  of  nuts  engaging  the  screw  threads  on  these  pins.  This 
method  of  supporting  drops  and  jacks  is  one  that  is  equally  adapt- 
able for  use  in  other  forms  of  boards,  such  as  the  simple  magneto 
switchboard. 

In  Fig.  344  is  shown  a  detail  photograph  of  the  key  shelf  wir- 
ing in  one  of  these  Monarch  magneto  switchboards.  In  this  the 
under  side  of  the  keys  is  shown,  the  key  shelf  being  raised  on  its  hinge 
for  that  purpose.  The  cable,  containing  all  of  the  insulated  wires 
leading  to  these  keys,  enters  the  space  under  the  key  shelf  at  the 
extreme  left  and  from  the  rear.  It  then  passes  to  the  right  of  this 
space  where  a  "knee"  is  formed,  after  which  the  cable  is  securely 
strapped  to  the  under  side  of  the  key  shelf.  By  this  construction 
sufficient  flexibility  is  provided  for  in  the  cable  to  permit  the  raising 
arid  lowering  of  the  key  shelf ,  the  long  reach  of  the  cable  between  the 
"knee"  and  the  point  of  entry  at  the  left  serving  as  a  torsion  member, 
so  that  the  raising  of  the  shelf  will  give  the  cable  a  slight  twist  rather 
than  bend  it  at  a  sharp  angle. 


CHAPTER  XXVI 


THE  COMMON=BATTERY  MULTIPLE  SWITCHBOARD 

Western  Electric  No.  1  Relay  Board.  The  common-battery 
multiple  switchboard  differs  from  the  simple  or  non-multiple  com- 
mon-battery switchboard  mainly  in  the  provision  of  multiple  jacks 
arid  in  the  added  features  which  are  involved  in  the  provision  for  a 
busy  test.  The  principles  of  signaling  and  of  supplying  current 
to  the  subscribers  for  talking  are  the  same  as  in  the  non-multiple 
common-battery  board.  For  purposes  of  illustrating  the  practical 
workings  of  the  common-battery  multiple  switchboard,  we  will 
take  the  standard  form  of  the  Western  Electric  Company,  choosing 
this  only  because  it  is  the  standard  with  nearly  all  the  Bell  operating 
companies  throughout  the  United  States. 

Line  Circuit.  We  will  first  consider  the  line  circuit  in  simpli- 
fied form,  as  shown  in  Fig.  345.  At  the  left  in  this  figure  the  com- 


<STAT/Ofi 


Fig.  345.     Line  Circuit  Western  Electric  No.  1.  Board 


mon-battery  circuit  is  shown  at  the  subscriber's  station,  and  at  the 
right  the  central-office  apparatus  is  indicated  so  far  as  equipment 
of  a  single  line  is  concerned.  In  this  simplified  diagram  no  attempt 
has  been  made  to  show  the  relative  positions  of  the  various  parts, 
these  having  been  grouped  in  this  figure  in  such  a  way  as  to  give  as 
clear  and  simple  an  idea  as  possible  of  the  circuit  arrangements. 
It  is  seen  at  a  glance  that  this  is  a  branch  terminal  board,  the  three 
contacts  of  each  jack  being  connected  by  separate  taps  or  legs  to 
three  wires  running  throughout  the  length  of  the  board,  these  three 


436  TELEPHONY 

wires  being  individual  to  the  jacks  of  one  line.  On  this  account 
this  line  circuit  is  commonly  referred  to  as  a  three-wire  circuit.  By 
the  same  considerations  it  will  be  seen  that  the  switchboard  line 
circuit  of  the  branch-terminal  multiple  magneto  system,  shown  in 
Fig.  338,  would  be  called  a  four-wire  circuit.  It  will  be  shown 
later  that  other  multiple  switchboards  in  wide  use  have  a  still  further 
reduction  in  the  number  of  wires  running  through  the  jacks,  or 
through  the  multiple  as  it  is  called,  such  being  referred  to  as  two- 
wire  switchboards. 

The  two  limbs  of  the  line  which  extend  from  the  subscriber's 
circuit,  beside  being  connected  by  taps  to  the  tip  and  sleeve  contacts 
of  the  jack  respectively,  connect  with  the  two  back  contacts  of  a 
cut-off  relay,  and  when  this  relay  is  in  its  normal  or  unenergized 
condition,  these  two  limbs  of  the  line  are  continued  through  the  wind- 
ings of  the  line  relay  and  thence  one  to  the  ungrounded  or  negative 
side  of  the  common-battery  and  the  other  to  the  grounded  side. 
The  subscriber's  station  circuit  being  normally  open,  no  current 
flows  through  the  line,  but  when  the  subscriber  removes  his  receiver 
for  the  purpose  of  making  a  call  the  line  circuit  is  completed  and 
current  flows  through  the  coil  of  the  line  relay,  thus  energizing  that 
relay  and  causing  it  to  complete  the  circuit  of  the  line  lamp.  The 
cut-off  relay  plays  no  part  in  the  operation  of  the  subscriber's  calling, 
but  merely  leaves  the  circuit  of  the  line  connected  through  to  the 
calling  relay  and  battery.  The  coil  of  the  cut-off  relay  is  connected 
to  ground  on  one  side  and  on  the  other  side  to  the  third  wire  run- 
ning through  the  switchboard  multiple  and  which  is  tapped  off  to 
each  of  the  test  rings  on  the  jacks.  As  will  be  shown  later,  when 
the  operator  plugs  into  the  jack  of  a  line,  such  a  connection  is  estab- 
lished that  the  test  ring  of  that  jack  will  be  connected  to  the  live  or 
negative  pole  of  the  common  battery,  which  will  cause  current  to 
flow  through  the  coil  of  the  cut-off  relay,  which  will  then  operate  to 
cut  off  both  of  the  limbs  of  the  line  from  their  normal  connection 
with  ground  and  the  battery  and  the  line  relay.  Hence  the  name 
cut-off  relay. 

The  use  of  the  cut-off  relay  to  sever  the  calling  apparatus  from 
the  line  at  all  times  when  the  line  is  switched  serves  to  make  possible 
a  very  much  simpler  jack  than  would  otherwise  be  required,  as  will 
be  obvious  to  anyone  who  tries  to  design  a  common-battery  multi- 


COMMON-BATTERY   MULTIPLE   SWITCHBOARD     437 

pie  system  without  a  cut-off  relay.  The  additional  complication 
introduced  by  the  cut-off  relay  is  more  than  offset  by  the  saving  in 
complexity  of  the  jacks.  It  is  desirable,  on  account  of  the  great 
number  of  jacks  necessarily  employed  in  a  multiple  switchboard,  that 
the  jacks  be  of  the  simplest  possible  construction,  thus  reducing 
to  a  minimum  their  first  cost  and  making  them  much  less  likely 
to  get  out  of  order. 

Cord  Circuit.  The  cord  circuit  of  the  Western  Electric  stand- 
ard multiple  common-battery  switchboard  is  shown  in  Fig.  346.  This 
cord  circuit  involves  the  use  of  three  strands  in  the  flexible  cords 
of  both  the  calling  and  the  answering  plugs.  Two  of  these  are  the 
ordinary  tip  and  ring  conductors  over  which  speech  is  transmitted 
to  the  connected  subscriber's  wire.  The  third,  the  sleeve  strand,  car- 
ries the  supervisory  lamps  and  has  associated  with  it  other  apparatus 
for  the  control  of  these  lamps  and  of  the  test  circuit. 


Fig.  346.     Cord  Circuit  Western  Electric  No.  1  Board 

The  system  of  battery  feed  is  the  well-known  split  repeating- 
coil  arrangement  already  discussed.  The  tip  strand  runs  straight 
through  to  the  repeating  coil,  while  the  ring  strand  contains,  in  each 
case,  the  winding  of  the  supervisory  relay  corresponding  to  either 
the  calling  or  the  answering  plug.  In  order  that  the  presence  in  the 
talking  circuit  of  a  magnet  winding  possessing  considerable  im- 
pedance may  not  interfere  with  the  talking  efficiency,  each  of  these 
supervisory  relay  windings  is  shunted  by  a  non-inductive  resistance. 
In  practice  the  supervisory  relay  windings  have  each  a  resistance 
of  about  20  ohms  and  the  shunt  around  them  each  a  resistance  of 
about  31  ohms.  In  the  third  strand  of  each  cord  is  placed  a  12- 
volt  supervisory  lamp,  and  in  series  with  it  a  resistance  of  about  80 
ohms.  Each  supervisory  relay  is  adapted,  when  energized,  to  close 


438  TELEPHONY 

a  40-ohm  shunt  about  its  supervisory  lamp.  The  arrangement  and 
proportion  of  these  resistances  is  such  that  when  a  plug  is  inserted 
into  the  jack  of  a  line  ^the  lamp  will  receive  current  from  a  circuit 
traced  from  the  negative  pole  of  the  battery  in  the  center  of  the  cord 
circuit  through  the  lamp  and  the  80-ohm  series  resistance,  through 
the  third  strand  of  the  cord  to  the  test  thimble  of  the  jack,  and  thence 
to  the  positive  or  grounded  pole  of  the  batteiy  through  the  third 
conductor  in  the  multiple  and  the  winding  of  the  cut-off  relay.  This 
current  always  flows  as  long  as  the  plug  is  inserted,  and  it  is  just 
sufficient  to  illuminate  the  lamp  when  the  supervisory  relay  arma- 
ture is  not  attracted.  When,  however,  the  supervisory  relay  arma- 
ture is  attracted,  the  shunting  of  the  lamp  by  the  40-ohm  resistance 
cuts  down  the  current  to  such  a  degree  as  to  prevent  the  illumination 
of  the  lamp,  although  some  current  still  flows  through  it. 

The  usual  ringing  and  listening  key  is  associated  with  the  call- 
ing plug,  and  in  some  cases  a  ring-back  key  is  associated  with  the 
answering  plug,  but  this  is  not  standard  practice. 

Operation.  The  operation  of  this  cord  circuit  in  conjunction 
with  the  line  circuit  of  Fig.  345  may  best  be  understood  by  reference 
to  Fig.  347.  This  figure  employs  a  little  different  arrangement  of 
the  line  circuit  in  order  more  clearly  to  indicate  how  the  two  lines 
may  be  connected  by  a  cord;  a  study  of  the  two  line  circuits,  how- 
ever, will  show  that  they  are  identical  in  actual  connections.  It  is 
to  be  remembered  that  all  of  the  battery  symbols  shown  in  this 
figure  represent  in  reality  the  same  battery,  separate  symbols  being 
shown  for  greater  simplicity  in  circuit  connections. 

We  will  assume  the  subscriber  at  Station  A  calls  for  the  sub- 
scriber at  Station  B.  The  operation  of  the  line  relay  and  the  conse- 
quent lighting  of  the  line  lamp,  and  also  the  operation  of  the  pilot 
relay  will  be  obvious  from  what  has  been  stated.  The  response  of 
the  operator  by  inserting  the  answering  plug  into  the  answering  jack, 
and  the  throwing  of  her  listening  key  so  as  to  bridge  her  talking  cir- 
cuit across  the  cord  in  order  to  place  herself  in  communication  with 
the  subscriber,  is  also  obvious.  The  insertion  of  the  answering 
plug  into  the  answering  jack  completed  the  circuit  through  the  third 
strand  of  the  cord  and  the  winding  of  the  cut-off  relay  of  the  calling 
line,  and  this  accomplishes  three  desirable  results.  The  circuit  so 
completed  may  be  traced  from  the  negative  or  ungrounded  side  of 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      439 

the  battery  to  the  center  portion  of  the  cord  circuit,  thence  through 
the  supervisory  lamp  1,  resistance  2,  to  the  third  conductor  on  the 
plug,  test  thimble  on  the  jack,  thence  through  the  winding  of  the 
cut-off  relay  to  ground,  which  forms  the  other  terminal  of  the  bat- 
tery. The  results  accomplished  by  the  closing  of  this  circuit  are: 
first,  the  energizing  of  the  cut-off  relay  to  cut  off  the  signaling  por- 
tion of  the  line;  second,  the  flowing  of  current  through  the  lamp  that 


s  TAT /on  -B- 


v -  "X" 


Fig.  347.     Western  Electric  No.  1  Board 


is  almost  sufficient  to  illuminate  it,  but  not  quite  so  because  of  the 
closure  of  the  shunt  about  it,  for  the  reason  that  will  be  described; 
third,  the  raising  of  the  potential  of  all  the  contact  thimbles  on  the 
jacks  from  zero  to  a  potential  different  from  that  of  the  ground  and 
equal  in  amount  to  the  fall  of  potential  through  the  winding  of  the 
cut-off  relay.  A  condition  is  thus  established  at  the  test  rings  such 
that  some  other  operator  at  some  other  section  in  testing  the  line 
will  find  it  busy  and  will  not  connect  with  it. 

The  reason  why  the  lamp  J,  connected  with  the  answering  plug, 
was  not  lighted  was  that  the  supervisory  relay  3,  associated  with  the 


440  TELEPHONY 

answering  plug,  became  energized  when  the  operator  plugged  in, 
due  to  the  flow  of  current  from  the  battery  through  the  calling  sub- 
scriber's talking  apparatus,  this  flow  of  current  being  permitted  by 
the  removal  of  the  calling  subscriber's  receiver  from  its  hook.  The 
energjzing  of  this  relay  magnet  by  causing  the  attraction  of  its  arm- 
ature, closed  the  shunt  about  the  lamp  1,  which  shunt  contains  the 
40-ohm  resistance  4,  and  thus  prevents  the  lamp  from  receiving  enough 
current  to  illuminate  it.  Obviously,  as  soon  as  the  calling  sub- 
scriber replaces  his  receiver  on  its  hook,  the  supervisory  relay  3  will 
be  de-energized,  the  shunt  around  the  lamp  will  be  broken,  and  the 
lamp  will  be  illuminated  to  indicate  to  the  operator  the  fact  that  the 
subscriber  with  whose  line  her  calling  plug  is  connected  has  re- 
placed his  receiver  on  its  hook. 

Testing — Called  Line  Idle.  Having  now  shown  how  the  oper- 
ator connects  with  the  calling  subscriber's  line  and  how  that  line 
automatically  becomes  guarded  as  soon  as  it  is  connected  with,  so 
that  no  other  operator  will  connect  with  it,  we  will  discuss  how  the 
operator  tests  the  called  line  and  subsequently  connects  with  that 
line,  if  it  is  found  proper  to  do  so.  If,  on  making  the  test  with  one 
of  the  multiple  jacks  of  the  line  leading  to  Station  B,  that  line  is  idle 
and  free  to  be  connected  with,  its  test  rings  will  all  be  at  zero  poten- 
tial because  of  the  fact  that  they  are  connected  with  ground  through 
the  cut-off  relay  winding  with  no  source  of  current  connected  with 
them.  The  tip  of  the  calling  plug  will  also  be  at  zero  potential  in 
making  this  test,  because  it  is  connected  to  ground  through  the  tip 
side  of  the  calling-plug  circuit  and  one  winding  of  the  cord-circuit 
repeating  coil.  As  a  result  no  flow  of  current  will  occur,  the  operator 
will  receive  no  click,  and  she  will  know  that  she  is  free  to  connect  with 
the  line.  As  soon  as  she  does  so,  by  inserting  the  plug,  the  third 
strand  of  the  cord  will  be  connected  with  the  test  thimble  of  the  calling 
line  and  the  resulting  flow  of  current  will  bring  about  three  results, 
two  of  which  are  the  same,  and  one  of  which  is  slightly  different  from 
those  described  as  .resulting  from  the  insertion  of  the  answering 
plug  into  the  jack  of  the  calling  line.  First,  the  cut-off  relay  will  be 
operated  and  cut  off  the  line  signaling  apparatus  from  the  called 
line;  second,  a  flow  of  current  will  result  through  the  calling  super- 
visory lamp  5,  which  in  this  case  will  be  sufficient  to  illuminate  that 
lamp  for  the  reason  that  the  called  subscriber  has  not  yet  responded, 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     441 

the  calling  supervisory  relay  6  has,  therefore,  not  yet  been  energized, 
and  the  lamp  has  not,  therefore,  been  shunted  by  its  associated  resist- 
ance 7;  third,  the  test  thimbles  of  the  called  line  will  be  raised  to  a 
potential  above  that  of  the  earth,  and  thus  the  line  will  be  guarded 
against  connection  at  another  section  of  the  switchboard.  As  soon 
as  the  called  subscriber  responds  to  the  ringing  current  sent  out 
by  the  operator,  current  will  flow  over  the  cord  circuit  and  over  his 
line  through  his  transmitter.  This  will  cause  the  calling  supervisory 
relay  to  be  energized  and  the  calling  lamp  to  be  extinguished.  Both 
lamps  1  and  5  remain  extinguished  as  long  as  the  connected  sub- 
scribers are  in  conversation,  but  as  soon  as  either  one  of  them  hangs  up 
his  receiver  the  corresponding  lamp  will  be  lighted,  due  to  the  de- 
energization  of  the  supervisory  relay  and  the  breaking  of  the  shunt 
around  the  lamp.  The  lighting  of  both  lamps  associated  with  a 
cord  circuit  is  a  signal  to  the  operator  for  disconnection. 

Testing — Called  Line  Busy.  If  we  now  assume  that  the  called 
line  was  already  busy,  by  virtue  of  being  connected  with  at  another 
section,  the  test  rings  of  that  line  would  accordingly  all  be  raised 
to  a  potential  above  that  of  the  earth.  As  a  result,  when  the  oper- 
ator applied  the  tip  of  her  calling  plug  to  a  test  thimble  on  that  line, 
current  would  flow  from  this  test  thimble  through  the  tip  of  the 
calling  plug  and  tip  strand  of  the  cord  and  through  one  winding 
of  the  cord-circuit  repeating  coil  to  ground.  This  would  cause  a 
slight  raising  of  potential  of  the  entire  tip  side  of  the  cord  circuit  and 
a  consequent  momentary  flow  of  current  through  the  secondary  of 
the  operator  s  circuit  bridged  across  the  cord  circuit  at  that  time. 

Operator's  Circuit  Details.  The  details  of  the  operator's  talk- 
ing circuit  shown  in  Fig.  347  deserve  some  attention.  The  battery 
supply  to  the  operator's  transmitter  is  through  an  impedance  coil  9. 
The  condenser  12  is  bridged  around  the  transmitter  and  the  two 
primary  windings  10  and  11,  which  windings  are  in  parallel  so  as  to 
afford  a  local  circuit  for  the  passage  of  fluctuating  currents  set  up  by 
the  transmitter.  .  The  two  primary  windings  10  and  11  are  on  sep- 
arate induction  coils,  the  secondary  windings  13  and  14  being,  there- 
fore, on  separate  cores.  The  winding  15,  in  circuit  with  the  second- 
ary winding  14  and  the  receiver,  is  a  non-inductive  winding  and  is 
supposed  to  have  a  resistance  about  equal  to  the  effective  resistance 
to  fluctuating  currents  of  a  subscriber's  line  of  average  length.  Ow- 


442  TELEPHONY 

ing  to  the  respective  directions  of  the  primary  and  secondary  windings 
10  and  11,  13  and  14,  the  result  is  that  the  outgoing  currents  set  up 
by  the  operator's  transmitter  are  largely  neutralized  in  the  operator's 
receiver.  Incoming  currents  from  either  of  the  connected  sub- 
scribers, however,  pass,  in  the  main,  through  the  secondary  coil  13 
and  the  operator's  receiver,  rather  than  through  the  shunt  path 
formed  by  the  secondary  14,  and  the  non-inductive  resistance  15. 
This  is  known  as  an  "anti-side  tone"  arrangement,  and  its  object  is 
to  prevent  the  operator  from  receiving  her  own  voice  transmission 
so  loudly  as  to  make  her  ear  insensitive  to  the  feebler  voice  currents 
coming  in  from  the  subscribers. 

Order-Wire  Circuits.  The  two  keys  16  and  17,  shown  in  con- 
nection with  the  operator's  talking  circuit  in  Fig.  347,  play  no  part 
in  the  regular  operation  of  connecting  two  local  lines,  as  described 
above.  They  are  order-wire  keys,  and  the  circuits  with  which  they 
connect  lead  to  the  telephone  sets  of  other  operators  at  distant  central 
offices,  and  by  pressing  either  one  of  these  keys  the  operator  is  en- 
abled to  place  herself  in  communication  over  these  so-called  order- 
wire  circuits  with  such  other  operators.  The  function  and  mode 
of  operation  of  these  order-wire  circuits  will  be  described  in  the  next 
chapter,  wherein  inter-office  connections  will  be  discussed. 

Wiring  of  Line  Circuit.  The  line  circuits  shown  in  Figs.  345 
and  347  are,  as  stated,  simplified  to  facilitate  understanding,  although 
the  connections  shown  are  those  which  actually  exist.  The  more 
complete  wiring  of  a  single  line  circuit  is  shown  in  Fig.  348.  The 
line  wires  are  shown  entering  at  the  left.  They  pass  immediately, 
upon  entering  the  central  office,  through  the  main  distributing  frame, 
the  functions  and  construction  of  which  will  be  considered  in  detail 
in  a  subsequent  chapter.  The  dotted  portions  of  the  circuit  shown 
in  connection  with  this  main  distributing  frame  indicate  the  path 
from  the  terminals  on  one  side  of  the  frame  to  those  on  the  other 
through  so-called  jumper  wires.  The  two  limbs  of  the  line  then 
pass  to  terminals  1  and  2  on  one  side  of  the  so-called  intermediate 
distributing  frame.  Here  the  circuit  of  each  limb  of  the  line  divides, 
passing,  on  the  one  hand,  to  the  tip  and  sleeve  springs  of  all  the  mul- 
tiple jacks  belonging  to  that  line;  and,  on  the  other  hand,  through 
the  jumper  wires  indicated  by  dotted  lines  on  the  intermediate  dis- 
tributing frame,  and  thence  to  the  tip  and  ring  contacts  of  the  an- 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     443 

swering  jack.  A  consideration  of  this  connection  will  show  that 
the  actual  electrical  connections  so  far  as  already  described  are 
exactly  those  of  Figs.  345  and  347,  although  those  figures  omitted  the 
main  and  intermediate  distributing  frames.  Only  two  limbs  of  the 
line  are  involved  in  the  main  frame.  In  the  intermediate  frame 
the  test  wire  running  through  the  multiple  is  also  involved.  This 
test  wire,  it  will  be  seen,  leads  from  the  test  thimbles  of  all  the 
multiple  jacks  to  the  terminal  3  on  the  intermediate  frame,  thence 
through  the  jumper  wire  to  the  terminal  6  of  this  frame,  and  to 
the  test  thimble  of  the  answering  jack.  Here  again  the  electrical 
connections  are  exactly  those  represented  in  Figs.  345  and  347,  al- 
though those  figures  do  not  show  the  intermediate  frame. 

The  two  terminals  4  and  5  of  the  intermediate  frame,  besides 
being  connected  to  the  tip  and  sleeve  springs  of  the  answering  jack, 
are  connected  to  the  contacts  of  the  cut-off  relay,  and  thence  through 
the  coils  of  the  line  relay  to  ground  on  one  side  and  to  battery  on 
the  other.  Thus  the  line  relay  and  battery  are  normally  included  in 
the  circuit  of  the  line.  The  contact  6  on  the  intermediate  distrib- 
uting frame,  besides  being  connected  to  the  test  thimble  of  all  the 
jacks,  is  connected  through  the  coil  of  the  cut-off  relay  to  ground, 
thus  establishing  a  path  by  which  current  is  supplied  to  the  cut-off 
relay  when  connection  is  made  to  the  line  at  any  jack.  There  is 
another  contact  7  on  the  intermediate  distributing  frame  which 
merely  forms  a  terminal  for  joining  one  side  of  the  line  lamp  to  the 
back  contact  of  the  line  relay. 

Functions  of  Distributing  Frames.  Since  the  line  circuit  thus 
far  described  in  connection  with  Fig.  348  is  exactly  the  same  as  that 
of  Fig.  345  in  its  electrical  connections,  it  becomes  obvious  that  the 
main  and  intermediate  distributing  frames  play  no  part  in  the  oper- 
ation of  the  circuit  any  more  than  a  binding  post  of  a  telephone 
plays  a  part  in  its  operation.  These  frames  carry  terminals  for 
facilitating  the  connection  of  the  various  wires  in  the  line  circuit  and, 
as  will  be  shown  later,  for  facilitating  certain  changes  in  the  line 
connection. 

Remembering  that  the  dotted  lines  in  Fig.  348  indicate  jumper 
wires  of  the  main  and  intermediate  distributing  frames,  and  that 
these  are  in  the  nature  of  temporary  or  readily  changeable  connec- 
tions, and  that  the  full  lines,  whether  heavy  or  light,  are  permanent 


144 


TELEPHONY 


connections  not  readily  changeable,  it  will  be  seen  that  the  wires 
leading  through  the  multiple  jacks  of  a  certain  line  are  permanently 
associated  with  each  other,  and  with  certain  terminals  on  the  main 
distributing  frame  and  certain  other  terminals  on  the  intermediate 


MA/M  DJSTR/BUT/MG 
T      FRA^ME 

L/rtE 


/MTERMED/ATE 
5Tff/BU 
FRAME 


TO  OTHER 
L/rtE  LAMPS 


CUT-OFF 
RELAY 


rt/GHTBELL. 
RELAY  COM  MOM 
TO  EttT/RE.  OFF/CE 


GErtERA  TORrfrt/GHT 
BELL 


Fig.  348.     Line  Circuit  No.  1  Board 

distributing  frame.  It  will  also  be  seen  that  the  line  lamp  and  the 
answering  jack,  together  with  the  cut-off  relay  and  line  relay,  are  per- 
manently associated  with  each  other  and  with  another  group  of 
terminals  4,  5,  6,  and  7  on  me  intermediate  distributing  frame.  It 
will  also  be  apparent  that  by  changing  the  jumper  wires  on  the  main 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     445 

frame,  any  outside  line  may  be  connected  with  any  different  set  of 
line  switchboard  equipment,  and  also  that  by  making  changes  in  the 
jumper  wires  on  the  intermediate  frame,  any  given  answering  jack 
and  line  lamp  with  its  associated  line  cut-off  relay  may  be  associated 
with  any  set  of  multiple  jacks. 

Pilot  Signals.  In  a  portion  of  the  circuit  leading  from  the 
battery  that  is  common  to  a  group  of  line  lamps  is  the  winding  of 
the  pilot  relay,  which  is  common  to  this  group  of  line  lamps.  This 
controls,  as  already  described,  the  circuit  of  the  pilot  lamp  common 
to  the  same  group  of  line  lamps.  In  addition,  a  night-bell  circuit 
is  sometimes  provided,  this  usually  being  in  the  form  of  an  ordinary 
polarized  ringer,  the  circuit  of  which  is  controlled  by  a  night-bell 
relay  common  to  the  entire  office.  Normally,  this  relay  is  shunted 
out  of  the  circuit  of  the  common  portion  of  the  lead  to  the  pilot 
relay  contacts  by  the  key  5,. but  when  the  key  8  is  opened  all  current 
that  is  fed  to  the  pilot  lamps  passes  through  the  night-bell  relay, 
and  thus,  whenever  any  pilot  lamp  is  lighted,  the  night-bell  relay  will 
attract  its  armature  and  thus  close  the  circuit  of  the  calling  generator 
through  the  night  bell. 

A  study  of  this  figure  will  make  clear  to  the  student  how  the 
portions  of  the  circuit  that  are  individual  to  the  line  are  associated 
with  such  things  as  the  battery,  that  are  common  to  the  entire  office, 
and  such  as  the  pilot  relay  and  lamp,  that  are  common  to  a  group  of 
lines  terminating  in  one  position. 

Modified  Relay  Windings.  In  some  cases,  the  line  relay  instead 
of  being  double  wound,  as  shown,  is  made  with  a  single  winding, 
this  winding  being  normally  included  between  the  ring  side  of  the 
cut-off  relay  and  the  battery,  the  tip  side  of  the  cut-off  relay  being 
run  direct  to  ground.  The  present  practice  of  the  Western  Electric 
Company  is  towards  the  double-wound  relay,  however,  and  that  is 
considered  standard  in  all  of  their  large  No.  1  multiple  boards,  except 
where  the  customer,  owing  to  special  reasons,  demands  a  single 
wound  relay  on  the  ring  side  of  the  line.  The  prime  reason  for  the 
two-winding  line  relay  is  the  lessened  click  in  the  calling-subscrib- 
er's receiver  which  occurs  when  the  operator  answers  All  line  re- 
lays prior  to  1902  were  single-wound,  but  after  that  they  were  made 
double  and  used  some  turns  of  resistance  wire  to  limit  the  normal 
calling  current. 


446  TELEPHONY 

Relay  Mounting.  In  the  standard  No.  1  relay  board  of  the 
Western  Electric  Company  and,  in  fact,  in  nearly  all  common- 
battery  multiple  boards  that  are  manufactured  by  other  companies, 
the  line  and  cut-off  relays  are  mounted  on  separate  racks  outside 
the  switchboard  room  and  adjacent  to  the  main  and  intermediate 
distributing  frames,  the  wiring  being  extended  from  the  relays  to 
the  jacks  and  lamps  on  the  switchboard  proper  by  means  of  suitable 
cables.  The  Western  Electric  Company  has  recently  instituted  a 
departure  from  this  practice  in  the  case  of  some  of  their  smaller  No. 
1  switchboard  installations.  Where  it  is  thought  that  the  ultimate 
capacity  required  by  the  board  will  not  be  above  3,000  lines,  the 
relay  rack  is  dispensed  with  and  all  of  the  line  and  cut-off  relays, 
as  well  as  the  supervisory  relays,  are  mounted  in  the  rear  of  the 
switchboard  frame.  For  this  purpose  the  line  and  cut-off  relays 
are  specially  made  with  the  view  to  securing  the  utmost  compact- 
ness. In  still  other  cases,  in  switchboards  of  relatively  small  ultimate 
capacity,  they  use  this  small  line  and  cut-off  relay  mounted  on  a  sepa- 
rate relay  rack,  in  which  case  the  board  is  the  standard  No.  1  board 
except  for  the  type  of  relays.  In  all  of  these  modifications  of  the 
No.  1  board  adapted  for  the  use  of  the  smaller  and  cheaper  relays,  the 
line  relay  has  but  a  single  winding,  the  small  size  of  the  relay  winding 
not  lending  itself  readily  to  double  winding  with  the  added  necessary 
coil  terminals. 

Capacity  Range.  The  No.  1  Western  Electric  board  is  made 
in  standard  sizes  up  to  an  ultimate  capacity  of  9,600  lines.  For  all 
capacities  above  4,900  lines,  a  f-inch  jack,  vertical  and  horizontal 
face  dimensions,  is  employed.  For  this  capacity  the  smaller  types 
of  cut-off  and  line  relays  are  not  employed.  Up  to  ultimate  capac- 
ities of  4,900  lines,  |-inch  jacks  are  employed,  and  either  the  small 
or  the  large  relays  mounted  on  a  separate  rack  are  available.  Up 
to  3,000  lines  ultimate  capacity,  the  |-inch  jack  is  employed,  and 
either  the  small  or  the  large  cut-off  and  line  relays  are  available, 
but  in  case  the  small  type  is  used  the  purchaser  has  the  option  of 
mounting  them  on  a  separate  relay  rack,  as  in  ordinary  practice,  or 
mounting  them  in  the  switchboard  cabinet  and  dispensing  with  the 
relay  rack. 

Western  Electric  No.  10  Board.  The  No.  1  common-battery 
multiple  switchboard,  regardless  of  its  size  and  type  of  arrangement 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      447 

of  line  and  cut-off  relays,  involves  two  relays  for  each  line,  the  line 
relay  energized  by  the  taking  of  the  receiver  off  its  hook,  and  the  cut- 
off relay  energized  by  the  act  of  the  operator  on  plugging  in  and  serv- 
ing to  remove  the  line  relay  from  the  circuit  whenever  and  as  long  as 
a  plug  is  inserted  into  any  jack  of  the  line.  This  seems  to  involve  a 
considerable  expense  in  relays,  but  this,  as  has  been  stated,  is  war- 
ranted by  the  greater  simplicity  in  jacks  which  the  use  of  the  cut-off 
relay  makes  possible.  In  addition  to  this  expense  of  investment  in 
the  line  and  cut-off  relays,  the  amount  of  current  required  to  hold  up 
the  cut-off  relays  during  conversations  foots  up  to  a  considerable  item 
of  expense,  particularly  as  the  system  of  supervisory  signals  is  one  in 
which  the  supervisory  lamp  takes  current  not  only  while  burning, 
but  its  circuit  takes  even  more  current  when  the  lamp  is  extinguished 
during  the  time  of  a  connection.  For  all  of  these  reasons,  and  some 
other  minor  ones,  it  was  deemed  expedient  by  the  engineers  of  the 
Western  Electric  Company  to  design  a  common-battery  multiple 
switchboard  for  small  and  medium-sized  exchanges  in  which  certain 
sacrifices  might  be  made  to  the  end  of  accomplishing  certain  savings. 
The  result  has  been  a  type  of  switchboard,  designated  the  No.  10, 
which  may  be  found  in  a  number  of  Bell  exchanges,  it  being  con- 
sidered particularly  adaptable  to  installations  of  from  500  to  3,000 
lines.  Although  this  board  has  been  subject  to  a  good  deal  of  ad- 
verse criticism,  and  although  it  seems  probable  that  even  for  the 
cheaper  boards  the  No.  1  type  with  some  of  the  modifications  just 
described  will  eventually  supersede  this  No.  10  board,  yet  the  present 
extent  of  use  of  the  No.  10  board  and  the  instructive  features  which 
its  type  displays  warrant  its  discussion  here. 

Circuits.  The  circuits  of  this  switchboard  are  shown  in  Fig. 
349,  this  indicating  two-line  circuits  and  a  connecting  cord  circuit, 
together  with  the  auxiliary  apparatus  employed  in  connection  with 
the  operator's  telephone  circuit,  the  pilot  and  night  alarm  circuits. 
The  most  noticeable  feature  is  that  cut-off  jacks  are  employed,  the 
circuit  of  the  line  normally  extending  through  the  sets  of  jack  springs 
in  the  multiple,  and  answering  jacks  to  the  line  relay  and  battery  on 
one  side  of  the  line,  and  to  ground  on  the  other  side.  Obviously, 
the  additional  complexity  of  the  jack  saves  the  use  of  a  cut-off  relay 
and  the  relay  equipment  of  each  line  consists,  therefore,  of  but  a  sin- 
gle line  relay,  which  controls  the  lamp  in  an  obvious  manner. 


448 


TELEPHONY 


The  cord  circuit  is  of  the  three-conductor  type,  the  two  talking 
strands  extending  to  the  usual  split  repeating-coil  arrangement,  and 
battery  current  for  talking  purposes  being  fed  through  these  windings 
as  in  the  standard  No.  1  board.  The  supervisory  relay  is  included 
in  the  ring  strand  of  the  cord  circuit  and  is  shunted  by  a  non-inductive 


Fig.  349.     Western  Electric  No.  10  Board 

resistance,  so  that  its  impedance  will  not  interfere  with  the  talking 
currents.  The  armature  of  the  supervisory  relay  closes  the  lamp 
contact  on  its  back  stroke,  so  that  the  lamp  is  always  held  extinguished 
when  the  relay  is  energized.  The  supervisory  lamp  is  included 
in  a  connection  between  the  back  contact  of  the  supervisory  relay 
and  ground,  this  connection  including  the  central-office  battery. 
As  a  result,  the  illumination  of  the  supervisory  lamp  is  impossible  un- 
til a  plug  has  been  inserted  into  a  jack,  in  which  case,  assuming  the 
supervisory  relay  to  be  de-energized,  the  lamp  circuit  is  completed 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      449 

through  the  wire  connecting  all  of  the  test  thimbles  and  the  resistance 
permanently  bridged  to  ground  from  that  wire. 

Test.  For  purposes  of  the  test  it  is  evident  that  the  test  rings 
of  an  idle  line  are  always  at  ground  potential,  due  to  their  connection 
to  ground  through  the  resistance  coil.  It  is  also  evident  that  the  tip 
of  an  unused  calling  plug  will  always  be  at  ground  potential  and, 
therefore,  that  the  testing  of  an  idle  line  will  result  in  no  click  in 
the  operator's  receiver.  When  a  line  is  switched,  however,  the  poten- 
tial of  all  the  test  rings  will  be  raised  due  to  their  being  connected 
with  the  live  pole  of  the  battery  through  the  third  strand  of  the  cord. 
When  the  operator  in  testing  touches  the  test  contact  of  the  jack  of  a 
busy  line,  a  current  will,  therefore,  flow  from  this  test  contact  to  the  tip 
strand  of  the  cord  and  thence  to  ground  through  one  of  the  repeating 
coil  windings.  The  potential  of  the  tip  side  of  the  cord  will,  there- 
fore, be  momentarily  altered,  and  this  will  result  in  a  click  in  the 
operator's  receiver  bridged  across  the  cord  circuit  at  the  time.  The 
details  of  the  operator's  cord  circuit  and  of  the  pilot  lamp  and  night 
alarm  circuits  will  be  clear  from  the  diagram. 

Operation.  A  brief  summary  of  the  operation  of  this  system  is 
as  follows: 

The  subscriber  removes  his  receiver  from  its  hook,  thus  drawing 
up  the  armature  of  the  line  relay  and  lighting  his  line  lamp.  The 
operator  answers.  The  line  lamp  is  extinguished  by  the  falling  back 
of  the  line-relay  armature,  due  to  the  breaking  of  the  relay  circuit 
at  the  jack  contacts.  The  subscriber  then  receives  current  for  his 
transmitter  through  the  cord-circuit  battery  connections.  The 
supervisory  relay  connected  with  the  answering  cord  is  not  lighted, 
because,  although  the  lamp-circuit  connection  is  completed  at  the 
jack,  the  supervisory  relay  is  operated  to  hold  the  lamp  circuit 
open.  Conversation  ensues  between  the  operator  and  the  sub- 
scriber, after  which  the  operator  tests  the  line  called  for  with  the 
tip  of  the  calling  plug  of  the  pair  used  in  answering.  If  the  called 
line  is  not  busy,  no  click  will  ensue,  because  both  the  tested  ring 
and  the  calling  plug  are  at  the  same  potential.  Finding  no  click, 
the  operator  will  insert  the  plug  and  ring  by  means  of  the  ringing 
key.  When  the  operator  plugs  in,  the  supervisory  lamp,  associated 
with  the  calling  plug,  becomes  lighted  because  the  circuit  is  completed 
at  the  jack  and  the  supervisory  relay  remains  de-energized,  since  the 


450  TELEPHONY 

line  circuit  is  open  at  the  subscriber's  station.  When  the  called 
subscriber  responds,  the  calling  supervisory  lamp  goes  out  because 
of  the  energization  of  the  supervisory  relay.  Both  lamps  remain  out 
during  the  conversation,  but  when  either  subscriber  hangs  up,  the 
corresponding  supervisory  lamp  will  be  lighted  because  of  the  fall- 
ing back  of  the  supervisory  relay  armature. 

If  the  called  line  is  busy,  a  click  will  be  heard,  for  the  reason  de- 
scribed, and  the  operator  will  so  inform  the  calling  subscriber.  It 
goes  without  saying,  that  in  any  multiple-switchboard  system  a 
plug  may  be  found  in  the  actual  multiple  jack  that  is  reached  for, 
in  which  case,  although  no  test  will  be  made,  the  busy  condition 
will  be  reported  back  to  the  calling  subscriber. 

Economy.  It  has  been  the  belief  of  the  Western  Electric  engi- 
neers that  a  real  economy  is  accomplished  in  this  type  of  board  by  the 
saving  in  relay  equipment.  It  is,  of  course,  apparent  at  a  glance 
that  with  a  switchboard  long  enough  and  of  sections  enough,  the 
cost  of  extra  jack  springs  and  their  platinum  contacts  must  become 
great  enough  to  offset  the  saving  accomplished  by  omitting  the  cut- 
off relay.  This  makes  it  apparent  that  if  there  is  any  economy  in 
this  type  of  multiple  switchboard,  it  must  be  found  in  the  very  small 
boards  where  there  are  but  few  jacks  per  line  and  where  the  extra 
cost  of  the  cut-off  jack  is  not  enough  to  offset  the  extra  cost  of  an 
added  relay.  It  is  the  growing  belief,  however,  among  engineers, 
that  the  multiple  switchboard  must  be  very  small  indeed  in  order  that 
the  added  complexity  of  the  cut-off  jacks  and  wiring  may  be  able 
to  save  anything  over  the  two-relay  type  of  line;  and  it  is  believed  that 
where  economy  is  necessary  in  small  boards,  it  may  be  best  effected 
by  employing  cheaper  and  more  compact  forms  of  relays  and  mount- 
ing them,  if  necessary,  directly  in  the  switchboard  cabinet. 

NOTE.  These  two  standard  types  of  common-battery  multiple  switch- 
boards of  the  Western  Electric  Company  represent  the  development  through 
long  years  of  careful  work  on  the  part  of  the  Western  Electric  and  Bell  engi- 
neers, credit  being  particularly  due  to  Scribner,  McBerty,  and  McQuarrie  of 
the  Western  Electric  Company,  and  Hayes  of  the  American  Telephone  and 
Telegraph  Company. 

Kellogg  Two=Wire  Multiple  Board.  The  simplicity  in  the  jacks 
permitted  by  the  use  of  the  cut-off  relay  in  the  Western  Electric 
common-battery  multiple  switchboard  for  larger  exchanges  was 
carried  a  step  further  by  Dunbar  and  Miller  in  the  development  of 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      451 

the  so-called  two-wire  common-battery  multiple  switchboard,  which 
for  many  years  has  been  the  standard  of  the  Kellogg  Switchboard 
and  Supply  Company.  The  particular  condition  which  led  to  the 
development  of  the  two-wire  system  was  the  demand  at  that  time  on 
the  Kellogg  Company  for  certain  very  large  multiple  switchboards, 
involving  as  many  as  18,000.  lines  in  the  multiple.  Obviously,  this 
necessitated  a  small  jack,  and  obviously  a  jack  having  only  two 
contacts,  a  tip  spring  and  a  sleeve,  could  be  made  more  easily  and 
with  greater  durability  of  this  very  small  size  than  a  jack  requiring 
three  or  more  contacts.  Other  reasons  that  were  considered  were, 
of  course,  cheapness  in  cost  of  construction  and  extreme  simplicity, 
which,  other  things  being  equal,  lends  itself  to  low  cost  of  mainte- 
nance. 

Line  Circuit.  Like  the  standard  Western  Electric  board  for 
large  offices,  the  Kellogg  two-wire  board  employs  two  relays  for 
each  line,  the  line  relay  under  the  control  of  the  subscriber  and  in 
turn  controlling  the  lamp,  and  a  cut-off  relay  under  the  control  of 
the  operator  and  in  turn  controlling  the  connection  of  the  line  relay 
with  the  line.  The  line  circuit  as  originally  developed  and  as  widely 
used  by  the  Kellogg  Company  is  shown  in  Fig.  350.  The  extreme 
simplicity  of  the  jacks  is  apparent,  as  is  also  the  fact  that  but  two 
wires  lead  through  the  multiple.  Another  distinguishing  feature  is, 
that  all  of  the  multiple  and  answering  jacks  are  normally  cut  off 
from  the  line  at  the  cut-off  relay,  but  when  the  cut-off  relay  oper- 
ates it  serves,  in  addition  to  cutting  off  the  line  relay,  to  attach  the 
two  limbs  of  the  line  to  the  two  wires  leading  through  the  multiple  and 
answering  jacks.  The  control  of  the  line  relay  by  the  subscriber's 
switch  hook  is  clear  from  the  figure.  The  control  of  the  cut-off 
relay  is  secured  by  attaching  one  terminal  of  the  cut-off  relay  wind- 
ing permanently  to  that  wire  leading  through  the  multiple  which  con- 
nects with  the  sleeve  contacts  of  the  jack,  the  other  terminal  of  the 
cut-off  relay  being  grounded.  The  way  in  which  this  relay  is  oper- 
ated will  be  understood  when  it  is  stated  that  the  sleeve  contacts  of 
both  the  answering  and  calling  plugs  always  carry  full  battery  poten- 
tial and,  therefore,  whenever  any  plug  is  inserted  into  any  jack,  current 
flows  from  the  sleeve  of  the  jack  through  the  sleeve  contact  of  the  jack 
to  ground,  through  the  winding  of  the  cut-off  relay,  which  relay  becomes 
energized  and  performs  the  functions  just  stated.  It  is  seen  that  the 


452 


TELEPHONY 


wire  running  through  the  multiple  to  which  the  sleeve  jack  contacts 
are  attached,  is  thus  made  to  serve  the  double  purpose  of  answering 
as  one  side  of  the  talking  circuit,  and  also  of  performing  the  functions 
carried  out  by  the  separate  or  third  wire  in  the  three-wire  system. 


Fig.  350.     Two-Wire  Line  Circuit 

It  will  be  shown  also  that,  in  addition,  this  wire  is  made  to  lend  itself 
to  the  purposes  of  the  busy  test  without  any  of  these  functions  inter- 
fering with  each  other  in  any  way. 

Cord  Circuit.  The  cord  circuit  in  somewhat  simplified  form  is 
shown  in  Fig.  351.  Here  again  there  are  but  two  conductors  to  the 
plugs  and  two  strands  to  the  cords.  This  greater  simplicity  is  in 
some  measure  offset  by  the  fact  that  four  relays  are  required,  two 
for  each  plug.  This  so-called  four-relay  cord  circuit  may  be  most 
readily  understood  by  considering  half  of  it  at  a  time,  since  the  two 
relays  associated  with  the  answering  plug  act  in  exactly  the  same 
way  as  those  connected  with  the  calling  plug. 

Associated  with  each  plug  of  a  pair  are  two  relays  1  and  2,  in  the 
case  of  the  answering  cord,  and  3  and  4  m  the  case  of  the  calling 
cord.  The  coils  of  the  relays  1  and  2  are  connected  in  series  and 


Fig.  351.     Two- Wire  Cord  Circuit 


bridged  across  the  answering  cord,  a  battery  being  included  between 
the  coils  in  this  circuit.  The  coils  of  the  relays  3  and  4  are  simi- 
larly connected  across  the  calling  cord.  A  peculiar  feature  of  the 
Kellogg  system  is  that  two  batteries  are  used  in  connection  with 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      453 

the  cord  circuit,  one  of  them  being  common  to  all  answering  cords 
and  the  other  to  all  calling  cords.  The  operation  of  the  system 
would,  however,  be  exactly  the  same  if  a  single  battery  were  substi- 
tuted for  the  two. 

Supervisory  Signals.  Considering  the  relays  associated  with 
the  answering  cord,  it  is  obvious  that  these  two  relays  1  and  2  to- 
gether control  the  circuit  of  the  supervisory  lamp  5,  the  circuit  of 
this  lamp  being  closed  only  when  the  relay  1  is  de-energized  and  the 
relay  2  is  energized.  We  will  find  in  discussing  the  operation  of 
these  that  the  relay  2  is  wholly  under  the  control  of  the  operator,  and 
that  the  relay  1,  after  its  plug  has  been  connected  with  a  line,  is  wholly 
under  the  control  of  the  subscriber  on  that  line.  It  is  through  the 
windings  of  these  two  relays  that  current  is  fed  to  the  line  of  the  sub- 
scriber connected  with  the  corresponding  cord. 

When  a  plug — the  answering  plug,  for  instance — is  inserted  into 
a  jack,  current  at  once  flows  from  the  positive  pole  of  the  left-hand 
battery  through  the  winding  of  the  relay  2  to  the  sleeve  of  the  plug, 
thence  to  the  sleeve  of  the  jack  and  through  the  cut-off  relay  to 
ground.  This  at  once  energizes  the  supervisory  relay  2  and  the  cut- 
off relay  associated  with  the  line.  The  cut-off  relay  acts,  as  stated, 
to  continue  the  tip  and  sleeve  wires  associated  with  the  jacks  to  the  line 
leading  to  the  subscriber,  and  also  to  cut  off  the  line  relay.  The  super- 
visory relay  2  acts  at  the  same  time  to  attract  its  armature  and  thus 
complete  its  part  in  closing  the  circuit  of  the  supervisory  lamp. 
Whether  or  not  the  lamp  will  be  lighted  at  this  time  depends  on  wheth- 
er the  relay  1  is  energized  or  not,  and  this,  it  will  be  seen,  depends 
on  whether  the  subscriber's  receiver  is  off  or  on  its  hook.  If  off  its 
hook,  current  will  flow  through  the  metallic  circuit  of  the  line  for 
energizing  the  subscriber's  transmitter,  and  as  whatever  current  goes 
to  the  subscriber's  line  must  flow  through  the  relay  1,  that  relay  will  be 
energized  and  prevent  the  lighting  of  the  supervisory  lamp  5.  If, 
on  the  other  hand,  the  subscriber's  receiver  is  on  its  hook,  no  cur- 
rent will  flow  through  the  line,  the  supervisory  relay  will  not  be  en- 
ergized, and  the  lamp  5  will  be  lighted. 

In  a  nutshell,  the  sleeve  supervisory  relay  normally  prevents  the 
lighting  of  the  corresponding  supervisory  lamp,  but  as  soon  as  the 
operator  inserts  a  plug  into  the  jack  of  the  line,  the  relay  2  establishes 
such  a  condition  as  to  make  possible  the  lighting  of  the  supervisory 


454  TELEPHONY 

lamp,  and  the  lighting  of  this  lamp  is  then  controlled  entirely  by  the 
relay  1,  which  is,  in  turn,  controlled  by  the  position  of  the  subscriber's 
switch-hook. 

Battery  Feed.  A  2-microfarad  condenser  is  included  in  each 
strand  of  the  cord,  and  battery  is  fed  through  the  relay  windings 
to  the  calling  and  called  subscribers  on  opposite  sides  of  these  con- 
densers, in  accordance  with  the  combined  impedance  coil  and  con- 
denser method  described  in  Chapter  XIII.  Here  the  relay  windings 
do  double  duty,  serving  as  magnets  for  operating  the  relays  and  as 
retardation  coils  in  the  system  of  battery  supply. 

Complete  Cord  and  Line  Circuits.  The  complete  cord  and 
line  circuits  of  the  Kellogg  two-wire  system  are  shown  in  Fig.  352. 
In  the  more  recent  installations  of  the  Kellogg  Company  the  cord 
and  line  circuits  have  been  slightly  changed  from  those  shown  in 
Figs.  350  and  351,  and  these  changes  have  been  incorporated  in  Fig. 
352.  The  principles  of  operation  described  in  connection  with  the 
simplified  figures  remain,  however,  exactly  the  same.  One  of  the 
changes  is,  that  the  tip  side  of  the  lines  is  permanently  connected  to 
the  tips  of  the  jacks  instead  of  being  normally  cut  off  by  the  cut-off 
relay,  as  was  done  in  the  system  as  originally  developed.  Another 
change  is,  that  the  line  relay  is  associated  with  the  tip  side  of  the 
line,  rather  than  with  the  sleeve  side,  as  was  formerly  done.  The 
cord  circuit  shown  in  Fig.  352  shows  exactly  the  same  arrangement 
of  supervisory  relays  and  exactly  the  same  method  of  battery  feed  as 
in  the  simplified  cord  circuit  of  Fig.  351,  but  in  addition  to  this  the 
detailed  connections  of  the  operator's  talking  set  and  of  her  order- 
wire  keys  are  indicated,  and  also  the  ringing  equipment  is  indicated 
as  being  adapted  for  four-party  harmonic  work. 

In  connection  with  this  ringing  key  it  may  be  stated  that  the 
springs  7,  8,  9,  and  10  are  individually  operated  by  the  pressure  of 
one  of  the  ringing  key  buttons,  while  the  spring  17,  connected 
with  the  sleeve  side  of  the  calling  plug,  is  always  operated  simultan- 
eously with  the  operation  of  any  one  of  the  other  springs.  As  a  re- 
sult the  proper  ringing  circuit  is  established,  it  being  understood 
that  the  upper  contacts  of  the  springs  7,  8,  9,  and  10  lead  to  the 
terminals  of  their  respective  ringing  generators,  the  other  terminals 
of  which  are  grounded.  The  circuit  is,  therefore,  from  the  generator, 
through  the  ringing  key,  out  through  the  tip  side  of  the  line,  back  over 


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456  TELEPHONY 

the  sleeve  side  of  the  line,  and  to  ground  through  the  spring  17,  re- 
sistance 11,  and  the  battery,  which  is  one  of  the  cord-circuit  batteries. 
The  object  of  this  coil  11  and  the  battery  connection  through  it  to  the 
ringing-key  spring  is  to  prevent  the  falling  back  of  the  cut-off  relay 
when  the  ringing  key  is  operated.  This  will  be  clear  when  it  is  re- 
membered that  the  cut-off  relay  is  energized  by  battery  current  fed 
over  the  sleeve  strand  of  the  cord,  and  obviously,  since  it  is  necessary 
when  the  ringing  key  is  operated  to  cut  off  the  supply  wire  back  of  the 
key,  this  would  de-energize  the  cut-off  relay  when  the  ringing  key 
was  depressed,  and  the  falling  back  of  the  cut-off  relay  contacts 
would  make  it  impossible  to  ring  because  the  sleeve  side  of  the  line 
would  be  cut  off.  The  battery  supply  through  the  resistance  11  is, 
therefore,  substituted  on  the  sleeve  strand  of  the  cord  for  the  battery 
supply  through  the  normal  connection. 

Busy  Test.  The  busy  test  depends  on  all  of  the  test  rings  being 
at  zero  potential  on  an  idle  line  and  at  a  higher  potential  on  a  busy 
line.  Obviously,  when  the  line  is  not  switched,  the  test  rings  are  at 
zero  potential  on  account  of  a  ground  through  the  cut-off  relay. 
When,  however,  a  plug  is  inserted  in  either  the  answering  or  multiple 
jacks,  the  test  rings  will  all  be  raised  in  potential  due  to  being  con- 
nected with  the  live  side  of  the  battery  through  the  sleeve  strand 
of  the  cord.  Conditions  on  the  line  external  to  the  central  office  can- 
not make  an  idle  line  test  busy  because,  owing  to  the  presence  of 
the  cut-off  relay,  the  sleeve  contacts  of  all  the  jacks  are  disconnected 
from  the  line  when  it  is  idle.  .  The  test  circuit  from  the  tip  of  the 
calling  plug  to  ground  at  the  operator's  set  passes  through  the  tip 
strand  of  the  cord,  thence  through  a  pair  of  normally  closed  extra 
contacts  on  the  supervisory  relay  4,  thence  in  series  through  all  the 
ringing  key  springs  10,  9,  8,  and  7,  thence  through  an  extra  pair 
of  springs  12  and  13  on  the  listening  key — closed  only  when  the  lis- 
tening key  is  operated — and  thence  to  ground  through  a  retardation 
coil  14-  No  battery  or  other  source  of  potential  exists  in  this  circuit 
between  ground  and  the  tip  of  the  calling  plug  and,  therefore,  the 
tip  is  normally  at  ground  potential.  The  sleeve  ring  of  the  jack 
being  at  ground  potential  if  the  line  is  idle,  no  current  will  flow 
and  no  click  will  be  produced  in  testing  such  a  line.  If,  however, 
the  line  is  busy,  the  test  ring  will  be  at  a  higher  potential  and,  there- 
fore, current  will  flow  from  the  tip  of  the  calling  plug  to  ground 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     457 

over  the  path  just  traced,  and  this  will  cause  a  rise  in  potential  at 
the  terminal  of  the  condenser  15  and  a  momentary  flow  of  cur- 
rent through  the  tertiary  winding  16  of  the  operator's  induction  coil; 
hence  the  click. 

Obviously  the  testing  circuit  from  the  tip  of  the  calling  plug  to 
ground  at  the  operator's  set  is  only  useful  during  the  time  when  the 
calling  plug  is  not  in  a  jack,  and  as  the  tip  strand  of  the  calling  plug 
has  to  do  double  duty  in  testing  and  in  serving  as  a  part  of  the  talking 
circuit,  the  arrangement  is  made  that  the  testing  circuit  will  be  auto- 
matically broken  and  the  talking  circuit  through  the  tip  strand  auto- 
matically completed  when  the  plug  is  inserted  into  a  jack  in  estab- 
lishing a  connection.  This  is  accomplished  by  means  of  the  extra 
contact  on  the  relay  4>  which  relay,  it  will  be  remembered,  is  held 
energized  when  its  corresponding  plug  is  inserted  in  a  jack.  During 
the  time  when  the  plug  is  not  inserted,  this  relay  is  not  energized 
and  the  test  circuit  is  completed  through  the  back  contact  of  its  right- 
hand  armature.  When  connection  is  made  at  the  jack,  this  relay 
becomes  energized  and  the  tip  strand  of  the  cord  circuit  is  made  com- 
plete by  the  right-hand  lever  being  pulled  against  the  front  contact 
of  this  relay.  The  keys  shown  to  the  right  of  the  operator's  set  are 
order-wire  keys. 

Summary  of  Operation.  We  may  give  a  brief  summary  of  the 
operation  of  this  system  as  shown  in  Fig.  352.  The  left-hand  station 
calls  and  the  line  relay  pulls  up,  lighting  the  lamp.  The  operator 
inserts  an  answering  plug  in  the  answering  jack,  thus  energizing  the 
cut-off  relay  which  operates  to  cut  off  the  line  relay  and  to  complete 
the  connection  between  the  jacks  and  the  external  line.  The  act  of 
plugging  in  by  the  operator  also  raises  the  potential  of  all  the  test 
rings  so  as  to  guard  the  line  against  intrusion  by  other  callers.  The 
supervisory  lamp  5  remains  unlighted  because,  although  the  relay 
2  is  operated,  the  relay  1  is  also  operated,  due  to  the  calling  subscriber's 
receiver  being  off  its  hook.  The  operator  throws  her  listening  key, 
communicates  with  the  subscriber,  and,  learning  that  the  right-hand 
station  is  wanted,  proceeds  to  test  that  line.  If  the  line  is  idle, 
she  will  get  no  click,  because  the  tip  of  her  calling  plug  and  the  tested 
ring  will  be  at  the  same  ground  potential.  She  then  plugs  in  and 
presses  the  proper  ringing-key  button  to  send  out  the  proper  fre- 
quency to  ring  the  particular  subscriber  on  the  line— if  there  be 


458  TELEPHONY 

more  than  one — the  current  from  the  battery  through  the  coil  11  and 
spring  17  serving  during  this  operation  to  hold  up  the  cut-off  relay. 

As  soon  as  the  operator  plugs  in  with  the  calling  plug,  the  super- 
visory lamp  6  lights,  assuming  that  the  called  subscriber  had  not 
already  removed  his  receiver  from  its  hook,  due  to  the  fact  that 
the  relay  4  is  energized  and  the  relay  3  is  not.  As  soon  as  the 
called  subscriber  responds,  the  relay  3  becomes  energized  and  the 
supervisory  lamp  goes  out.  If  the  line  called  for  had  been  busy 
by  virtue  of  being  plugged  at  another  section,  the  tip  of  the  oper- 
ator's plug  in  testing  would  have  found  the  test  ring  raised  to  a 
potential  above  the  ground,  and,  as  a  consequence,  current  would 
have  flowed  from  the  tip  of  this  plug  through  the  back  contact  of 
the  right-hand  lever  of  relay  ^,  thence  through  the  ringing  key  springs 
and  the  auxiliary  listening-key  springs  to  ground  through  the  retard- 
ation coil  14'  This  would  have  produced  a  click  by  causing  a  mo- 
mentary flow  of  current  through  the  tertiary  winding  16  of  the  oper- 
ator's set. 

Wiring  of  Line  Circuit.  The  more  complete  wiring  diagram  of 
a  single  subscriber's  line,  Fig.  353,  shows  the  placing  in  the  circuits 
of  the  terminals  and  jumper  wires  of  the  main  distributing  frame 
and  of  the  intermediate  distributing  frame,  and  also  shows  how  the 
pilot  lamps  and  night-alarm  circuits  are  associated  with  a  group 
of  lines.  The  main  distributing  frame  occupies  the  same  relative 
position  in  this  line  circuit  as  in  the  Western  Electric,  being  located 
in  the  main  line  circuit  outside  of  all  the  switchboard  apparatus. 
The  intermediate  distributing  frame  occupies  a  different  relative 
position  from  that  in  the  Western  Electric  line.  It  will  be  recalled 
by  reference  to  Fig.  348  that  the  line  lamp  and  the  answering  jack  were 
permanently  associated  with  the  line  and  cut-off  relays,  such  muta- 
tions of  arrangement  as  were  possible  at  the  intermediate  distribut- 
ing frame  serving  only  to  vary  the  connection  between  the  multiple 
of  a  line  and  one  of  the  various  groups  of  apparatus  consisting  of  an 
answering  jack  and  line  lamp  and  associated  relays.  In  the  Kellogg 
arrangement,  Fig.  353,  the  line  and  cut-off  relays,  instead  of  being 
permanently  associated  with  the  answering  jack  and  line  lamp, 
are  permanently  associated  with  the  multiple  jacks,  no  changes,  of 
which  the  intermediate  or  main  frames  are  capable,  being  able  to 
alter  the  relation  between  a  group  of  multiple  jacks  and  its  associated 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      459 

line  and  cut-off  relays.  In  this  Kellogg  arrangement  the  interme- 
diate distributing  frame  may  only  alter  the  connection  of  an  answer- 
ing jack  and  line  lamp  with  the  multiple  and  its  permanently  asso- 
ciated relays.  The  pilot  and  night  alarm  arrangements  of  Fig.  353 
should  be  obvious  from  the  description  already  given  of  other  sim- 
ilar systems. 

Dean  Multiple  Board.     In  Fig.  354  are  shown  the  circuits  of  the 
multiple  switchboard  of  the  Dean  Electric  Company.    The  subscrib- 


MVLT:  JACXS 


Fig.  353.     Kellogg  Two- Wire  Line  Circuit 

er's  station  equipment  shown  at  Station  A  and  Station  B  will  be 
recognized  as  the  Wheatstone-bridge  circuit  of  the  Dean  Company. 
Line  Circuit.  The  line  circuit  is  easily  understood  in  view  of 
what  has  been  said  concerning  the  Western  Electric  line  circuit,  the 
line  relay  1  being  single  wound  and  between  the  live  side  of  the 
battery  and  the  ring  side  of  the  line.  The  cut-off  relay  2  is  operated 
whenever  a  plug  is  inserted  in  a  jack  and  serves  to  sever  the  con- 
nection of  the  line  with  the  normal  line  signaling  apparatus. 


460  TELEPHONY 

Cord  Circuit.  The  cord  circuit  is  of  the  four-relay  type,  but 
employs  three  conductors  instead  of  two,  as  in  the  two-wire  system. 
The  relay  3,  being  in  series  between  the  battery  and  the  sleeve  con- 
tact on  the  plug,  is  energized  whenever  a  plug  is  inserted  in  the 
jack,  its  winding  being  placed  in  series  with  the  cut-off  relay  of  the 
line  with  which  the  plug  is  connected.  This  completes  the  circuit 
through  the  associated  supervisory  lamp  unless  the  relay  4  is  ener- 
gized, the  local  lamp  circuit  being  controlled  by  the  back  contact  of 
relay  4  and  the  front  contact  of  relay  3.  It  is  through  the  two  wind- 
ings of  the  relay  4  that  current  is  fed  to  the  subscriber's  station,  and, 
therefore,  the  armature  of  this  relay  is  responsive  to  the  movements 
of  the  subscriber's  hook.  As  the  relay  3  holds  the  supervisory  lamp 
circuit  closed  as  long  as  a  plug  is  inserted  in  a  jack  of  the  line,  it 
follows  that  during  a  connection  the  relay  4  w^l  have  entire  control 
of  the  supervisory  lamp. 

Listening  Key.  The  listening  key,  as  usual,  serves  to  connect 
the  operator's  set  across  the  talking  strands  of  the  cord  circuit,  and 
the  action  of  this  in  connection  with  the  operator's  set  needs  no  fur- 
ther explanation. 

Ringing  Keys.  The  ringing-key  arrangement  illustrated  is 
adapted  for  use  with  harmonic  ringing,  the  single  springs  5,  6,  7, 
and  8  each  being  controlled  by  a  separate  button  and  serving  to 
select  the  particular  frequency  that  is  to  be  sent  to  line.  The  two 
springs  9  and  10  always  act  to  open  the  cord  circuit  back  of  the  ring- 
ing keys,  whenever  any  one  of  the  selective  buttons  is  depressed,  in 
order  to  prevent  interference  by  ringing  current  with  the  other  oper- 
ations of  the  circuit. 

Two  views  of  these  ringing  keys  are  shown  in  Figs.  355  and  356. 
Fig.  356  is  an  end  view  of  the  entire  set.  In  Fig.  355  the  listening  key 
is  shown  at  the  extreme  right  and  the  four  selective  buttons  at  the 
left.  When  a  button  is  released  it  rises  far  enough  to  cause  the  dis- 
engagement of  the  contacts,  but  remains  partially  depressed  to  serve 
as  an  indication  that  it  was  last  used.  The  group  of  springs  at  the 
extreme  left  of  Fig.  355  are  the  ones  represented  at  9  and  10  in  Fig. 
354  and  by  the  anvils  with  which  those  springs  co-operate. 

Test.  The  test  in  this  Dean  system  is  simple,  and,  like  the 
Western  Electric  and  Kellogg  systems,  it  depends  on  the  raising  of 
the  potential  of  the  test  thimbles  of  all  the  line  jacks  of  a  line  when 


w 

I 


462 


TELEPHONY 


a  connection  is  made  with  that  line  by  a  plug  at  any  position.  When 
an  operator  makes  a  test  by  applying  the  tip  of  the  calling  plug  to 
the  test  thimble  of  a  busy  line,  current  passes  from  the  test  thimble 


Fig.  355.     Dean  Party  Line  R'nging  Key 

through  the  tip  strand  of  the  cord  to  ground  through  the  left-hand 
winding  of  the  calling  supervisory  relay  4-  The  drop  of  potential 
through  this  winding  causes  the  tip  strand  of  the  cord  to  be  raised  to 
a  higher  potential  than  it  was  before,  and  as  a  result 
the  upper  plate  of  the  condenser  11  is  thus  altered  in 
potential  and  this  change  in  potential  across  the  con- 
denser results  in  a  click  in  the  operator's  ear. 

Stromberg=Carlson  Multiple  Board.  Line  Circuit. 
In  Fig.  357  is  shown  the  multiple  common-battery 
switchboard  circuits  employed  by  the  Stromberg- 
Carlson  Telephone  Manufacturing  Company.  The 
subscriber's  line  circuits  shown  in  this  drawing  are 
of  the  three-wire  type  and,  with  the  exception  of 
the  subscriber's  station,  are  the  same  as  already 
described  for  the  Western  Electric  Company's  sys- 
tem. 

Cord  Circuit.  The  cord  circuit  employed  is  of  the 
two-conductor  type,  the  plugs  being  so  constructed  as 
to  connect  the  ring  and  thimble  contacts  of  the  jack  when  inserted. 
This  cord  circuit  is  somewhat  similar  to  that  employed  by  the 
Kellogg  Switchboard  and  Supply  Company,  shown  in  Fig.  352, 


Fig.  356.     Dean 
Party     Line 
Ringing  Key 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      463 

except  that  only  one  battery  is  employed,  and  that  certain  functions 
of  this  circuit  are  performed  mechanically  by  the  inter-action  of  the 
armatures  of  the  relays. 

Supervisory  Signals.  When  the  answering  plug  is  inserted 
in  a  jack,  in  response  to  a  call,  the  current  passing  to  the  subscriber's 
station  and  also  through  the  cut-off  relay  must  flow  through  the  relay 

1,  thus  energizing  it.     As  the  calling  subscriber's  receiver  is  at  this 
time  removed  from  the  hook  switch;  the  path  for  current  will  be  com- 
pleted through  the  tip  of  the  jack,  thence  through  the  tip  of  the  plug, 
through  relay  2  to  ground,  causing  relay  2  to  be  operated  and  to  break 
the  circuit  of  the  answering  supervisory  lamp.     The  two  relays  /  and 
2  are  so  associated  mechanically  that  the  armature  of  1  controls  the 
armature  of  2  in  such  a  manner  as  to  normally  hold  the  circuit  of 
the  answering  supervisory  lamp  open.     But,  however,  when  the  plug 
is  inserted  in  a  jack,  relay  1  is  operated  and  allows  the  operation  of 
relay  2  to  be  controlled  by  the  hook  switch  at  the  subscriber's  sta- 
tion.    The  supervisory  relay  3  associated  with  the  calling  cord  is 
operated  when  the  calling  plug  is  placed  in  a  jack,  and  this  relay 
normally  holds  the  armature  of  relay  4  m  an  operated  position  in  a 
similar  manner  as  the  armature  of  relay  1  controlled  that  of  relay 

2.  Supervisory  relay  4  ls  under  the  control  of  the  hook  switch  at 
the  called  subscriber's  station. 

Test.  In  this  circuit,  as  in  several  previously  described,  when 
a  plug  is  inserted  in  a  jack  of  a  line,  the  thimble  contacts  of  the  jacks 
associated  with  that  line  are  raised  to  a  higher  potential  than  that 
which  they  normally  have.  The  operator  in  testing  a  busy  line,  of 
course  having  previously  moved  the  listening  key  to  the  listening 
position,  closes  a  path  from  the  test  thimble  of  the  jack,  through 
the  tip  of  the  calling  plug,  through  the  contacts  of  the  relay  4>  the 
inside  springs  of  the  listening  key,  thence  through  a  winding  of  the 
induction  coil  associated  with  her  set  to  ground.  The  circuit  thus 
established  allows  current  to  flow  from  the  test  thimble  of  the  jack 
through  the  winding  of  her  induction  coil  to  ground,  causing  a  click 
in  her  telephone  receiver.  The  arrangement  of  the  ringing  circuit 
does  not  differ  materially  from  that  already  described  for  other  sys- 
tems and,  therefore,  needs  no  further  explanation. 

Multiple  Switchboard  Apparatus.  Coming  now  to  a  discussion 
of  the  details  of  apparatus  emploj'ed  in  multiple  switchboards,  it  may 


e 

I 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     465 

be  stated  that  much  of  the  apparatus  used  in  the  simpler  types  is 
capable  of  doing  duty  in  multiple  switchboards,  although,  of  course, 
modification  in  detail  is  often  necessary  to  make  the  apparatus  fit 
the  particular  demands  of  the  system  in  which  it  is  to  be  used. 

Jacks.  Probably  the  most  important  piece  of  apparatus  in  the 
multiple  switchboard  is  the  jack,  its  importance  being  increased  by 
the  fact  that  such  very  large  numbers  of  them  are  sometimes  necessary. 
Switchboards  having  hundreds  of  thousands  of  jacks  are  not  un- 
common. The  multiple  jacks  are  nearly  always  mounted  in  strips  of 
twenty  and  the  answering  jacks  usually  in  strips  of  ten,  the  length  of 
the  jack  strip  being  the  same  in  each  case  in  the  same  board  and, 
therefore,  giving  twice  as  wide  a  spacing  in  the  answering  as  in  the 
multiple  jacks.  The  distance  between  centers  in  the  multiple  jacks 
varies  from  a  quarter  of  an  inch — which  is  perhaps  the  extreme  min- 
imum— to  half  an  inch,  beyond  which  larger  limit  there  seems  to  be 
no  need  of  going  in  any  case.  It  is  customary  that  the  jack  strip 
shall  be  made  of  the  same  total  thickness  as  the  distance  between 
the  centers  of  two  of  its  jacks,  and  from  this  it  follows  that  the  strips 
when  piled  one  upon  the  other  give  the  same  vertical  distance 
between  jack  centers  as  the  horizontal  distance. 

In  Fig.  358  is  shown  a  strip  of  multiple  and  a  strip  of  answer- 
ing jacks  of  Western  Electric  make,  this  being  the  type  employed  in 
the  No.  1  standard  switchboards  for  large  exchanges.  In  Fig.  359  are 
shown  the  multiple  and  answering  jacks  employed  in  the  No.  10 
Western  Electric  switchboard.  The  multiple  jacks  in  the  No.  1 
switchboard  are  mounted  on  f-inch  centers,  the  jacks  having  three 
branch  terminal  contacts.  The  multiple  jacks  of  the  No.  10  switch- 
board indicated  in  Fig.  359  are  mounted  on  ^-inch  centers,  each 
jack  having  five  contacts  as  indicated  by  the  requirement  of  the  cir- 
cuits in  Fig.  349. 

In  Fig.  360  are  shown  the  answering  and  multiple  jacks  of  the 
Kellogg  Switchboard  and  Supply  Company's  two-wire  system.  The 
extreme  simplicity  of  these  is  particularly  well  shown  in  the  cut  of 
the  answering  jack,  and  these  figures  also  show  clearly  the  customary 
method  of  numbering  jacks.  In  very  large  multiple  boards  it  has 
been  the  practice  of  the  Kellogg  Company  to  space  the  multiple 
jacks  on  TVmch  centers,  and  in  their  smaller  multiple  work,  they 
employ  the  ^-inch  spacing.  With  the  y^-inch  spacing  that  company 


466 


TELEPHONY 


has  been  able  to  build  boards  having  a  capacity  of  18,000  lines,  that 
many  jacks  being  placed  within  the  reach  of  each  operator. 

In  all  modem  multiple  switchboards  the  test  thimble  or  sleeve 
contacts  are  drawn  up  from  sheet  brass  or  German  silver  into  tubular 
form  and  inserted  in  properly  spaced  borings  in  strips  of  hard  rubber 
forming  the  faces  of  the  jacks.  These  strips  sometimes  are  rein- 
forced by  brass  strips  on  their  under  sides.  The  springs  forming  the 
other  terminals  of  the  iack  are  mounted  in  milled  slots  in  another 


Fig.  358.     Answering  and  Multiple  Jacks  for  No.  1  Board 

strip  of  hard  rubber  mounted  in  the  rear  of  and  parallel  to  the  front 
strip  and  rigidly  attached  thereto  by  a  suitable  metal  framework.  In 
this  way  desired  rigidity  and  high  insulation  between  the  various 
parts  is  secured. 

Lamp  Jacks.  The  lamp  jacks  employed  in  multiple  work  need 
no  further  description  in  view  of  what  has  been  said  in  connection 
with  lamp  jacks  for  simple  common-battery  boards.  The  lamp 
jack  spacing  is  always  the  same  as  the  answering  jack  spacing,  so  that 
the  lamps  will  come  in  the  same  vertical  alignment  as  their  corre- 
sponding answering  jacks  when  the  lamp  strips  and  answering  jack 
strips  are  mounted  in  alternate  layers. 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      467 

Relays.  Next  in  order  of  importance  in  the  matter  of  individ- 
ual parts  for  multiple  switchboards  is  the  relay.  The  necessity  for 
reliability  of  action  in  these  is  apparent,  and  this  means  that  they 


Fig.  359.     Answering  and  Multiple  Jacks  for  No   10  Board 

must  not  only  be  well  constructed,  but  that  they  must  be  protected 
from  dust  and  moisture  and  must  have  contact  points  of  such  a  na- 
ture as  not  to  corrode  even  in  the  presence  of  considerable  sparking 


/  I  III  111  I II 1 1 1 1  till* 


Fig.  360.     Answering  and  Multiple  Jacks  for  Kellogg  Two- Wire  Board 

and  of  the  most  adverse  atmospheric  conditions.  Economy  of  space 
is  also  a  factor  and  has  led  to  the  almost  universal  adoption  of  the 
single-magnet  type  of  relay  for  line  and  cut-off  as  well  as  supervisory 
purposes. 


468 


TELEPHONY 


The  Western  Electric  Company  employs  different  types  of  re- 
lays for  line,  cut-off,  and  supervisory  purposes.  This  is  contrary 
to  the  practice  of  most  of  the  other  companies  who  make  the  same 
general  type  of  relay  serve  for  all  of  these  purposes.  A  good  idea  of 
the  type  of  Western  Electric  line  relay,  as  employed  in  its  No.  1  board, 
may  be  had  from  Fig.  361.  As  is  seen  this  is  of  the  tilting  armature 


Fig.  361.     Type  of  Line  Relay 


type,  the  armature  rocking  back  and  forth  on  a  knife-edge  contact  at 
its  base,  the  part  on  which  it  rests  being  of  iron  and  of  such  form  as 
to  practically  complete,  with  the  armature  and  core,  the  magnetic 
circuit.  The  cut-off  relay,  Fig.  362,  is  of  an  entirely  different  type. 
The  armature  in  this  is  loosely  suspended  by  means  of  a  flexible 


Fig.  362.     Type  of  Cut-Ofl  Relay 

spring  underneath  two  L-shaped  polar  extensions,  one  extending  up 
from  the  rear  end  of  the  core  and  the  other  from  the  front  end. 
When  energized  this  armature  is  pulled  away  from  the  core  by  these 
L-shaped  pieces  and  imparts  its  motion  through  a  hard-rubber  pin 
to  the  upper  pair  of  springs  so  as  to  effect  the  necessary  changes  in 
the  circuit. 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD    469 

Much  economy  in  space  and  in  wiring  is  secured  in  the  type 
of  switchboards  employing  cut-off  as  well  as  line  relays  by  mounting 
the  two  relays  together  and  in  making  of  them,  in  fact,  a  unitary 


Fig.  363.     Western  Electric  Combined  Line  and  Cut-off  Relay 

piece  of  apparatus.  Since  the  line  relay  is  always  associated  with 
the  cut-off  relay  of  the  same  line  and  with  no  other,  it  is  obvious 
that  this  unitary  arrangement  effects  a  great  saving  in  wiring  and 


Fig.  364.     Western  Electric  Supervisory  Relay 

also  secures  a  great  advantage  in  the  matter  of  convenience  of 
inspection.  Such  a  combined  cut-off  and  line  relay,  employed  in 
the  Western  Electric  No.  1  relay  board,  is  shown  in  Fig.  363. 


Fig.  365.     Line  Relay  No.  10  Board 


These  are  mounted  in  banks  of  ten  pairs,    a   common    dust   cap 
of  sheet  iron  covering  the  entire  group. 


470 


TELEPHONY 


The  Western  Electric  supervisory  relay,  Fig.  364,  is  of  the  tilting 
armature  type  and  is  copper  clad.  The  dust  cap  in  this  case  fits 
on  with  a  bayonet  joint  as  clearly  indicated.  In  Fig.  365  is  shown 
the  line  relay  employed  in  the  Western  Electric  No.  10  board. 


Fig.  366.     Kellogg  Line  and  Cut-off  Relays 

The  Kellogg  Company  employs  the  type  of  relay  of  which  the 
magnetic  circuit  was  illustrated  in  Fig.  95.  In  its  multiple  boards  it 
commonly  mounts  the  line  and  cut-off  relays  together,  as  shown  in 
Fig.  366.  A  single,  soft  iron  shell  is  used  to  cover  both  of  these, 
thus  serving  as  a  dust  shield  and  also  as  a  magnetic  shield  to  prevent 
cross-talk  between  adjacent  relays — an  important  feature,  since 
it  will  be  remembered  the  cut-off  relays  are  left  permanently  con- 
nected with  the  talking  circuit.  Fig.  367,  which  shows  a  strip  of 
twenty  such  pairs  of  relays,  from  five  of  which  the  covers  have  been 


Fig.  367.     Strip  of  Kellogg  Line  and  Cut-Off  Relays 

removed,  is  an  excellent  detail  view,  of  the  general  practice  in  this 
respect;  obviously,  a  very  large  number  of  such  relays  may  be  mounted 
in  a  comparatively  small  space.  The  mounting  strip  shown  in  this 
cut  is  of  heavy  rolled  iron  and  is  provided  with  openings  through 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD     471 

which  the  connection  terminals— shown  more  clearly  in  Fig.  366 — 
project.  On  the  back  of  this  mounting  strip  all  the  wiring  is  done 
and  much  of  this  wiring — that  connecting  adjacent  terminals  on  the 
back  of  the  relay  strip — is  made  by  means  of  thin  copper  wires 
without  insulation,  the  wires  being  so  short  as  to  support  themselves 
without  danger  of  crossing  with  other  wires.  When  these  wires  are 
adjacent  to  ground  or  battery  wires  they  may  be  protected  by  sleev- 
ing, so  as  to  prevent  crosses. 

An   interesting  feature   in   relay  construction  is   found  in  the 
relay  of   the  Monarch  Telephone  Manufacturing  Company  shown 


Fig.  368.     Monarch  Relay 

in  Figs.  368  and  369.  The  assembled  relay  and  its  mount- 
ing strip  and  cap  are  shown  in  Fig.  368.  This  relay  is  so  con- 
structed that  by  the  lifting  of  a  single  latch  not  only  the  armature 
but  the  coil  may  be  bodily  removed,  as  shown  in  Fig.  369,  in 
which  the  latch  is  shown  in  its  raised  position.  As  seen,  the 
armature  has  an  L-shaped  projection  which  serves  to  operate  the 
contact  springs  lying  on  the  iron  plate  above  the  coil.  The  sim- 
plicity of  this  device  is  attractive,  and  it  is  of  convenience  not  only 
from  the  standpoint  of  easy  repairs  but  also  from  the  standpoint 
of  factory  assembly,  since  by  manufacturing  standard  coils  with 
different  characters  of  windings  and  standard  groups  of  springs,  it 
is  possible  to  produce  without  special  manufacture  almost  any  com- 
bination of  relay. 


472 


TELEPHONY 


Assembly.  The  arrangement  of  the  key  and  jack  equipment  in 
complete  multiple  switchboard  sections  is  clearly  shown  in  Fig.  370, 
which  shows  a  single  three-position  section  of  one  of  the  small 
multiple  switchboards  of  the  Kellogg  Switchboard  and  Supply  Com- 
pany. The  arrangement  of  keys  and  plugs  on  the  key  shelf  is  sub- 
stantially the  same  as  in  simple  common-battery  boards.  As  in  the 
simple  switchboards  the  supervisory  lamps  are  usually  mounted  on 
the  hinged  key  shelf  immediately  in  the  rear  of  the  ~  listening  and 
ringing  keys  and  with  such  spacing  as  to  lie  immediately  in  front  of 


Fig.  369.     Monarch  Relay 

the  plugs  to  which  they  correspond.  The  reason  for  mounting  the 
supervisory  lamps  on  the  key  shelf  is  to  make  them  easy  of  access 
in  case  of  the  necessity  of  lamp  renewals  or  repairs  on  the  wiring 
The  space  at  the  bottom  of  the  vertical  panels,  containing  the  jacks, 
is  left  blank,  as  this  space  is  obstructed  by  the  standing  plugs  in 
front  of  it.  Above  the  plugs,  however,  are  seen  the  alternate 
strips  of  line  lamps  and  answering  jacks,  the  lamps  in  each  case 
being  directly  below  the  corresponding  answering  jacks.  Above 
the  line  lamps  and  answering  jacks  in  the  two  positions  at  the 
right  there  are  blank  strips  into  which  additional  line  lamps  and 
jacks  may  be  placed  in  case  the  future  needs  of  the  system  demand 
it.  The  space  above  these  is  the  multiple  jack  space,  and  it  is  evi- 


COMMON-BATTERY  MULTIPLE  SWITCHBOARD      473 

dent  from  the  small  number  of  multiple  jacks  in  this  little  switch- 
board that  the  present  equipment  of  the  board  is  small.  It  is  also 
evident  from  the  amount  of  blank  space  left  for  future  installations 
of  multiple  jacks  that  a  considerable  growth  is  expected.  Thus, 
while  there  are  but  four  banks  of  100  multiple  jacks,  or  400  in  all, 
there  is  room  in  the  multiple  for  300  banks  of  100  multiple  jacks,  or 
3,000  in  all.  The  method  of  grouping  the  jacks  in  banks  of  100  and 
of  providing  for  their  future  growth  is  clearly  indicated  in  this  figure. 
The  next  section  at  the  right  of  the  one  shown  would  contain  a  dupli- 
cate set  of  multiple  jacks  and  also  an  additional  equipment  of  an- 
swering jacks  and  lamps* 


Fig.  370.    Small  Multiple  Board  Section 

For  ordinary  local  service  no  operator  would  sit  at  the  left-hand 
position  of  the  section  shown,  that  being  the  end  position,  since 
the  operator  there  would  not  be  able  easily  to  reach  the  extreme 
right-hand  portion  of  the  third  position  and  would  have  nothing  to 
reach  at  her  left.  This  end  position  in  this  particular  board  illus- 
trated is  provided  with  toll-line  equipment,  a  practice  not  uncom- 
mon in  small  multiple  boards.  To  prevent  confusion  let  us  assume 
that  the  multiple  jack  space  contains  its  full  equipment  of  3,000 


474  TELEPHONY 

jacks  on  each  section.  The  operator  in  the  center  position  of  the 
section  shown  could  easily  reach  any  one  of  the  jacks  on  that  sec- 
tion. The  operator  at  the  third  position  could  reach  any  jack  on  the 
second  and  third  position  of  her  section,  but  could  not  well  reach 
multiple  jacks  in  the  first  position.  She  would,  however,  have  a 
duplicate  of  the  multiple  jacks  in  this  first  position  in  the  section  at 
her  right,  i.  e.,  in  the  fourth  position,  and  it  makes  no  difference  on 
what  portion  of  the  switchboard  she  plugs  into  the  multiple  so  long 
as  she  plugs  into  a  jack  of  the  right  line. 


CHAPTER  XXVII 
TRUNKINQ  IN  MULTI=OFFICE  SYSTEMS 

It  has  been  stated  that  a  single  exchange  may  involve  a  number 
of  offices,  in  which  case  it  is  termed  a  multi-office  exchange.  In  a 
multi-office  exchange,  switchboards  are  necessary  at  each  office  in 
which  the  subscribers'  lines  of  the  corresponding  office  district  ter- 
minate. Means  for  intercommunication  between  the  subscribers 
in  one  office  and  those  in  any  other  office  are  afforded  by  inter-office 
trunks  extended  between  each  office  and  each  of  the  other  offices. 

If  the  character  of  the  community  is  such  that  each  of  the  offices 
has  so  few  lines  as  to  make  the  simple  switchboard  suffice  for  its  local 
connections,  then  the  trunking  between  the  offices  may  be  carried 
out  in  exactly  the  same  way  as  explained  between  the  various  simple 
switchboards  in  a  transfer  system,  the  only  difference  being  that  the 
trunks  are  long  enough  to  reach  from  one  office  to  another  instead  of 
being  short  and  entirely  local  to  a  single  office.  Such  a  condition  of 
affairs  would  only  be  found  in  cases  where  several  small  communi- 
ties were  grouped  closely  enough  together  to  make  them  operate  as 
a  single  exchange  district,  and  that  is  rather  unusual. 

The  subject  of  inter-office  trunking  so  far  as  manual  switchboards 
are  concerned  is,  therefore,  confined  mainly  to  trunking  between 
a  number  of  offices  each  equipped  with  a  manual  multiple  switch- 
board. 

Necessity  for  Multi=0ffice  Exchanges.  Before  taking  up  the 
details  of  the  methods  and  circuits  employed  in  trunking  in  multi- 
office  systems,  it  may  be  well  to  discuss  briefly  why  the  multi-office 
exchange  is  a  necessity,  and  why  it  would  not  be  just  as  well  to  serve 
all  of  the  subscribers  in  a  large  city  from  a  single  huge  switchboard 
in  which  all  of  the  subscribers'  lines  would  terminate.  It  cannot  be 
denied,  when  other  things  are  equal,  that  it  is  better  to  have  only 
one  operator  involved  in  any  connection  which  means  less  labor  and 
less  liability  of  error. 


476  TELEPHONY 

The  reasons,  however,  why  this  is  not  feasible  in  really  large 
exchanges  are  several.  The  main  one  is  that  of  the  larger  invest- 
ment required.  Considering  the  investment  first  from  the  stand- 
point of  the  subscriber's  line,  it  is  quite  clear  that  the  average  length 
of  subscriber's  line  will  be  very  much  greater  in  a  given  community 
if  all  of  the  lines  are  run  to  a  single  office,  than  will  be  the  case  if  the 
exchange  district  is  divided  into  smaller  office  districts  and  the  lines  run 
merely  from  the  subscribers  to  the  nearest  office.  There  is  a  direct 
and  very  large  gain  in  this  respect,  in  the  multi-office  system  over  the 
single  office  system  in  large  cities,  but  this  is  not  a  net  gain,  since 
there  is  an  offsetting  investment  necessary  in  the  trunk  lines  between 
the  offices,  which  of  course  are  separate  from  the  subscribers'  lines. 

Approaching  the  matter  from  the  standpoint  of  switchboard 
construction  and  operation,  another  strong  reason  becomes  apparent 
for  the  employment  of  more  than  one  office  in  large  exchange  dis- 
tricts. Both  the  difficulties  of  operation  and  the  expense  of  construc- 
tion and  maintenance  increase  very  rapidly  when  switchboards 
grow  beyond  a  certain  rather  well-defined  limit.  Obviously,  the 
limitation  of  the  multiple  switchboard  as  to  size  involves  the  number 
of  multiple  jacks  that  it  is  feasible  to  place  on  a  section.  Multiple 
switchboards  have  been  constructed  in  this  country  in  which  the 
sections  had  a  capacity  of  18,000  jacks.  Schemes  have  been  pro- 
posed and  put  into  effect  with  varying  success,  for  doubling  and 
quadrupling  the  capacity  of  multiple  switchboards,  one  of  these 
being  the  so-called  divided  multiple  board  devised  by  the  late  Milo 
G.  Kellogg,  and  once  used  in  Cleveland,  Ohio,  and  St.  Louis,  Mis- 
souri. Each  of  these  boards  had  an  ultimate  capacity  of  24,000  lines, 
and  each  has  been  replaced  by  a '  "straight"  multiple  board  of  smaller 
capacity.  In  general,  the  present  practice  in  America  does  not 
sanction  the  building  of  multiple  boards  of  more  than  about  10,000 
lines  capacity,  and  as  an  example  of  this  it  may  be  cited  that  the 
largest  standard  section  manufactured  for  the  Bell  companies  has  an 
ultimate  capacity  of  9,600  lines. 

European  engineers  have  shown  a  tendency  towards  the  oppo- 
site practice,  and  an  example  of  the  extreme  in  this  case  is  the  multiple 
switchboard  manufactured  by  the  Ericsson  Company,  and  installed 
in  Stockholm,  in  which  the  jacks  have  been  reduced  to  such  small 
dimensions  as  to  permit  an  ultimate  capacity  of  60,000  lines, 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  477 

The  reasons  governing  the  decision  of  American  engineers  in 
establishing  the  practice  of  employing  no  multiple  switchboards  of 
greater  capacity  than  about  lO/XX)  lines,  briefly  outlined,  are  as 
follows :  The  building  of  switchboards  with  larger  capacity,  while 
perfectly  possible,  makes  necessary  either  a  very  small  jack  or  some 
added  complexity,  such  as  that  of  the  divided  multiple  switchboard, 
either  of  which  i:  considered  objectionable.  Extremely  small  jacks 
and  large  multiples  introduce  difficulties  as  to  the  durability  of  the 
jacks  and  the  plugs,  and  also  they  tend  to  slow  down  the  work  of 
operators  and  to  introduce  errors.  They  also  introduce  the  necessity 
of  a  smaller  gauge  of  wire  through  the  multiple  than  it  has  been  found 
desirable  to  employ.  Considered  from  the  standpoint  of  expense, 
it  is  evident  that  as  a  multiple  switchboard  increases  in  number  of 
lines,  its  size  increases  in  two  dimensions,  i.  e.,  in  length  of  board 
and  height  of  section,  and  this  element  of  expense,  therefore,  is  a 
function  of  the  square  of  the  number  of  lines. 

The  matter  of  insurance,  both  with  respect  to  the  risk  as  to 
property  loss  and  the  risk  as  to  breakdown  of  the  service,  also  points 
distinctly  in  the  direction  of  a  plurality  of  offices  rather  than  one. 
Both  from  the  standpoint  of  risk  against  fire  and  other  hazards, 
which  might  damage  the  physical  property,  and  of  risk  against  in- 
terruption to  service  due  to  a  breakdown  of  the  switchboard  itself, 
or  a  failure  of  its  sources  of  current,  or  an  accident  to  the  cable  ap- 
proaches, the  single  office  practice  is  like  putting  all  one's  eggs  in  one 
basket. 

Another  factor  that  has  contributed  to  the  adoption  of  smaller 
switchboard  capacities  is  the  fact  that  in  the  very  large  cities  even  a 
40,000  line  multiple  switchboard  would  still  not  remove  the  necessity 
of  multi-office  exchanges  with  the  consequent  certainty  that  a  large 
proportion  of  the  calls  would  have  to  be  trunked  anyway. 

Undoubtedly,  one  of  the  reasons  for  the  difference  between 
American  and  European  practice  is  the  better  results  that  American 
operating  companies  have  been  able  to  secure  in  the  handling  of 
calls  at  the  incoming  end  of  trunks.  This  is  due,  no  doubt,  in  part 
to  the  differences  in  social  and  economic  conditions  under  which  ex- 
changes are  operated  in  this  country  and  abroad,  and  also  in  part  to 
the  characteristics  of  the  English  tongue  when  compared  to  some 
of  the  other  tongues  in  the  matter  of  ease  with  which  numbers  may 


478  TELEPHONY 

be  spoken.  In  America  it  has  been  found  possible  to  so  perfect  the 
operation  of  trunking  under  proper  operating  conditions  and  with 
good  equipment  as  to  relieve  multi-office  practice  of  many  of  the 
disadvantages  which  have  been  urged  against  it. 

Classification.  Broadly  speaking  there  are  two  general  meth- 
ods that  may  be  employed  in  trunking  between  exchanges.  The 
first  and  simplest  of  these  methods  is  to  employ  so-called  two-way 
trunks.  These,  as  their  name  indicates,  may  be  used  for  completing 
connections  between  offices  in  either  direction,  that  is,  whether  the 
call  originates  at  one  end  or  the  other.  The  other  way  is  by  the 
use  of  one-way  trunks,  wherein  each  trunk  carries  traffic  in  one  di- 
rection only.  Where  such  is  the  case,  one  end  of  the  trunk  is  always 
used  for  connecting  with  the  calling  subscriber's  line  and  is  termed 
the  outgoing  end,  and  the  other  end  is  always  used  in  completing  the 
connection  with  the  called  subscriber's  line,  and  is  referred  to  as  the 
incoming  end.  Traffic  in  the  other  direction  is  handled  by  another 
set  of  trunks  differing  from  the  first  set  only  in  that  their  outgoing 
and  incoming  ends  are  reversed. 

As  has  already  been  pointed  out,  a  system  of  trunks  employing 
two-way  trunks  is  called  a  single-track  system,  and  a  system  involving 
two  sets  of  one-way  trunks  is  called  a  double-track  system.  It  is 
to  be  noted  that  the  terms  outgoing  and  incoming,  as  applied  to  the 
ends  of  trunks  and  also  as  applied  to  traffic,  always  refer  to  the  di- 
rection in  which  the  trunk  handles  traffic  or  the  direction  in  which 
the  traffic  is  flowing  with  respect  to  the  particular  office  under  con- 
sideration at  the  time.  Thus  an  incoming  trunk  at  one  office  is  an 
outgoing  trunk  at  the  other. 

Two-Way  Trunks.  Two-way  trunks  are  nearly  always  employed 
where  the  traffic  is  very  small  and  they  are  nearly  always  operated 
by  having  the  A  -operator  plug  directly  into  the  jack  at  her  end  of  the 
trunk  and  displaying  a  signal  at  the  other  end  by  ringing  over  the 
trunk  as  she  would  over  an  ordinary  subscriber's  line.  The  oper- 
ator at  the  distant  exchange  answers  as  she  would  on  an  ordinary 
line,  by  plugging  into  the  jack  of  that  trunk,  and  receives  her  orders 
over  the  trunk  either  from  the  originating  operator  or  from  the  sub- 
scriber, and  then  completes  the  connection  with  the  called  subscriber. 
Such  trunks  are  often  referred  to  as  "ring-down"  trunks,  and  their 
equipment  consists  in  a  drop  and  jack  at  each  end.  In  case  there 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  479 

is  a  multiple  board  at  either  01  both  of  the  offices,  then  the  equip- 
ment at  each  end  of  the  trunk  would  consist  of  a  drop  and  answering 
jack,  together  with  the  full  quota  of  multiple  jacks.  It  is  readily 
seen  that  this  mode  of  operation  is  slow,  as  the  work  that  each  oper- 
ator has  to  do  is  the  same  as  that  in  connecting  two  local  subscribers, 
plus  the  time  that  it  takes  for  the  operators  to  communicate  with 
each  other  over  the  trunk. 

One-Way  Trunks.  Where  one-way  trunks  are  employed  in  the 
double-track  system,  the  trunks,  assuming  that  they  connect  multi- 
ple boards,  are  provided  with  multiple  jacks  only  at  their  outgoing 
ends,  so  that  any  operator  ma)  reach  them  for  an  outgoing  connection, 
and  at  their  incoming  ends  they  terminate  each  in  a  single  plug  and 
in  suitable  signals  and  ringing  keys,  the  purpose  of  which  will  be 
explained  later.  Over  such  trunks  there  is  no  verbal  communication 
between  the  operators,  the  instructions  passing  between  the  opera- 
tors over  separate  order-wire  circuits.  This  is  done  in  order  that 
the  trunk  may  be  available  as  much  as  possible  for  actual  conver- 
sation between  the  subscribers.  It  may  be  stated  at  this  point 
that  the  duration  of  the  period  from  the  time  when  a  trunk  is  appro- 
priated by  the  operators  for  the  making  of  a  certain  connection  Am  til 
the  time  when  the  trunk  is  finally  released  and  made  available  for 
another  connection  is  called  the  holding  timef  and  this  holding  time 
includes  not  only  the  period  while  the  subscribers  are  in  actual  con- 
versation over  it,  but  also  the  periods  while  the  operators  are  making 
the  connection  and  afterwards  while  they  are  taking  it  down.  It. 
may  be  said,  therefore,  that  the  purpose  of  employing  separate  or- 
der wires  for  communication  between  the  operators  is  to  make  the 
holding  time  on  the  trunks  as  small  as  possible  and,  therefore,  for 
the  purpose  of  enabling  a  given  trunk  to  take  part  in  as  many 
connections  in  a  given  time  as  possible. 

In  outline  the  operation  of  a  one-way  trunk  between  common- 
battery,  manual,  multiple  switchboards  is,  with  modifications  that 
will  be  pointed  out  afterwards,  as  follows:  When  a  subscriber's 
line  signal  is  displayed  at  one  office,  the  operator  in  attendance  at 
that  position  answers  and  finding  that  the  call  is  for  a  subscriber  in 
another  office,  she  presses  an  order-wire  key  and  thereby  connects 
her  telephone  set  directly  with  that  of  a  5-operator  at  the  proper 
other  office.  Unless  she  finds  that  other  operators  are  talking  over  the 


480  TELEPHONY 

order  wire,  she  merely  states  the  number  of  the  called  subscriber, 
and  the  B-operator  whose  telephone  set  is  permanently  connected 
with  that  order  wire  merely  repeats  the  number  of  the  called  sub- 
scriber and  follows  this  by  designating  the  number  of  the  trunk 
which  the  yl-operator  is  to  employ  in  making  the  connection.  The 
^4-operator,  thereupon,  immediately  and  without  testing,  inserts  the 
calling  plug  of  the  pair  used  in  answering  the  call  into  the  trunk  jack 
designated  by  the  5-operator;  the  jB-operator  simultaneously  tests 
the  multiple  jack  of  the  called  subscriber  and,  if  she  finds  it  not  busy, 
inserts  the  plug  of  the  designated  trunk  into  the  multiple  jack  of  the 
called  subscriber  and  rings  his  bell  by  pressing  the  ringing  key  as- 
sociated with  the  trunk  cord  used.  The  work  on  the  part  of  the  A- 
operator  in  connecting  with  the  outgoing  end  of  the  trunk  and  on 
the  part  of  the  B -operator  in  connecting  the  incoming  end  of  the 
trunk  with  the  line  goes  on  simultaneously,  and  it  makes  no  differ- 
ence which  of  these  operators  completes  the  connection  first. 

It  is  the  common  practice  ot  the  Bell  operating  companies  in 
this  country  to  employ  what  is  called  automatic  or  machine  ring- 
ing in  connection  with  the  5-operator's  work.  When  the  B-oper- 
ator  presses  the  ringing  key  associated  with  the  incoming  trunk  cord, 
she  pays  no  further  attention  to  it,  and  she  has  no  supervisory  lamp 
to  inform  her  as  to  whether  or  not  the  subscriber  has  answered.  The 
ringing  key  is  held  down,  after  its  depression  by  the  operator,  either 
by  an  electromagnet  or  by  a  magnet-controlled  latch,  and  the  ring- 
ing of  the  subscriber's  bell  continues  at  periodic  intervals  as  controlled 
by  the  ringing  commutator  associated  with  the  ringing  machine. 
When  the  subscriber  answers,  however,  the  closure  of  his  line  cir- 
cuit results  in  such  an  operation  of  the  magnet  associated  with  the 
ringing  key  as  .to  release  the  ringing  key  and  thus  to  automatically 
discontinue  the  ringing  current. 

When  a  connection  is  established  between  two  subscribers 
through  such  a  trunk  the  supervision  of  the  connection  falls  entirely 
upon  the  yl-operator  who  established  it.  This  means  that  the  calling 
supervisory  lamp  at  the  ^4-operator's  position  is  controlled  over  the 
trunk  from  the  station  of  the  called  subscriber,  the  answering  super- 
visory lamp  being,  of  course,  under  the  control  of  the  calling  sub- 
scriber as  in  the  case  of  a  local  connection.  It  is,  therefore,  the  A- 
operator  who  always  initiates  the  taking  down  of  a  trunk  connection, 


TRUNKING  IN  MULTI-OFFICE   SYSTEMS  481 

• 

and  when,  in  response  to  the  lighting  of  the  two  lamps,  she  with- 
draws her  calling  plug  from  the  trunk  jack,  the  supervisory  lamp  as- 
sociated with  the  incoming  end  of  the  trunk  at  the  other  office  is 
lighted,  and  the  5-operator  obeys  it  by  pulling  down  the  plug. 

If,  upon  testing  the  multiple  jack  of  the  called  subscriber's 
line,  the  5-operator  finds  the  line  to  be  busy,  she  at  once  inserts  the 
trunk  plug  into  a  so-called  "busy-back"  jack,  which  is  merely  a  jack 
whose  terminals  are  permanently  connected  to  a  circuit  that  is  in- 
termittently opened  and  closed,  and  which  also  has  impressed  upon  it 
an  alternating  current  of  such  a  nature  as  to  produce  the  familiar 
"buzz-buzz"  in  a  telephone  receiver.  The  opening  and  closing  of 
this  circuit  causes  the  calling  supervisory  lamp  of  the  ^4-operator  to 
flash  at  periodic  intervals  just  as  if  the  called  subscriber  had  raised 
and  lowered  his  receiver,  but  more  regularly.  This  is  the  indication 
to  the  ^4-operator  that  the  line  called  for  is  busy.  The  buzzing 
sound  is  repeated  back  through  the  cord  circuit  of  the  yl-operator 
to  the  calling  subscriber  and  is  a  notification  to  him  that  the  line  is 
busy. 

Sometimes,  as  is  practiced  in  New  York  City,  for  instance,  the 
buzzing  feature  is  omitted,  and  the  only  indication  that  the  calling 
subscriber  receives  that  the  called-for  line  is  busy  is  being  told  so  by 
the  yl-operator.  This  may  be  considered  a  special  feature  and  it  is 
employed  in  New  York  because  there  the  custom  exists  of  telling  a 
calling  subscriber,  when  the  line  he  has  called  for  has  been  found 
busy,  that  the  party  will  be  secured  for  him  and  that  he,  the  calling 
subscriber,  \vill  be  called,  if  he  desires. 

A  modification  of  this  busy-back  feature  that  has  been  employed 
in  Boston,  and  perhaps  in  other  places,  is  to  associate  with  the  busy- 
back  jack  at  the  5-operator's  position  a  phonograph  which,  like  a 
parrot,  keeps  repeating  "Line  busy — please  call  again."  Where 
this  is  done  the  calling  subscriber,  if  he  understands  what  the  phono- 
graph says,  is  supposed  to  hang  up  his  receiver,  at  which  time  the 
^4-operator  takes  down  the  connection  and  the  5-operator  follows 
in  response  to  the  notification  of  her  supervisory  lamp.  The  phono- 
graph busy-back  scheme,  while  ingenious,  has  not  been  a  success  and 
has  generally  been  abandoned, 

As  a  rule  the  independent  operating  companies  in  this  country 
have  not  employed  automatic  ringing,  and  in  this  case  the  5-oper- 


482  TELEPHONY 

• 

ators  have  been  required  to  operate  their  ringing  keys  and  to  watch 
for  the  response  of  the  called  subscriber.  In  order  to  arrange  for 
this,  another  supervisory  lamp,  termed  the  ringing  lamp,  is  associated 
with  each  incoming  trunk  plug,  the  going  out  of  this  lamp  being  a 
notification  to  the  5-operator  to  discontinue  ringing. 

Western  Electric  Trunk  Circuits.  The  principles  involved 
in  inter-office  trunking  with  automatic  ringing,  are  well  illustrated 
in  the  trunk  circuit  employed  by  the  Western  Electric  Company  in 
connection  with  its  No.  1  relay  boards.  The  dotted  dividing  line 
through  the  center  of  Fig.  371  represents  the  separating  space  be- 
tween two  offices.  The  calling  subscriber's  line  in  the  first  office  is 
shown  at  the  extreme  left  and  the  called  subscriber's  line  in  the  sec- 
ond office  is  shown  at  the  extreme  right.  Both  of  these  lines  are  stand- 
ard multiple  switchboard  lines  of  the  form  already  discussed.  The 
equipment  illustrated  in  the  first  office  is  that  of  an  ^4-board,  the 
cord  circuit  shown  being  that  of  the  regular  .4-operator.  The 
outgoing  trunk  jacks  connecting  with  the  trunk  leading  to  the  other 
office  are,  it  will  be  understood,  multipled  through  the  ^-sections 
of  the  board  and  contain  no  relay  equipment,  but  the  test  rings  are 
connected  to  ground  through  a  resistance  coil  1,  which  takes  the 
place  of  the  cut-off  relay  winding  of  a  regular  line  so  far  as  test  con- 
ditions and  supervisory  relay  operation  are  concerned.  The  equip- 
ment illustrated  in  the  second  office  is  that  of  a  5-board,  it  being 
understood  that  the  called  subscriber's  line  is  multipled  through 
both  the  A-  and  J5-boards  at  that  office.  The  part  of  the  equipment 
that  is  at  this  point  unfamiliar  to  the  reader  is,  therefore,  the  cord 
circuit  at  the  5-operator's  board.  This  includes,  broadly  speaking, 
the  means:  (1)  for  furnishing  battery  current  to  the  called  subscriber; 
(2)  for  accomplishing  the  ringing  of  the  called  subscriber  and  for 
automatically  stopping  the  ringing  when  he  shall  respond;  (3)  for 
performing  the  ordinary  switching  functions  in  connection  with  the 
relays  of  the  called  subscriber's  line  in  just  the  same  way  that  an  A- 
operator's  cord  carries  out  these  functions;  and  (4)  for  causing  the 
operation  of  the  calling  supervisory  relay  of  the  yl-operator's  cord 
circuit  in  just  the  same  manner,  under  control  of  the  connected  called 
subscriber,  as  if  that  subscriber's  line  had  been  connected  directly 
to  the  yl-operator's  cord  circuit. 

The  operation  of  these  devices  in  the  .B-operator's  cord  circuit 


484  TELEPHONY 

may  be  best  understood  by  following  the  establishment  of  the  con- 
nection. Assuming  that  the  calling  subscriber  in  the  first  office 
desires  a  connection  with  the  subscriber's  line  shown  in  the  second 
office, and  that  the  /1-operator  at  the  first  office  has  answered  the  call, 
she  will  then  communicate  by  order  wire  with  the  J5-operator  at  the 
second  office,  stating  the  number  of  the  called  subscriber  and  receiving 
from  that  operator  in  return  the  number  of  the  trunk  to  be  employed. 
The  two  operators  will  then  proceed  simultaneously  to  establish 
the  connection,  the  /1-operator  inserting  the  calling  plug  into  the 
outgoing  trunk  jack,  and  the  B-operator  inserting  the  trunk  plug 
into  the  multiple  jack  of  the  called  subscriber's  line  after  testing. 
We  will  assume  at  first  that  the  called  subscriber's  line  is  found  idle 
and  that  both  of  the  operators  complete  their  respective  portions  of 
the  work  at  the  same  time  and  we  will  consider  first  the  condition 
of  the  calling  supervisory  relay  at  the  /1-opera  tor's  position. 

The  circuit  of  the  calling  supervisory  lamp  will  have  been  closed 
through  the  resistance  coil  1  connected  with  the  outgoing  trunk 
jacks  and  the  lamp  will  be  lighted  because,  as  will  be  shown,  it  is 
not  yet  shunted  out  by  the  operation  of  its  associated  supervisory 
relay.  Tracing  the  circuit  of  the  calling  supervisory  relay  of  the 
yl-operator's  circuit,  it  will  be  found  to  pass  from  the  live  side  of  the 
battery  to  the  ring  side  of  the  trunk  circuit  through  one  winding  of  the 
repeating  coil  of  the  5-operator's  cord;  beyond  this  the  circuit  is 
open,  since  no  path  exists  through  the  condenser  2  bridged  across 
the  trunk  circuit  or  through  the  normally  open  contacts  of  the  relay 
3  connected  in  the  talking  circuit  of  the  trunk.  The  association  of 
this  relay  3  with  the  repeating  coil  and  the  battery  of  the  trunk  is 
seen  to  be  just  the  same  as  that  of  a  supervisory  relay  in  the  A-oper- 
ator's  cord,  and  it  is  clear,  therefore,  that  this  relay  3  will  not  be 
energized  until  the  called  subscriber  has  responded.  When  it  is 
energized  it  will  complete  the  path  to  ground  through  the  /1-operator's 
calling  supervisory  relay  and  operate  to  shunt  out  the  ^4-operator's 
calling  supervisory  lamp  in  just  the  same  manner  as  if  the  yl-oper- 
ator's calling  plug  had  been  connected  directly  with  the  line  of  the 
calling  subscriber.  In  other  words,  the  called  subscriber  in  the 
second  office  controls  the  relay  3,  which,  in  turn,  controls  the  calling 
supervisory  relay  of  the  yl-operator,  which,  in  turn,  shunts  out  its 
lamp. 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  485 

The  connection  being  completed  between  the  two  subscribers, 
the  5-operator  depresses  one  or  the  other  of  the  ringing  keys  5  or  6, 
according  to  which  party  on  the  line  is  called,  assuming  that  it  is  a 
two-party  line.  It  will  be  noticed  that  the  springs  of  these  ringing 
keys  are  not  serially  arranged  in  the  talking  circuit,  but  the  cutting 
off  of  the  trunk  circuit  back  of  the  ringing  keys  is  accomplished 
by  the  set  of  springs  shown  just  at  the  left  of  the  ringing  keys,  which 
set  of  springs  7  is  operated  whenever  either  one  of  the  ringing  keys 
is  depressed.  An  auxiliary  pair  of  contacts,  shown  just  below 
the  group  of  springs  7,  is  also  operated  mechanically  whenever 
either  one  of  the  ringing  keys  is  depressed,  and  this  serves  to 
close  one  of  two  normally  open  points  in  the  circuit  of  the  ringing- 
key  holding  magnet  8.  This  holding  magnet  8  is  so  arranged  with 
respect  to  the  contacts  of  the  ringing  key  that  whenever  any  one  of 
them  is  depressed  by  the  operator,  it  will  be  held  depressed  as  long 
as  the  magnet  is  energized  just  the  same  as  if  the  operator  kept  her 
finger  on  the  key.  The  other  normally  open  point  in  the  circuit  of 
the  holding  magnet  8  is  at  the  lower  pair  of  contacts  of  the  test  and 
holding  relay  9.  This  relay  is  operated  whenever  the  trunk  plug 
is  inserted  in  the  jack  of  a  called  line,  regardless  of  the  position 
of  the  subscriber's  equipment  on  that  line.  The  circuit  may  be 
traced  from  the  live  side  of  the  battery  through  the  trunk  discon- 
nect lamp  4,  coil  9,  sleeve  strand  of  cord,  and  to  ground  through 
the  cut-off  relay  of  the  line.  The  insertion  of  the  trunk  plug  into 
the  jack  thus  leaves  the  completion  of  the  holding-magnet  circuit 
dependent  only  upon  the  auxiliary  contact  on  the  ringing  key,  and, 
therefore,  as  soon  as  the  operator  presses  either  one  of  these  keys,  the 
clutch  magnet  is  energized  and  the  key  is  held  down,  so  that  ringing 
current  continues  to  flow  at  regular  intervals  to  the  called  subscriber's 
station. 

The  ringing  current  issues  from  the  generator  10,  but  the  supply 
circuit  from  it  is  periodically  interrupted  by  the  commutator 
11  geared  to  the  ringing-machine  shaft.  This  periodically  inter- 
rupted ringing  current  passes  to  the  ringing-key  contacts  through 
the  coil  of  the  ringing  cut-off  relay  12,  and  thence  to  the  subscriber's 
line.  The  ringing  current  is,  however,  insufficient  to  cause  the 
operation  of  this  relay  12  as  long  as  the  high  resistance  and  impedance 
of  the  subscriber's  bell  and  condenser  are  in  the  circuit.  It  is,  how- 


486  TELEPHONY 

ever,  sufficiently  sensitive  to  be  operated  by  this  ringing  current 
when  the  subscriber  responds  and  thus  substitutes  the  comparatively 
low  resistance  and  impedance  path  of  his  talking  apparatus  for  the 
previous  path  through  his  bell.  The  pulling  up  of  the  ringing  cut- 
off relay  12  breaks  a  third  normally  closed  contact  in  the  circuit  of 
the  holding  coil  8,  de-energizing  that  coil  and  releasing  the  ringing 
key,  thus  cutting  off  ringing  current.  There  is  a  third  brush  on  the 
commutator  11  connected  with  the  live  side  of  the  central  battery, 
and  this  is  merely  for  the  purpose  of  assuring  the  energizing  of  the 
ringing  cut-off  relay  12,  should  the  subscriber  respond  during  the 
interval  while  the  commutator  11  held  the  ringing  current  cut  off. 
The  relay  12  may  thus  be  energized  either  from  the  battery,  if  the 
subscriber  responds  during  a  period  of  silence  of  his  ringer,  or  from 
the  generator  10,  if  the  subscriber  responds  during  a  period  while 
his  bell  is  sounding;  in  either  case  the  ringing  current  will  be  promptly 
cut  off  by  the  release  of  the  ringing  key. 

The  trunk  operator's  "disconnect  lamp''  is  shown  at  4>  and  it  is 
to  be  remembered  that  this  lamp  is  lighted  only  when  the  .4-operator 
takes  down  the  connection  at  her  end,  and  also  that  this  lamp  is 
entirely  out  of  the  control  of  the  subscribers,  the  conditions  which 
determine  its  illumination  being  dependent  on  the  positions  of  the 
operators'  plugs  at  the  two  ends  of  the  trunk.  With  both  plugs  up, 
the  lamp  4  will  receive  current,  but  will  be  shunted  to  prevent  its 
illumination.  The  path  over  which  it  receives  this  current  may  be 
traced  from  battery  through  the  lamp  4,  thence  through  the  coil  of 
the  relay  9  and  the  cut-off  relay  of  the  called  subscriber's  line.  This 
current  would  be  sufficient  to  illuminate  the  lamp,  but  the  lamp  is 
shunted  by  a  circuit  which  may  be  traced  from  the  live  side  of  bat- 
tery through  the  contact  of  the  relay  13,  closed  at  the  time,  and 
through  the  coil  of  the  trunk  cut-off  relay  coil  14*  The  resistance 
of  this  coil  is  so  proportioned  to  the  other  parts  of  the  circuit  as  to 
prevent  the  illumination  of  the  lamp  just  exactly  as  in  the  case  of  the 
shunting  resistances  of  the  lamps  in  the  .4-operator's  cord.  It 
will  be  seen,  therefore,  that  the  supply  of  current  to  the  trunk  dis- 
connect lamp  is  dependent  on  the  trunk  plug  being  inserted  into  the 
jack  of  the  subscriber's  line  and  that  the  shunting  out  of  this  lamp 
is  dependent  on  the  energization  of  the  relay  13.  This  relay  13  is 
energized  as  long  as  the  J-operator's  plug  is  inserted  into  the  out- 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  487 

going  trunk  jack,  the  path  of  the  energizing  circuit  being  traced 
from  the  live  side  of  the  battery  at  the  second  office  through  the 
right-hand  winding  of  this  relay,  thence  over  the  tip  side  of  the  trunk 
to  ground  at  the  first  office.  From  this  it  follows  that  as  long  as  both 
plugs  are  up,  the  disconnect  lamp  will  receive  current  but  will  be 
shunted  out,  and  as  soon  as  the  /1-operator  pulls  down  the  connec- 
tion, the  relay  13  will  be  de-energized  and  will  thus  remove  the  shunt 
from  about  the  lamp,  allowing  its  illumination.  The  left-hand  winding 
of  the  relay  13  performs  no  operating  function,  but  is  merely  to  main- 
tain the  balance  of  the  talking  circuit,  it  being  bridged  during  the 
connection  from  the  ring  side  of  the  trunk  to  ground  in  order  to 
balance  the  bridge  connection  of  the  right-hand  coil  from  the  live 
side  of  battery  to  the  tip  side  of  the  trunk  circuit. 

The  relay  14,  already  referred  to  as  forming  a  shunt  for  the 
trunk  disconnect  lamp,  has  for  its  function  the  keeping  of  the  talking 
circuit  through  the  trunk  open  until  such  time  as  the  relay  13  oper- 
ates, this  being  purely  an  insurance  against  unnecessary  ringing  of 
a  subscriber  in  case  the  ^4-operator  should  by  mistake  plug  into  the 
wrong  trunk.  It  is  not,  therefore,  until  the  ^4-operator  has  plugged 
into  the  trunk  and  the  relay  13  has  been  operated  to  cause  the 
energization  of  the  relay  14  that  the  ringing  of  the  called  subscriber 
can  occur,  regardless  of  what  the  5-operator  may  have  done. 

The  relay  9  has  an  additional  function  to  that  of  helping  to  con- 
trol the  circuit  of  the  ringing-key  holding  magnet.  This  is  the 
holding  of  the  test  circuit  complete  until  the  operator  has  tested  and 
made  a  connection  and  then  automatically  opening  it.  The  test 
circuit  of  the  S-operatcr's  trunk  may  be  traced,  at  the  time  of  testing, 
from  the  thimble  of  the  multiple  jack  under  test,  through  the  tip  of 
the  cord,  thence  through  the  uppermost  pair  of  contacts  of  the  relay 
9  to  ground  through  a  winding  of  the  5-operator's  induction  coil. 
After  the  test  has  been  made  and  the  plug  inserted,  the  relay  9, 
which  is  operated  by  the  insertion  of  the  plug,  acts  to  open  this 
test  circuit  and  at  the  same  time  complete  the  tip  side  of  the  cord 
circuit. 

In  the  upper  portion  of  Fig.  371  the  order-wire  connections,  by 
which  the  ^4-operator  and  the  5-operator  communicates  are  indi- 
cated. It  must  be  remembered  in  connection  with  these  that  the 
^-operator  only  has  control  of  this  connection,  the  5-operator 


488 


TELEPHONY 


being  compelled  necessarily  to  hear  whatever  the  ^4-operators  have 
to  say  when  the  ^4-operators  come  in  on  the  circuit. 

The  incoming  trunk  circuit  employed  by  the  Western  Electric 
Company  for  four-party  line  ringing  is  shown  in  Fig.  372,  it  being 


PULSA77MG  - 
CURRENT 


Fig.  372.     Incoming  Trunk  Circuit 


necessarily  somewhat  modified  from  that  shown  in  Fig.  371,  which 
is  adapted  for  two-party  line  ringing  only.  In  addition  to  the  pro- 
vision of  the  four-party  line  ringing  keys,  by  which  positive  or  nega- 
tive pulsating  current  is  received  over  either  limb  of  the  line,  and  to 


Fig.  373.     Western  Electric  Trunk  Ringing  Key 

the  provision  of  the  regular  alternating  current  ringing  key  for 
ringing  on  single  party  lines,  it  is  necessary  in  the  ringing  cut-off 
relay  to  provide  for  keeping  the  alternating  and  the  pulsating  ring- 
ing currents  entirely  separate.  For  this  reason,  the  ringing  cut- 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS 


489 


off  relay  12  is  provided  with  two  windings,  that  at  the  right  being 
in  the  path  of  the  alternating  ringing  currents  that  are  supplied 
to  the  alternating  current  key,  and  that  at  the  left  being  in  the 


Fig.  374     Trunk  Relay 

ground  return  path  for  all  of  the  pulsating  ringing  currents  supplied 
to  the  pulsating  keys.  With  this  explanation  it  is  believed  that  this 
circuit  will  be  understood  from  what  has  been  said  in  connection  with 
Fig.  371.  The  operation  of  the  holding  coil  8  is  the  same  in  each 
case,  the  holding  magnet  in  Fig.  372  serving  to  hold  depressed  any 
one  of  the  five  ringing  keys  that  may  have  been  used  in  calling  the 
subscriber. 

The  standard  four-party  line,  trunk  ringing  key  of  the  Western 
Electric  Company  is  shown  in  Fig.  373.     In  this  the  various  keys 


Fig.  375.     Trunk  Relay 


operate  not  by  pressure  but  rather  by  being  pulled  by  the  finger  of 
the  operator  in  such  a  way  as  to  subject  the  key  shaft  to  a  twisting 


490 


TELEPHONY 


movement.  The  holding  magnet  lies  on  the  side  opposite  to  that 
shown  in  the  figure  and  extends  along  the  full  length  of  the  set  of 
keys,  each  key  shaft  being  provided  with  an  armature  which  is  held 
by  this  magnet  until  the  magnet  is  de-energized  by  the  action  of  the 
ringing  cut-off  relay. 

The  standard  trunk  relays  employed  by  the  Western  Electric 
Company  in  connection  with  the  circuits  just  described  are  shown 
in  Figs.  374  and  375.  In  each  case  the  dust-cap  or  shield  is  also 
shown.  The  relay  of  Fig.  374  is  similar  to  the  regular  cut-off  relay 
and  is  the  one  used  for  relays  9  and  14  of  Figs.  371  and  372.  The 
relay  of  Fig.  375  is  somewhat  similar  to  the  subscriber's  line  relay 
in  that  it  has  a  tilting  armature,  and  is  the  one  used  at  13  in  Figs. 
371  and  372.  The  trunk  relay  3  in  Figs.  371  and  372  is  the  same 
as  the  ^-operator's  supervisory  relays  already  discussed. 

It  has  been  stated  that  under  certain  circumstances  5-operator's 
trunk  circuits  devoid  of  ringing  keys,  and  consequently  of  all  keys, 
may  be  employed.  This,  so  far  as  the  practice  of  the  Bell  companies 


Fig.  376.     Keyless  Trunk 

is  concerned,  is  true  only  in  offices  where  there  .are  no  party  lines, 
or  where,  as  in  many  of  the  Chicago  offices,  the  party  lines  are 
worked  on  the  "jack  per  station"  basis.  In  "jack  per  station" 
working,  the  selection  of  the  station  on  a  party  line  is  determined 
by  the  jack  on  which  the  plug  is  put,  rather  than  by  a  ringing  key, 
and  hence  the  keyless  trunk  may  be  employed. 

A  keyless  trunk  as  used  in  New  York  is  shown  in  Fig.  376 
This  has  no  manually  operated  keys  whatever,  and  the  relay  17, 
when   it   is   operated,   establishes   connection   between    the   ringing 
generator  and  the  conductors  of  the  trunk  plug.    The  relays  3,  18, 
and  12  operate  in  a  manner  identical  with  those  bearing  corre- 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  491 

spending  numbers  in  Fig.  371.  As  soon  as  the  trunk  operator  plugs 
into  the  multiple  jack  of  the  called  subscriber,  the  relay  16  will  op- 
erate for  the  same  reason  that  the  relay  9  operated  in  connection 
with  Fig.  371.  The  trunk  disconnect  lamp  will  receive  current, 
but  if  the  operator  has  already  established  connection  with  the  other 
end  of  the  trunk,  this  lamp  will  not  be  lighted  because  shunted  by 
the  relay  17,  due  to  the  pulling  up  of  the  armature  of  the  relay  13. 
The  relay  15  plays  no  part  in  the  operation  so  far  described,  because 
of  the  fact  that  its  winding  is  short-circuited  by  its  own  contacts  and 
those  of  relay  12,  when  the  latter  is  not  energized.  As  a  result  of 
the  operation  of  the  relay  17,  ringing  current  is  sent  to  line,  the  sup- 
ply circuit  including  the  coil  of  the  relay  12.  As  soon  as  the  sub- 
scriber responds  to  this  ringing  current,  the  armature  of  the  relay  12 
is  pulled  up,  thus  breaking  the  shunt  about  the  relay  15,  which,  there- 
fore, starts  to  operate  in  series  with  the  relay  17,  but  as  its  armatures 
assume  their  attracted  position,  the  relay  17  is  cut  out  of  the  circuit, 
the  coil  of  the  relay  15  being  substituted  for  that  of  the  relay  17  in  the 
shunt  path  around  the  lamp  4-  The  relay  17  falls  back  and  cuts  off 
the  ringing  current.  The  relay  15  now  occupies  the  place  with  respect 
to  the  shunt  around  the  lamp  4  that  the  relay  17  formerly  did,  the 
continuity  of  this  shunt  being  determined  by  the  energization  of  the 
relay  13.  When  the  ^4-operator  at  the  distant  exchange  withdraws 
the  calling  plug  from  the  trunk  jack,  this  relay  13  becomes  de-ener- 
gized, breaking  the  shunt  about  the  lamp  4  and  permitting  the  dis- 
play of  that  lamp  as  a  signal  to  the  operator  to  take  down  the  con- 
nection. It  may  be  asked  why  the  falling  back  of  relay  15  will  not 
again  energize  relay  17  and  thus  cause  a  false  ring  on  the  called  sub- 
scriber. This  will  not  occur  because  both  the  relays  15  and  17  de- 
pend for  their  energization  on  the  closure  of  the  contacts  of  the  relay 
13,  and  when  this  falls  back  the  relay  17  cannot  again  be  energized 
even  though  the  relay  15  assumes  its  normal  position. 

Kellogg  Trunk  Circuits.  The  provision  for  proper  working  of 
trunk  circuits  in  connection  with  the  two-wire  multiple  switchboards 
is  not  an  altogether  easy  matter,  owing  particularly  to  the  smaller  num- 
ber of  wires  available  in  the  plug  circuits.  It  has  been  worked  out 
in  a  highly  ingenious  way,  however,  by  the  Kellogg  Company,  and  a 
diagram  of  their  incoming  trunk  circuit,  together  with  the  associated 
circuits  involved  in  an  inter-office  connection,  is  shown  in  Fig.  377. 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  493 

This  figure  illustrates  a  connection  from  a  regular  two-wire 
multiple  subscriber's  line  in  one  office,  through  an  ^4-operator's 
cord  circuit  there,  to  the  outgoing  trunk  jacks  at  that  office,  thence 
through  the  incoming  trunk  circuit  at  the  other  office  to  the  regular 
two-wire  multiple  subscriber's  line  at  that  second  office.  The  por- 
tion of  this  diagram  to  be  particularly  considered  is  that  of  the  B- 
operator's  cord  circuit.  The  trunk  circuit  terminates  in  the  multipled 
outgoing  trunk  jacks  at  the  first  office,  the  trunk  extending  between 
offices  consisting,  of  course,  of  but  two  wires.  We  will  first  consider 
the  control  of  the  calling  supervisory  lamp  in  the  .4-operator's  cord 
circuit,  it  being  remembered  that  this  control  must  be  from  the  called 
subscriber's  station.  It  will  be  noticed  that  the  left-hand  armature  of 
the  relay  1  serves  normally  to  bridge  the  winding  of  relay  2  across  the 
cord  circuit  around  the  condenser  3.  When,  however,  the  relay  1  pulls 
up,  the  coil  of  relay  4  ls  substituted  in  this  bridge  connection  across 
the  trunk.  The  relay  2  has  a  very  high  resistance  winding — about 
15,000  ohms — and  this  resistance  is  so  great  that  the  tip  supervisory 
relay  of  the  ^-operator's  cord  will  not  pull  up  through  it.  As  a  re- 
sult, when  this  relay  is  bridged  across  the  trunk  circuit,  the  tip  relay 
on  the  calling  side  of  the  .4-operator's  cord  circuit  is  de-energized, 
just  as  if  the  trunk  circuit  were  open,  and  this  results  in  the  lighting 
of  the  yl-operator's  calling  supervisory  lamp.  The  winding  of  the 
relay  4>  however,  is  of  low  resistance — about  50  ohms — and  when 
this  is  substituted  for  the  high-resistance  winding  of  the  relay  2,  the 
tip  relay  on  the  calling  side  of  the  ^4-operator's  cord  is  energized, 
resulting  in  the  extinguishing  of  the  calling  supervisory  lamp.  The 
illumination  of  the  yl-operator's  calling  supervisory  lamp  depends, 
therefore,  on  whether  the  high-resistance  relay  2,  or  the  low-resistance 
relay  4,  is  bridged  across  the  trunk,  and  this  depends  on  whether  the 
relay  1  is  energized  or  not.  The  relay  1,  being  bridged  from  the  tip 
side  of  the  trunk  circuit  to  ground  and  serving  as  the  means  of  supply 
of  battery  current  to  the  called  subscriber,  is  operated  whenever 
the  called  subscriber's  receiver  is  removed  from  its  hook.  There- 
fore, the  called  subscriber's  hook  controls  the  operation  of  this  relay 
1,  which,  in  turn,  controls  the  conditions  which  cause  the  illumination 
or  darkness  of  the  calling  supervisory  lamp  at  the  distant  office. 

Assuming  that  the  A  -operator  has  received  and  answered  a  call, 
and  has  communicated  with  the  B-operator,  telling  her  the  number 


494  TELEPHONY 

of  the  called  subscriber,  and  has  received,  in  turn,  the  number  of 
the  trunk  to  be  used,  and  that  both  operators  have  put  up  the  con- 
nection, then  it  will  be  clear  from  what  has  been  said  that  the  calling 
supervisory  lamp  of  the  yl-operator  will  be  lighted  until  the  called 
subscriber  removes  his  receiver  from  its  hook,  because  the  tip  relay 
in  the  .4-operator's  cord  circuit  will  not  pull  up  through  the  15,000- 
ohm  resistance  winding  of  the  relay  2.  As  soon  as  the  subscriber 
responds,  however,  the  relay  7  will  be  operated  by  the  current  which 
supplies  his  transmitter.  This  will  substitute  the  low-resistance 
winding  of  the  relay  4  f°r  the  high-resistance  winding  of  the  relay 
2,  and  this  will  permit  the  energizing  of  the  tip  supervisory  relay 
of  the  .4-operator  and  put  out  the  calling  supervisory  lamp  at  her 
position.  As  in  the  Western  Electric  circuit,  therefore,  the  con- 
trol of  the  ^4-operator's  calling  supervisory  lamp  is  from  the  called 
subscriber's  station  and  is  relayed  back  over  the  trunk  to  the  orig- 
inating office. 

In  this  circuit,  manual  instead  of  automatic  ringing  is  employed, 
therefore,  unlike  the  Western  Electric  circuit,  means  must  be  pro- 
vided for  notifying  the  B-operator  when  the  calling  subscriber  has 
answered.  This  is  done  by  placing  at  the  5-operator's  position  a 
ringing  lamp  associated  with  each  trunk  cord,  which  is  illuminated 
when  the  5-operator  places  the  plug  of  the  incoming  trunk  into  the 
multiple  jack  of  the  subscriber's  line,  and  remains  illuminated  until 
the  subscriber  has  answered  This  is  accomplished  in  the  following 
manner:  when  the  operator  plugs  into  the  jack  of  the  line  called,  re- 
lay 5  is  energized  but  is  immediately  de-energized  by  the  disconnect- 
ing of  the  circuit  of  this  relay  from  the  sleeve  conductor  of  the  cord 
when  the  ringing  key  is  depressed,  the  selection  of  the  ringing  key 
being  determined  by  the  particular  party  on  the  line  desired.  These 
ringing  keys  have  associated  with  them  a  set  of  springs  9,  which 
springs  are  operated  when  any  one  of  the  ringing  keys  is  depressed. 
Thus,  with  a  ringing  key  depressed  and  the  relay  5  de-energized, 
the  ringing  lamp  will  be  illuminated  by  means  of  a  circuit  as  fol- 
lows :  from  the  live  side  of  the  battery,  through  the  ringing  lamp  12, 
through  the  back  contact  and  armature  of  the  relay  6,  through  the 
armature  and  contact  of  relay  4,  then  through  the  armature  and 
front  contact  of  relay  2 — which  at  this  time  is  the  relay  bridged 
across  the  trunk  and,  therefore,  energized — and  thence  through  the 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  495 

back  contact  and  armature  of  relay  5  to  ground.  When  the  sub- 
scriber removes  his  receiver  from  the  hook,  the  relay  1  will  become 
energized  as  previously  described,  and  will,  therefore,  operate  relay 
6  to  break  the  circuit  of  the  ringing  lamp.  The  circuit  thus  estab- 
lished by  the  operation  of  relay  1  is  as  follows:  from  the  live  side  of 
battery,  through  the  winding  of  relay  6,  through  the  armature  and 
contact  of  relay  1,  through  the  armature  and  contact  of  relay  4> 
through  the  armature  and  front  contact  of  relay  2,  thence  through 
the  armature  and  back  contact  of  relay  5  to  ground.  As  soon  as  the 
5-operator  notes  that  the  ringing  lamp  has  gone  out,  she  knows  that 
no  further  ringing  is  required  on  that  line,  thus  allowing  the  oper- 
ation of  relay  5  and  accomplishing  the  locking  out  of  the  ringing  lamp 
during  the  remainder  of  that  connection.  The  relay  6,  after  having 
once  pulled  up,  remains  locked  up  through  the  rear  contact  of  the 
left-hand  armature  of  relay  5  and  ground,  until  the  plug  is  removed 
from  the  jack. 

At  the  end  of  the  conversation,  when  the  ^4-operator  has  dis- 
connected her  cord  circuit  on  the  illumination  of  the  supervisory 
signals,  both  relays  2  and  4  will  be  in  an  unoperated  condition  and 
will  provide  a  circuit  for  illuminating  the  disconnect  lamp  asso- 
ciated with  the  5-operator's  cord.  This  circuit  may  be  traced  as 
follows:  from  battery  through  the  disconnect  lamp,  through  the 
armatures  and  contacts  of  relays  2  and  4,  thence  through  the  front 
contact  and  armature  of  relay  5  to  ground,  thus  illuminating  the  dis- 
connect lamp.  The  ringing  lamp  will  not  be  re-illuminated  at  this 
time,  due  to  the  fact  that  it  has  been  previously  locked  out  by  relay 
6.  The  operator  then  removes  the  plug  from  the  jack  of  the  line 
called,  and  the  apparatus  in  the  trunk  circuit  is  restored  to  normal 
condition. 

In  the  circuit  shown  only  keys  are  provided  for  ringing  two  par- 
ties. This  circuit,  however,  is  not  confined  to  the  use  of  two-party 
lines,  but  may  be  extended  to  four  parties  by  simply  duplicating  the 
ringing  keys  and  by  connecting  them  with  the  proper  current  for 
selectively  ringing  the  other  stations. 

The  method  of  determining  as  to  whether  the  called  line  is  free 
or  busy  is  similar  to  that  previously  described  for  the  yl-operator's 
cord  circuit  when  making  a  local  connection,  and  differs  only  in  the 
fact  that  in  the  case  of  the  trunk  cord  the  test  circuit  is  controlled 


496  TELEPHONY 

through  the  contacts  of  a  relay,  whereas  in  the  case  of  the  ^4-operator's 
cord,  the  test  circuit  was  controlled  through  the  contacts  of  the  lis- 
tening key.  The  function  of  the  resistance  10  and  the  battery  con- 
nected thereto  is  the  same  as  has  been  previously  described. 

The  general  make-up  of  trunking  switchboard  sections  is  not 
greatly  different  from  that  of  the  ordinary  switchboard  sections  where 
no  trunking  is  involved.  In  small  exchanges  where  ring-down 
trunks  are  employed,  the  trunk  line  equipment  is  merely  added  to 
the  regular  jack  and  drop  equipment  of  the  switchboard.  In  com- 
mon-battery multiple  switchboards  the  yl-boards  differ  in  no  respect 
from  the  standard  single  office  multiple  boards,  except  that  imme- 
diately above  the  answering  jacks  and  below  the  multiple  there  are 
arranged  in  suitable  numbers  the  jacks  of  the  outgoing  trunks. 

Where  the  offices  are  comparatively  small,  the  incoming  trunk 
portions  of  the  5-boards  are  usually  merely  a  continuance  of  the  A- 
boards,  the  subscriber's  multiple  being  continuous  with  and  differing 
in  no  respect  from  that  on  the  A  -sections.  Instead  of  the  usual  pairs 
of  ^4-operators'  plugs,  cords,  and  supervisory  equipment,  there  are 
on  the  key  and  plug  shelves  of  these  5-sections  the  incoming  trunk 
plugs  and  their  associated  equipment. 

In  large  offices  it  is  customary  to  make  the  5-board  entirely 
separate  from  the  yl-board,  although  the  general  characteristics  of 
construction  remain  the  same.  The  reason  for  separate  A-  and  B- 
switchboards  in  large  exchanges  is  to  provide  for  independent  growth 
of  each  without  the  growth  of  either  interfering  with  the  other. 

A  portion  of  an  incoming  trunk,  or  5-board,  is  shown  in  Fig. 
378.  The  multiple  is  as  usual,  and,  of  course,  there  are  no  outgo- 
ing trunk  jacks  nor  regular  cord  pairs.  Instead  the  key  and  plug 
shelves  are  provided  with  the  incoming-trunk  plug  equipments, 
thirty  of  these  being  about  the  usual  quota  assigned  to  each  opera- 
tor's position. 

In  multi-office  exchanges,  employing  many  central  offices, 
such,  for  instance,  as  those  in  New  York  or  Chicago,  it  is  frequently 
found  that  nearly  all  of  the  calls  that  originate  in  one  office  are  for 
subscribers  whose  lines  terminate  in  some  other  office  In  other 
words,  the  number  of  ca-lls  that  have  to  be  trunked  to  other  offices 
is  greatly  in  excess  of  the  number  of  calls  that  tnay  be  handled 
through  the  multiple  of  the  Aboard  in  which  they  originate.  It  is 


TRUNKIXG  IX  MULTI-OFFICE  SYSTEMS  497 


498 


TELEPHONY 


TRUNKING  IN  MULTI-OFFICE  SYSTEMS  499 

not  infrequent  to  have  the  percentage  of  trunked  calls  run  as  high  as 
75  per  cent  of  the  total  number  of  calls  originating  in  any  one  office, 
and  in  some  of  the  offices  in  the  larger  cities  this  percentage  runs 
higher  than  90  per  cent. 

This  fact  has  brought  up  for  consideration  the  problem  as  to 
whether,  when  the  nature  of  the  traffic  is  such  that  only  a  very  small 
portion  of  the  calls  can  be  handled  in  the  office  where  they  originate, 
it  is  worth  while  to  employ  the  multiple  terminals  for  the  subscribers' 
lines  on  the  /1-boards.  In  other  words,  if  so  great  a  proportion  as 
90  per  cent  of  the  calls  have  to  be  trunked  any  way,  is  it  worth  while 
to  provide  the  great  expense  of  a  full  multiple  on  all  the  sections  of 
the  yl-board  in  order  to  make  it  possible  to  handle  the  remaining 
10  per  cent  of  the  calls  directly  by  the  .4-operators? 

As  a  result  of  this  consideration  it  has  been  generally  conceded 
that  where  such  a  very  great  percentage  of  trunking  was  necessary, 
the  full  multiple  of  the  subscribers'  lines  on  each  section  was  not 
warranted,  and  what  is  known  as  the  partial  multiple  board  has 
come  into  existence  in  large  manual  exchanges.  In  these  the  reg- 
ular subscribers'  multiple  is  entirely  omitted  from  the  ^4-board,  all 
subscribers'  calls  being  handled  through  outgoing  trunk  jacks  con- 
nected by  trunks  to  5-boards  in  the  same  as  well  as  other  offices. 
In  these  partial  multiple  ^4-boards,  the  answering  jacks  are  multi- 
pled  a  few  times,  usually  twice,  so  that  calls  on  each  line  may  be 
answered  from  more  than  one  position.  This  multipling  of  an- 
swering jacks  does  not  in  any  way  take  the  place  of  the  regular  mul- 
tipling in  full  multiple  boards,  since  in  no  case  are  the  calls  completed 
through  the  multiple  jacks.  It  is  done  merely  for  the  purpose  of 
contributing  to  team  work  between  the  operators. 

A  portion  of  such  a  partial  multiple  ^4-board  is  shown  in  Fig. 
379.  This  view  shows  slightly  more  than  one  section,  and  the  regu- 
lar answering  jacks  and  lamps  may  be  seen  at  the  bottom  of  the 
jack  space  just  above  the  plugs.  Above  these  are  placed  the  out- 
going trunk  jacks,  those  that  are  in  use  being  indicated  with  white 
designation  strips.  Above  the  outgoing  trunk  jacks  are  placed  the 
multiples  of  the  answering  jacks,  these  not  being  provided  with 
lamps. 

The  partial  multiple  A  -section  of  Fig.  379  is  a  portion  of  the 
switchboard  equipment  of  the  same  office  to  which  the  trunking  sec- 


500 


TELEPHONY 


tion  shown  in  Fig.  378  belongs.  That  this  is  a  large  multiple  board 
may  be  gathered  from  the  number  of  multiple  jacks  in  the  trunking 
section,  8,400  being  installed  with  room  for  10,500.  That  the  board 
is  a  portion  of  an  equipment  belonging  to  an  exchange  of  enormous 
proportions  may  be  gathered  from  the  number  of  outgoing  trunk 
jacks  shown  in  the  A  -board,  and  in  the  great  number  of  order-wire 
keys  shown  between  each  of  the  sets  of  regular  cord-circuit  keys. 
The  switchboards  illustrated  in  these  two  figures  are  those  of  one  of 
the  large  offices  of  the  New  York  Telephone  Company  on  Manhat- 
tan Island,  and  the  photographs  were  taken  especially  for  this  work 

by  the  Western  Electric  Company. 

Cable  Color  Code.  A  great  part  of  the  wiring  of  switchboards  requires 
to  be  done  with  insulated  wires  grouped  into  cables.  In  the  wiring  of  manual 
switchboards  as  described  in  the  seven  preceding  chapters,  and  of  automatic 
and  automanual  systems  and  of  private  branch-exchange  and  intercommunicat- 
ing systems  described  in  succeeding  chapters,  cables  formed  as  follows  are 
widely  used: 

Tinned  soft  copper  wires,  usually  of  No.  22  or  No.  24  B.  &  S.  gauge,  are 
insulated,  first  with  two  coverings  of  silk,  then  with  one  covering  of  cotton. 
The  outer  (cotton)  insulation  of  each  wire  is  made  of  white  or  of  dyed  threads. 
If  dyed,  the  color  either  is  solid  red,  black,  blue,  orange,  green,  brown,  or 
slate,  or  it  is  striped,  by  combining  one  of  those  colors  with  white  or  a  remaining 
color.  The  object  of  coloring  the  wires  is  to  enable  them  to  be  identified  by 
sight  instead  of  by  electrical  testing. 

Wires  so  insulated  are  twisted  into  pairs,  choosing  the  colors  of  the 
"line"  and  "mate"  according  to  a  predetermined  plan.  An  assortment  of  these 
pairs  then  is  laid  up  spirally  to  form  the  cable  core,  over  which  are  placed 
certain  wrappings  and  an  outer  braid.  A  widely  used  form  of  switchboard 
cable  has  paper  and  lead  foil  wrappings  over  the  core,  and  the  outer  cotton 
braid  finally  is  treated  with  a  fire-resisting  paint. 

STANDARD  COLOR  CODE  FOR  CABLES 


LINE    WIRE 

MATE 

White 

Red 

Black 

Red-  White 

Black-White 

Blue 

1 

21 

41 

61 

81 

Orange 

2 

22 

42 

62 

82 

Green 

3 

23 

43 

63 

83 

Brown 

4 

24 

44 

64 

84 

Slate 

5 

25 

45 

65 

85 

Blue-  White 

6 

26 

46 

66 

86 

Blue-Orange 

7 

27 

47 

67 

87 

Blue-Green 

8 

28 

48 

68 

88 

Blue-Brown 

9 

29 

49 

69 

89 

Blue-Slate 

10 

30 

50 

70 

90 

Orange-  White 

11 

31 

51 

71 

91 

Orange-Green 

12 

32 

52 

72 

92 

Orange-Brown 

13 

33 

53 

73 

93 

Orange-Slate 

14 

34 

54 

74 

94 

Green-  White 

15 

35 

55 

75 

95 

Green-Brown 

16 

36 

56 

76 

96 

Green-Slate 

17 

37 

57 

77 

97 

Brown-  White 

18 

38 

58 

78 

98 

Brown-Slate 

19 

39 

59 

79 

99 

Slate-  White 

20 

40 

60 

80 

100 

The  numerals  represent  the  pair  numbers  in  the  cable. 

The  wires  of  spare  pairs  usually  are  designated  by  solid  red  with  white  mate  for 
first  spare  pair,  and  solid  black  with  white  mate  for  second  spare  pair.  Individual  spare 
wires  usually  are  colored  red-white  for  first  individual  spare,  and  black-white  for  second 
individual  spare. 


CHAPTER  XX VII I 
FUNDAMENTAL  CONSIDERATIONS  OF  AUTOMATIC  SYSTEMS 

Definition.  The  term  automatic,  as  applied  to  telephone 
systems,  has  come  to  refer  to  those  systems  in  which  machines  at 
the  central  office,  under  the  guidance  of  the  subscribers,  do  the  work 
that  is  done  by  operators  in  manual  systems.  In  all  automatic 
telephone  systems,  the  work  of  connecting  and  disconnecting  the 
lines,  of  ringing  the  called  subscriber,  even  though  he  must  be  se- 
lected from  among  those  on  a  party  line,  of  refusing  to  connect  with 
a  line  that  is  already  in  use,  and  informing  the  calling  subscriber 
that  such  line  is  busy,  of  making  connections  to  trunk  lines  and 
through  them  to  lines  in  other  offices  and  doing  the  same  sort  of  things 
there,  of  counting  and  recording  the  successful  calls  made  by  a  sub- 
scriber, rejecting  the  unsuccessful,  and  nearly  all  the  thousand  and 
one  other  acts  necessary  in  telephone  service,  are  performed  without 
the  presence  of  any  guiding  intelligence  at  the  central  office. 

The  fundamental  object  of  the  automatic  system  is  to  do  away 
with  the  central-office  operator.  In  order  that  each  subscriber  may 
control  the  making  of  his  own  connections  there  is  added  to  his  sta- 
tion equipment  a  call  transmitting  device  by  the  manipulation  of  which 
he  causes  the  central-office  mechanisms  to  establish  the  connections 
he  desires. 

We  think  that  the  automatic  system  is  one  of  the  most  astonish- 
ing developments  of  human  ingenuity.  The  workers  in  this  develop- 
ment are  worthy  of  particular  notice.  From  occupying  a  position 
in  popular  regard  in  common  with  long-haired  men  and  short-haired 
women  they  have  recently  appeared  as  sane,  reasonable  men  with  the 
courage  of  their  convictions  and,  better  yet,  with  the  ability  to  make 
their  convictions  come  true.  The  scoffers  have  remained  to  pray. 

Arguments  Against  Automatic  Idea.  Naturally  there  has  been 
a  bitter  fight  against  the  automatic.  Those  who  have  opposed  it 
have  contended: 


502  TELEPHONY 

First:  that  it  is  too  complicated  and,  therefore,  could  be  neither 
reliable  or  economical. 

Second:  that  it  is  too  expensive,  and  that  the  necessary  first  cost 
could  not  be  justified. 

Third:  that  it  is  too  inflexible  and  could  not  adapt  itself  to 
special  kinds  of  service. 

Fourth:  that  it  is  all  wrong  from  the  subscribers'  point  of  view 
as  the  public  will  not  tolerate  "doing  its  own  operating." 

Complexity.  This  first  objection  as  to  complexity,  and  con- 
sequent alleged  unreliability  and  lack  of  economy  should  be  care- 
fully analyzed.  It  too  often  happens  that  a  new  invention  is  cast 
into  outer  darkness  by  those  whose  opinions  carry  weight  by  such 
words  as  "it  cannot  work;  it  is  too  complicated."  Fortunately  for  the 
world,  the  patience  and  fortitude  which  men  must  possess  before 
they  can  produce  meritorious,  though  intricate  inventions,  are  us- 
ually sufficient  to  prevent  their  being  crushed  by  any  such  offhand 
condemnation,  and  the  test  of  time  and  service  is  allowed  to  become 
the  real  criterion. 

It  would  be  difficult  to  find  an  art  that  has  gone  forward  as 
rapidly  as  telephony.  Within  its  short  life  of  a  little  over  thirty 
years  it  has  grown  from  the  phase  of  trifl  ing  with  a  mere  toy  to  an 
affair  of  momentous  importance  to  civilization.  There  has  been  a 
tendency,  particularly  marked  during  recent  years,  toward  greater 
complexity;  and  probably  every  complicated  new  system  or  piece 
of  apparatus  has  been  roundly  condemned,  by  those  versed  in  the 
art  as  it  was,  as  being  unable  to  survive  on  account  of  its  compli- 
cation. 

To  illustrate:  A  prominent  telephone  man,  in  arguing  against 
the  nickel-in-the-slot  method  of  charging  for  telephone  service  once 
said,  partly  in  jest,  "The  Lord  never  intended  telephone  service  to 
be  given  in  that  way."  This,  while  a  little  off  the  point,  is  akin  to 
the  sweeping  aside  of  new  telephone  systems  on  the  sole  ground  that 
they  are  complicated.  These  are  not  real  reasons,  but  rather  con- 
venient ways  of  disposing  of  vexing  problems  with  a  minimum  amount 
of  labor.  Important  questions  lying  at  the  very  root  of  the  develop- 
ment of  a  great  industry  may  not  be  put  aside  permanently  in  this 
offhand  way.  The  Lord  has  never,  so  far  as  we  know,  indicated 
just  what  his  intentions  were  in  the  matter  of  nickel  service;  and  no 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS  503 

one  has  ever  shown  yet  just  what  degree  of  complexity  will  prevent 
a  telephone  system  from  working. 

It  is  safe  to  say  that,  if  other  things  are  equal,  the  simpler  a  ma- 
chine is,  the  better;  but  simplicity,  though  desirable,  is  not  all- 
important.  Complexity  is  warranted  if  it  can  show  enough  advan- 
tages. 

If  one  takes  a  narrow  view  of  the  development  of  things  mechan- 
ical and  electrical,  he  will  say  that  the  trend  is  toward  simplicity. 
The  mechanic  in  designing  a  machine  to  perform  certain  functions  tries 
to  make  it  as  simple  as  possible.  He  designs  and  re-designs,  mak- 
ing one  part  do  the  work  of  two  and  contriving  schemes  for  reducing 
the  complexity  of  action  and  form  of  each  remaining  part.  His  whole 
trend  is  away  from  complication,  and  this  is  as,  it  should  be.  Other 
things  being  equal,  the  simpler  the  better.  A  broad  view,  however, 
will  show  that  the  arts  are  becoming  more  and  more  complicated. 
Take  the  implements  of  the  art  of  writing:  The  typewriter  is  vastly 
more  complicated  than  the  pen,  whether  of  steel  or  quill,  yet  most 
of  the  writing  of  today  is  done  on  the  typewriter,  and  is  done  better 
and  more  economically.  The  art  of  printing  affords  even  more 
striking  examples. 

In  telephony,  while  every  effort  has  been  made  to  simplify  the 
component  parts  of  the  system,  the  system  itself  has  ever  developed 
from  the  simple  toward  the  complex.  The  adoption  of  the  multi- 
ple switchboard,  of  automatic  ringing,  of  selective  ringing  on  party 
lines,  of  measured-service  appliances,  and  of  automatic  systems 
have  all  constituted  steps  in  this  direction.  The  adoption  of  more 
complicated  devices  and  systems  in  telephony  has  nearly  always 
followed  a  demand  for  the  performance  by  the  machinery  of  the 
system  of  additional  or  different  functions.  As  in  animal  and  plant 
life,  so  in  mechanics — the  higher  the  organism  functionally  the  more 
complex  it  becomes  physically. 

Greater  intricacy  in  apparatus  and  in  methods  is  warranted  when 
it  is  found  desirable  to  make  the  machine  perform  added  functions. 
Once  the  functions  are  determined  upon,  then  the  whole  trend  of 
the  development  of  the  machine  for  carrying  them  out  should  be 
toward  simplicity.  When  the  machine  has  reached  its  highest  stage 
of  development  some  one  proposes  that  it  be  required  to  do  something 
that  ,has  hitherto  been  done  manually,  or  by  a  separate  machine, 


504  TELEPHONY 

or  not  at  all.  With  this  added  function  a  vast  added  complication 
may  come,  after  which,  if  it  develops  that  the  new  function  may  with 
economy  be  performed  by  the  machine,  the  process  of  simplification 
again  begins,  the  whole  design  finally  taking  on  an  indefinable  ele- 
gance which  appears  only  when  each  part  is  so  made  as  to  be  best 
adapted  in  composition,  form,  and  strength  to  the  work  it  is  to  per- 
form. 

Achievements  in  the  past  teach  us  that  a  machine  may  be  made 
to  do  almost  anything  automatically  if  only  the  time,  patience,  skill, 
and  money  be  brought  to  bear.  This  is  also  true  of  a  telephone  sys- 
tem. The  primal  question  to  decide  is,  what  functions  the  system  is  to 
perform  within  itself,  automatically,  and  what  is  to  be  done  manually 
or  with  manual  aid.  Sometimes  great  complications  are  brought 
into  the  system  in  an  attempt  to  do  something  which  may  very  easily 
and  cheaply  be  done  by  hand.  Cases  might  be  pointed  out  in  which 
fortunes  and  life-works  have  been  wasted  in  perfecting  machines 
for  which  there  was  no  real  economic  need.  It  is  needless  to  cite 
cases  where  the  reverse  is  true.  The  matter  of  wisely  choosing  the 
functions  of  the  system  is  of  fundamental  importance.  In  choosing 
these  the  question  of  complication  is  only  one  of  many  factors  to  be 
considered. 

One  of  the  strongest  arguments  against  intricacy  in  telephone 
apparatus  is  its  greater  initial  cost,  its  greater  cost  of  maintenance, 
and  its  liability  to  get  out  of  order.  Greater  complexity  of  apparatus 
usually  means  greater  first  cost,  but  it  does  not  necessarily  mean 
greater  cost  of  up-keep  or  lessened  reliability.  A  dollar  watch  is 
more  simple  than  an  expensive  one.  The  one,  however,  does  its 
work  passably  and  is  thrown  away  in  a  year  or  so;  the  other  does 
its  work  marvelously  well  and  may  last  generations,  being  handed 
down  from  father  to  son.  Merely  reducing  the  number  of  parts 
in  a  machine  does  not  necessarily  mean  greater  reliability.  Fre- 
quently the  attempt  to  make  one  part  do  several  diverse  things  re- 
sults in  such  a  sacrifice  in  the  simplicity  of  action  of  that  part  as  to 
cause  undue  strain,  or  wear,  or  unreliable  action.  Better  results  may 
be  attained  by  adding  parts,  so  that  each  may  have  a  comparatively 
simple  thing  to  do. 

The  stage  of  development  of  an  art  is  a  factor  in  determining  the 
degree  of  complexity  that  may  be  allowed  in  the  machinery  of  that 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS         505 

art.  A  linotype  machine,  if  constructed  by  miracle  several  hun- 
dred years  ago,  would  have  been  of  no  value  to  the  printer's  art  then. 
The  skill  was  not  available  to  operate  and  maintain  it,  nor  was  the 
need  of  the  public  sufficiently  developed  to  make  it  of  use.  Simi- 
larly the  automatic  telephone  exchange  would  have  been  of  little 
value  thirty  years  ago.  The  knowledge  of  telephone  men  was  not 
sufficiently  developed  to  maintain  it,  telephone  users  were  not  suffi- 
ciently numerous  to  warrant  it,  and  the  public  was  not  sufficiently 
trained  to  use  it.  Industries,  like  human  beings,  must  learn  to  creep 
before  they  can  walk. 

Another  factor  which  must  be  considered  in  determining  the 
allowable  degree  of  complexity  in  a  telephone  system  is  the  character 
of  the  labor  available  to  care  for  and  manage  it.  Usually  the  conditions 
which  make  for  unskilled  labor  also  lend  themselves  to  the  use  of 
comparatively  simple  systems.  Thus,  in  a  small  village  remote  from 
large  cities  the  complexity  inherent  in  a  common-battery  multiple 
switchboard  would  be  objectionable.  The  village  would  probably 
not  afford  a  man  adequately  skilled  to  care  for  it,  and  the  size  of  the 
exchange  would  not  warrant  the  expense  of  keeping  such  a  man. 
Fortunately  no  such  switchboard  is  needed.  A  far  simpler  device, 
the  plain  magneto  switchboard — so  simple  that  the  girl  who  manipu- 
lates it  may  also  often  care  for  its  troubles — is  admirably  adapted 
to  the  purpose.  So  it  is  with  the  automatic  telephone  system;  even 
its  most  enthusiastic  advocate  would  be  foolish  indeed  to  contend  that 
for  all  places  and  purposes  it  was  superior  to  the  manual. 

These  remarks  are  far  from  being  intended  as  a  plea  for  complex 
telephone  apparatus  and  systems;  every  device,  every  machine, 
and  every  system  should  be  of  the  simplest  possible  nature  consistent 
with  the  functions  it  has  to  perform.  They  are  rather  a  protest 
against  the  broadcast  condemnation  of  complex  apparatus  and  sys- 
tems just  because  they  are  complicated,  and  without  regard  to  other 
factors.  Such  condemnation  is  detrimental  to  the  progress  of  teleph- 
ony. \Yhere  would  the  printing  art  be  today  if  the  linotype,  the 
cylinder  press,  and  other  modern  printing  machinery  of  marvelous 
intricacy  had  been  put  aside  on  account  of  the  fact  that  they  were 
more  complicated  than  the  printing  machinery  of  our  forefathers? 

That  the  automatic  telephone  system  is  complex,  exceedingly 
complex,  cannot  be  denied,  but  experience  has  amply  proven  that 


506  TELEPHONY 

its  complexity  does  not  prevent  it  from  giving  reliable  service,  nor 
from  being  maintained  at  a  reasonable  cost. 

Expense.  The  second  argument  against  the  automatic — that 
it  is  too  expensive — is  one  that  must  be  analyzed  before  it  means 
anything.  It  is  true  that  for  small  and  medium-sized  exchanges  the 
total  first  cost  of  the  central  office  and  subscribers'  station  equipment, 
is  greater  than  that  for  manual  exchanges  of  corresponding  sizes. 
The  prices  at  which  various  sizes  of  automatic  exchange  equipments 
may  be  purchased  vary,  however,  almost  in  direct  proportion  to 
the  number  of  lines,  whereas  in  manual  equipment  the  price  per  line 
increases  very  rapidly  as  the  number  of  lines  increases.  From 
this  it  follows  that  for  very  large  exchanges  the  cost  of  automatic  ap- 
paratus becomes  as  low,  and  may  be  even  lower  than  for  manual. 
Roughly  speaking  the  cost  of  telephones  and  central-office  equip- 
ment for  small  exchanges  is  about  twice  as  great  for  automatic  as  for 
manual,  and  for  very  large  exchanges,  of  about  10,000  lines,  the 
cost  of  the  two  for  switchboards  and  telephones  is  about  equal. 

For  all  except  the  largest  exchanges,  therefore,  the  greater  first 
cost  of  automatic  apparatus  must  be  put  down  as  one  of  the  factors 
to  be  weighed  in  making  the  choice  between  automatic  and  manual, 
this  factor  being  less  arid  less  objectionable  as  the  size  of  the  equip- 
ment increases  and  finally  disappearing  altogether  for  very  large 
equipments.  Greater  first  cost  is,  of  course,  warranted  if  the  fixed 
charges  on  the  greater  investment  are  more  than  offset  by  the  econ- 
omy resulting.  The  automatic  screw  machine,  for  instance,  costs 
many  times  more  than  the  hand  screw  machine,  but  it  has  largely  dis- 
placed the  hand  machine  nevertheless. 

Flexibility.  The  third  argument  against  the  automatic  telephone 
system — its  flexibility — is  one  that  only  time  and  experience  has 
been  able  to  answer.  Enough  time  has  elapsed  and  enough  experi- 
ence has  been  gained,  however,  to  disprove  the  validity  of  this  ar- 
gument. In  fact,  the  great  flexibility  of  the  automatic  system  has 
been  one  of  its  surprising  developments.  No  sooner  has  the  state- 
ment been  made  that  the  automatic  system  could  not  do  a  certain 
thing  than  forthwith  it  has  done  it.  It  was  once  quite  clear  that 
the  automatic  system  was  not  practicable  for  party-line  selective 
ringing;  yet  today  many  automatic  systems  are  working  successfully 
with  this  feature;  the  selection  between  the  parties  on  a  line  being 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS         507 

accomplished  with  just  as  great  certainty  as  in  manual  systems. 
Again  it  has  seemed  quite  obvious  that  the  automatic  system  could 
not  hope  to  cope  with  the  reverting  call  problem,  i.  e.,  enabling  a 
subscriber  on  a  party  line  to  call  back  to  reach  another  subscriber 
on  the  same  line;  yet  today  the  automatic  system  may  do  this  in  a 
way  that  is  perhaps  even  more  satisfactory  than  the  way  in  which 
it  is  done  in  multiple  manual  switchboards.  It  is  true  that  the  auto- 
matic system  has  not  done  away  with  the  toll  operator  and  it  prob- 
ably never  will  be  advantageous  to  require  it  to  do  so  for  the  simple 
reason  that  the  work  of  the  toll  operator  in  recording  the  connections 
and  in  bringing  together  the  subscribers  is  a  matter  that  requires 
not  only  accuracy  but  judgment,  and  the  latter,  of  course,  no  machine 
can  supply.  It  is  probable  also  that  the  private  branch-exchange 
operator  will  survive  in  automatic  systems.  This  is  not  because  the 
automatic  system  cannot  readily  perform  the  switching  duties,  but 
the  private  branch-exchange  operator  has  other  duties  than  the  mere 
building  up  and  taking  down  of  connections.  She  is,  as  it  were,  a 
door-keeper  guarding  the  telephone  door  of  a  business  establish- 
ment; like  the  toll  operator  she  must  be  possessed  of  judgment  and 
of  courtesy  in  large  degree,  neither  of  which  can  be  supplied  by 
machinery. 

In  respect  to  toll  service  and  private  branch-exchange  service 
where,  as  just  stated,  operators  are  required  on  account  of  the  nature 
of  the  service,  the  automatic  system  has  again  shown  its  adaptability 
and  flexibility.  It  has  shown  its  capability  of  working  in  harmony 
with  manual  switchboards,  of  whatever  nature,  and  there  is  a  growing 
tendency  to  apply  automatic  devices  and  automatic  principles  of 
operation  to  manual  switchboards,  whether  toll  or  private  branch 
or  other  kinds,  even  though  the  services  of  an  operator  are  required, 
the  idea  being  to  do  by  machinery  that  portion  of  the  work  which  a 
machine  is  able  to  do  better  or  more  economically  than  a  human 
being. 

Attitude  of  Public.  The  attitude  of  the  public  toward  the  auto- 
matic is  one  that  is  still  open  to  discussion;  at  least  there  is  still  much 
discussion  on  it.  A  few  years  ago  it  did  seem  reasonable  to  suppose 
that  the  general  telephone  user  would  prefer  to  get  his  connection  by 
merely  asking  for  it  rather  than  to  make  it  himself  by  "spelling"  it 
out  on  the  dial  of  his  telephone  instrument.  We  have  studied  this 


508  TELEPHONY 

point  carefully  in  a  good  many  different  communities  and  it  is  OUT 
opinion  that  the  public  finds  no  fault  with  being  required  to  make 
its  own  connections.  To  our  minds  it  is  proven  beyond  question  that 
either  the  method  employed  in  the  automatic  or  that  in  the  manual 
system  is  satisfactory  to  the  public  as  long  as  good  service  results, 
and  it  is  beyond  question  that  the  public  may  get  this  with  either. 

Subscriber's  Station  Equipment.  The  added  complexity  of  the 
mechanism  at  the  subscriber's  station  is  in  our  opinion  the  most 
valid  objection  that  can  be  urged  against  the  automatic  system  as 
it  exists  today.  This  objection  has,  however,  been  much  reduced 
by  the  greater  simplicity  and  greater  excellence  of  material  and  work- 
manship that  is  employed  in  the  controlling  devices  in  modern  auto- 
matic systems.  However,  the  automatic  system  must  always  suffer 
in  comparison  with  ,the  manual  in  respect  of  simplicity  of  the  sub- 
scriber's equipment.  The  simplest  conceivable  thing  to  meet  all 
of  the  requirements  of  telephone  service  at  a  subscriber's  station  is 
the  modern  common-battery  manual  telephone.  The  automatic 
telephone  differs  from  this  only  in  the  addition  of  the  mechanism 
for  enabling  the  subscriber  to  control  the  central-office  apparatus 
in  the  making  of  calls.  From  the  standpoint  of  maintenance,  sim- 
plicity at  the  subscriber's  station  is,  of  course,  to  be  striven  for  since 
the  proper  care  of  complex  devices  scattered  all  over  a  community 
is  a  much  more  serious  matter  than  where  the  devices  are  centered 
at  one  point,  as  in  the  central  office.  Nevertheless,  as  pointed  out, 
complexity  is  not  fatal,  and  it  is  possible,  as  has  been  proven,  to  so 
design  and  construct  the  required  apparatus  in  connection  with  the 
subscribers'  telephones  as  to  make  them  subject  to  an  amount  of 
trouble  that  is  not  serious. 

Comparative  Costs.  A  comparison  of  the  total  costs  of  own- 
ing, operating,  and  maintaining  manual  and  automatic  systems 
usually  results  in  favor  of  the  automatic,  except  in  small  exchanges. 
This  seems  to  be  the  consensus  of  opinion  among  those  who  have 
studied  the  matter  deeply.  Although  the  automatic  usually  re- 
quires a  larger  investment,  and  consequently  a  larger  annual  charge 
for  interest  and  depreciation,  assuming  the  same  rates  for  each  case, 
and  although  the  automatic  requires  a  somewhat  higher  degree  of 
skill  to  maintain  it  and  to  keep  it  working  properly  than  the  manual, 
the  elimination  of  operators  or  the  reduction  in  their  number  and  the 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS        509 

consequent  saving  of  salaries  and  contributory  expenses  together 
with  other  items  of  saving  that  will  be  mentioned  serves  to  throw 
the  balance  in  favor  of  the  automatic. 

The  ease  with  which  the  automatic  system  lends  itself  to  inter- 
office trunking  makes  feasible  a  greater  subdivision  of  exchange 
districts  into  office  districts  and  particularly  makes  it  economical, 
where  such  would  not  be  warranted  in  manual  working.  All  this 
tends  toward  a  reduction  in  average  length  of  subscribers'  lines  and 
it  seems  probable  that  this  possibility  will  be  worked  upon  in  the 
future,  more  than  it  has  been  in  the  past,  to  effect  a  considerable 
saving  in  the  cost  of  the  wire  plant,  which  is  the  part  of  a  telephone 
plant  that  shows  least  and  costs  most. 

Automatic  vs.  Manual.  Taking  it  all  in  all  the  question  of  auto- 
matic versus  manual  may  not  and  can  not  be  disposed  of  by  a- con- 
sideration of  any  single  one  of  the  alleged  features  of  superiority  or 
inferiority  of  either.  Each  must  be  looked  at  as  a  practical  way  of 
giving  telephone  service,  and  a  decision  can  be  reached  only  by  a 
careful  weighing  of  all  the  factors  which  contribute  to  economy,  re- 
liability, and  general  desirability  from  the  standpoint  of  the  public. 
Public  sentiment  must  neither  be  overlooked  nor  taken  lightly,  since, 
in  the  final  analysis,  it  is  the  public  that  must  be  satisfied. 

Methods  of  Operation.  In  all  of  the  automatic  telephone  sys- 
tems that  have  achieved  any  success  whatever,  the  selection  of  the 
desired  subscriber's  line  by  the  calling  subscriber  is  accomplished  by 
means  of  step-by-step  mechanism  at  the  central  office,  controlled 
by  impulses  sent  or  caused  to  be  sent  by  the  acts  of  the  subscriber. 

Strowger  System.  In  the  so-called  Strowger  system,  manu- 
factured by  the  Automatic  Electric  Company  of  Chicago,  the  sub- 
scriber, in  calling,  manipulates  a  dial  by  which  the  central-office 
switching  mechanism  is  made  to  build  up  the  connection  he  wants. 
The  dial  is  moved  as  many  times  as  there  are  digits  in  the  called 
subscriber's  number  and  each  movement  sends  a  series  of  impulses 
to  the  central  office  corresponding  in  number  respectively  to  the  digits 
in  the  called  subscriber's  number.  During  each  pause,  except  the 
last  one,  between  these  series  of  impulses,  the  central-office  mechan- 
ism operates  to  shift  the  control  of  the  calling  subscriber's  line  from 
one  set  of  switching  apparatus  at  the  central  office  to  another. 

In  case  a  four-digit  number  is  being  selected  first,  the  move- 


510  TELEPHONY 

ment  of  the  dial  by  the  calling  subscriber  will  correspond  to  the 
thousands  digit  of  the  number  being  called,  and  the  resulting  move- 
ment of  the  central-office  apparatus  will  continue  the  calling  sub- 
scriber's line  through  a  trunk  to  a  piece  of  apparatus  capable  of 
further  extending  his  line  toward  the  line  terminals  of  the  thousand 
subscribers  whose  numbers  begin  with  the  digit  chosen.  The  next 
movement  of  the  dial  corresponding  to  the  hundreds  digit  of  the 
called  number  will  operate  this  piece  of  apparatus  to  again  extend 
the  calling  subscriber's  line  through  another  trunk  to  apparatus 
representing  the  particular  hundred  in  which  the  called  subscriber's 
number  is.  The  third  movement  of  the  dial  corresponding  to  the 
tens  digit  will  pick  out  the  group  of  ten  containing  the  called  sub- 
scriber's line,  and  the  fourth  movement  corresponding  to  the  units 
digit  will  pick  out  and  connect  with  the  particular  line  called. 

Larimer  System.  In  the  Lorimer  automatic  system  invented  by 
the  Lorimer  Brothers,  and  now  being  manufactured  by  the  Cana- 
dian Machine  Telephone  Company  of  Toronto,  Canada,  the  sub- 
scriber sets  up  the  number  he  desires  complete  by  moving  four 
levers  on  his  telephone  so  that  the  desired  number  appears  visibly 
before  him.  He  then  turns  a  handle  and  the  central-office  apparatus, 
under  the  control  of  the  electrical  conditions  thus  set  up  by  the  sub- 
scriber, establishes  the  connection.  In  this  system,  unlike  the  Strow- 
ger  system,  the  controlling  impulses  are  not  caused  by  the  movement 
of  the  subscriber's  apparatus  in  returning  to  its  normal  position  after 
being  set  by  the  subscriber.  Instead,  the  conditions  established  at 
the  subscriber's  station  by  the  subscriber  in  setting  up  the  desired 
number,  merely  determine  the  point  in  the  series  of  impulses  corre- 
sponding to  each  digit  at  which  the  stepping  impulses  local  to  the 
central  office  shall  cease,  and  in  this  way  the  proper  number  of  im- 
pulses in  the  series  corresponding  to  each  digit  is  determined. 

Magnet-  vs.  Power-Driven  Switches.  These  two  systems  differ 
radically  in  another  respect.  In  the  Strowger  system  it  is  the  electrical 
impulses  initiated  at  the  subscriber's  apparatus  that  actually  cause 
the  movement  of  the  switching  parts  at  the  central  office,  these  im- 
pulses energizing  electromagnets  which  move  the  central-office 
switching  devices  a  step  at  a  time  the  desired  number  of  steps.  In 
the  Lorimer  system  the  switches  are  all  power-driven  and  the  im- 
pulses under  the  control  of  the  subscriber's  instrument  merely  serve 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS         511 

to  control  the  application  of  this  power  to  the  various  switching 
mechanisms.  These  details  will  be  more  fully  dealt  with  in  sub- 
sequent chapters. 

Multiple  vs.  Trunking.  It  has  been  shown  in  the  preceding 
portion  of  this  work  that  the  tendency  in  manual  switchboard  prac- 
tice has  been  away  from  trunking  between  the  various  sections  or 
positions  of  a  board,  and  toward  the  multiple  idea  of  operating, 
wherein  each  operator  is  able  to  complete  the  connection  with  any 
line  in  the  same  office  without  resorting  to  trunks  or  to  the  aid  of 
other  operators.  Strangely  enough  the  reverse  has  been  true  in  the 
development  of  the  automatic  system.  As  long  as  the  inventors  tried 
to  follow  the  most  successful  practice  in  manual  working,  failure 
resulted.  The  automatic  systems  of  today  are  essentially  trunking 
systems  and  while  they  all  involve  multiple  connections  in  greater 
or  less  degree,  all  of  them  depend  fundamentally  upon  the  extending 
of  the  calling  line  by  separate  lengths  until  it  finally  reaches  and 
connects  with  the  called  line. 

Grouping  of  Subscribers.  In  this  connection  we  wish  to  point 
out  here  two  very  essential  features  without  which,  so  far  as  we  are 
aware,  no  automatic  telephone  system  has  been  able  to  operate 
successfully.  The  first  of  these  is  the  division  of  the  total  number  of 
lines  in  any  office  of  the  exchange  into  comparatively  small  groups 
and  the  employment  of  correspondingly  small  switch  units  for  each 
group.  Many  of  the  early  automatic  systems  that  were  proposed 
involved  the  idea  of  having  each  switch  capable  in  itself  of  making 
connection  with  any  line  in  the  entire  office.  As  long  as  the  num- 
ber of  lines  was  small — one  hundred  or  thereabouts — this  might  be 
all  right,  but  where  the  lines  number  in  the  thousands,  it  is  readily 
seen  that  the  switches  would  be  of  prohibitive  size  and  cost. 

Trunking  between  Groups.  This  feature  made  necessary  the 
employment  of  trunk  connections  between  groups.  By  means  of 
these  the  lines  are  extende  i  a  step  at  a  time,  first  entering  a  large 
group  of  groups,  containing  the  desired  subscriber;  then  entering 
the  smaller  group  containing  that  subscriber;  and  lastly  entering  into 
connection  with  the  line  itself.  The  carrying  out  of  this  idea  was 
greatly  complicated  by  the  necessity  of  providing  for  many  simul- 
taneous connections  through  the  switchboard.  It  was  compara- 
tively easy  to  accomplish  the  extension  of  one  line  through  a  series 


512  TELEPHONY 

of  links  or  trunks  to  another  line,  but  it  was  not  so  easy  to  do  this 
and  still  leave  it  possible  for  any  other  line  to  pick  out  and  con- 
nect with  any  other  idle  line  without  interference  with  the  first  con- 
nection. A  number  of  parallel  paths  must  be  provided  for  each 
possible  connection.  Groups  of  trunks  are,  therefore,  provided 
instead  of  single  trunks  between  common  points  to  be  connected. 
The  subscriber  who  operates  his  instrument  in  making  a  call  knows 
nothing  of  this  and  it  is,  of  course,  impossible  for  him  to  give  any 
thought  to  the  matter  as  to  which  one  of  the  possible  paths  he  shall 
choose.  It  was  by  a  realization  of  these  facts  that  the  failures  of 
the  past  were  turned  into  the  successes  of  the  present.  The  sub- 
scriber by  setting  his  signal  transmitter  was  made  to  govern  the  ac- 
tion of  the  central-office  apparatus  in  the  selection  of  the  proper 
group  of  trunks.  The  group  being  selected,  the  central-office  appa- 
ratus was  made  to  act  at  once  automatically  to  pick  out  and  connect 
with  the  first  idle  trunk  of  such  group.  Thus,  we  may  say  that  the 
subscriber  by  the  act  performed  on  his  signal  transmitter,  voluntarily 
chooses  the  group  of  trunks,  and  immediately  thereafter  the  central- 
office  apparatus  without  the  volition  of  the  subscriber  picks  out  the  first 
idle  one  of  this  group  of  trunks  so  chosen.  This  fundamental  idea, 
so  far  as  we  are  aware,  underlies  all  of  the  successful  automatic 
telephone-exchange  systems.  It  provides  for  the  possibility  of  many 
simultaneous  connections  through  the  switchboard,  and  it  provides 
against  the  simultaneous  appropriation  of  the  same  path  by  two 
or  more  calling  subscribers  and  thus  assures  against  interference 
in  the  choice  of  the  paths. 

Outline  of  Action.  In  order  to  illustrate  this  point  we  may 
briefly  outline  the  action  of  the  Strowger  automatic  system  in  the 
making  of  a  connection.  Assume  that  the  calling  subscriber  desires 
a  connection  with  a  subscriber  whose  line  bears  the  number  9,567. 
The  subscriber  in  making  the  call  will,  by  the  first  movement  of  his 
dial,  transmit  nine  impulses  over  his  line.  This  will  cause  the  se- 
lective apparatus  at  the  central  office,  that  is  at  the  time  associated 
with  the  calling  subscriber's  line,  to  move  its  selecting  fingers  oppo- 
site a  group  of  terminals  representing  the  ends  of  a  group  of  trunk 
lines  leading  to  apparatus  employed  in  connecting  with  the  ninth 
thousand  of  the  subscribers'  lines. 

While  the  calling  subscriber  is  getting  ready  to  transmit  the 


FUNDAMENTALS  OF  AUTOMATIC  SYSTEMS         513 

next  digit,  the  automatic  apparatus,  without  his  volition,  starts  to 
pick  out  the  first  idle  one  of  the  group  of  trunks  so  chosen.  Having 
found  this  it  connects  with  it  and  the  calling  subscriber's  line  is  thus 
extended  to  another  selective  apparatus  capable  of  performing  the 
same  sort  of  function  in  choosing  the  proper  hundreds  group. 

In  the  next  movement  of  his  dial  the  calling  subscriber  will 
send  five  impulses.  This  will  cause  the  last  chosen  selective  switch 
to  move  its  selective  fingers  opposite  a  group  of  terminals  represent- 
ing the  ends  of  a  group  of  trunks  each  leading  to  a  switch  that  is 
capable  of  making  connection  with  any  one  of  the  lines  in  the  fifth 
hundred  of  the  ninth  thousand.  Again  during  the  pause  by  the 
subscriber,  the  switch  that  chose  this  group  of  trunks  will  start  auto- 
matically to  pick  out  and  connect  with  the  first  idle  one  of  them,  and 
will  thus  extend  the  line  to  a  selective  switch  that  is  capable  of  reach- 
ing the  desired  line,  since  it  has  access  to  all  of  the  lines  in  the  chosen 
hundred.  The  third  movement  of  the  dial  sends  six  impulses  and 
this  causes  this  last  chosen  switch  to  move  opposite  the  sixth  group 
of  ten  terminals,  so  that  there  has  now  been  chosen  the  nine  hundred 
and  fifty-sixth  group  of  ten  lines.  The  final  movement  of  the  dial 
sends  seven  impulses  and  the  last  mentioned  switch  connects  with 
the  seventh  line  terminal  in  the  group  of  ten  previously  chosen  and 
the  connection  is  complete,  assuming  that  the  called  line  was  not 
already  engaged.  If  it  had  been  found  busy,  the  final  switch  would 
have  been  prevented  from  connecting  with  it  by  the  electrical  condi- 
tion of  certain  of  its  contacts  and  the  busy  signal  would  have  been 
transmitted  back  to  the  calling  subscriber. 

Fundamental  Idea.  This  idea  of  subdividing  the  subscribers' 
lines  in  an  automatic  exchange,  of  providing  different  groups  of 
trunks  so  arranged  as  to  afford  by  combination  a  number  of  possible 
parallel  paths  between  any  two  lines,  of  having  the  calling  subscriber 
select,  by  the  manipulation  of  his  instrument,  the  proper  group  of 
trunks  any  one  of  which  might  be  used  to  establish  the  connection 
he  desires,  and  of  having  the  central-office  apparatus  act  automat- 
ically to  choose  and  connect  with  an  idle  one  in  this  chosen  group, 
should  be  firmly  grasped.  It  appears,  as  we  have  said,  in  every 
successful  automatic  system  capable  of  serving  more  than  one  small 
group  of  lines,  and  until  it  was  evolved  automatic  telephony  was  not 
a  success. 


514  TELEPHONY 

Testing.  As  each  trunk  is  chosen  and  connected  with,  condi- 
tions are  established,  by  means  not  unlike  the  busy  test  in  multiple 
manual  switchboards,  which  will  guard  that  trunk  and  its  asso- 
ciated apparatus  against  appropriation  by  any  other  line  or  appara- 
tus as  long  as  it  is  held  in  use.  Likewise,  as  soon  as  any  subscriber's 
line  is  put  into  use,  either  by  virtue  of  a  call  being  originated  on  it, 
or  by  virtue  of  its  being  connected  with  as  a  called  line,  conditions 
are  automatically  established  which  guard  it  against  being  connected 
with  any  other  line  as  long  as  it  is  busy.  These  guarding  conditions 
of  both  trunks  and  lines,  as  in  the  manual  board,  are  established  by 
making  certain  contacts,  associated  with  the  trunks  or  lines,  assume 
a  certain  electrical  condition  when  busy  that  is  different  from  their 
electrical  condition  when  idle;  but  unlike  the  manual  switchboard 
this  different  electrical  condition  does  not  act  to  cause  a  click  in 
any  one's  ear,  but  rather  to  energize  or  de-energize  certain  electro- 
magnets which  will  establish  or  fail  to  establish  the  connection  ac- 
cording to  whether  it  is  proper  or  improper  to  do  so. 

Local  and  Inter-Office  Trunks.  The  groups  of  trunks  that  are 
used  in  building  up  connections  between  subscribers'  lines  may  be 
local  to  the  central  office,  or  they  may  extend  between  different 
offices.  The  action  of  the  two  kinds  of  trunks,  local  or  inter-office, 
is  broadly  the  same. 


CHAPTER   XXIX 
THE  AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM 

Almost  wherever  automatic  telephony  is  to  be  found — and 
its  use  is  extensive  and  rapidly  growing — the  so-called  Strowger 
system  is  employed.  It  is  so  named  because  it  is  the  outgrowth  of 
the  work  of  Almon  B.  Strowger,  an  early  inventor  in  the  automatic 
telephone  art.  That  the  system  should  bear  the  name  of  Strowger, 
however,  gives  too  great  prominence  to  his  work  and  too  little  to  that 
of  the  engineers  of  the  Automatic  Electric  Company  under  the  leader- 
ship of  Alexander  E.  Keith. 

Principles  of  Selecting  Switch.  The  underlying  features  of 
this  automatic  system  have  already  been  referred  to  in  the  abstract. 
A  better  grasp  of  its  principles  may,  however,  be  had  by  considering 
a  concrete  example  of  its  most  important  piece  of  apparatus — the 
selecting  switch.  The  bare  skeleton  of  such  a  switch,  sufficient 
only  to  illustrate  the  salient  point  in  its  mode  of  operation,  is  shown 
in  Fig.  380.  The  essential  elements  of  this  are  a  vertical  shaft  ca- 
pable of  both  longitudinal  and  rotary  motion;  a  pawl  and  ratchet 
mechanism  actuated  by  a  magnet  for  moving  the  shaft  vertically 
a  step  at  a  time;  another  pawl  and  ratchet  mechanism  actuated  by 
another  magnet  for  rotating  the  shaft  a  step  at  a  time;  an  arm  carry- 
ing wiper  contacts  on  its  outer  end,  mounted  on  and  moving  with 
the  shaft;  and  a  bank  of  contacts  arranged  on  the  inner  surface  of  a 
section  of  a  cylinder  adapted  to  be  engaged  by  the  wiper  contacts  on 
this  movable  arm. 

These  various  elements  are  indicated  in  the  merest  outline 
and  with  much  distortion  in  Fig.  380,  which  is  intended  to  illustrate 
the  principles  of  operation  rather  than  the  details  as  they  actually 
are  in  the  system.  In  the  upper  left-hand  corner  of  this  figure,  the 
magnet  shown  will,  if  energized  by  impulses  of  current,  attract  and 
release  its  armature  and,  in  doing  so,  cause  the  pawl  controlled  by 
this  magnet  to  move  the  vertical  shaft  of  the  switch  up  a  step  at  a  time, 


516 


TELEPHONY 


as  many  steps  as  there  are  impulses  of  current.  The  vertical  move- 
ment of  this  shaft  will  carry  the  wiper  arm,  attached  to  the  lower  end 
of  the  shaft,  up  the  same  number  of  steps  and,  in  doing  so3  will  bring 
the  contacts  of  this  wiper  arm  opposite,  but  not  engaging,  the  cor- 
responding row  of  stationary  contacts  in  the  semi-cylindrical  bank. 
Likewise,  through  the  ratchet  cylinder  on  the  intermediate  portion 
of  the  shaft,  the  magnet  shown  at  the  right-hand  portion  of  this  fig- 
ure will,  when  energized  by  a  succession  of  electrical  impulses,  ro- 


Fig.  380.     Principles  of  Automatic  Switch 

tate  the  shaft  a  step  at  a  time,  as  many  steps  as  there  are  impulses. 
This  will  thus  cause  the  contacts  of  the  wiper  arm  to  move  over  the 
successive  contacts  in  the  row  opposite  to  which  the  wiper  had  been 
carried  in  its  vertical  movement. 

At  the  lower  left-hand  corner  of  this  figure,  there  is  shown  a 
pair  of  keys  either  one  of  which,  when  operated,  will  complete  the 
circuit  of  the  magnet  to  which  it  is  connected,  this  circuit  including 
a  common  battery.  In  a  certain  rough  way  this  pair  of  key  switches 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM      .  o!7 

in  the  lower  left-hand  corner  of  the  drawing  may  be  taken  as  repre- 
senting the  call-transmitting  apparatus  at  the  subscriber's  station, 
and  the  two  wires  extending  therefrom  may  be  taken  as  representing 
the  line  wires  connecting  that  subscriber's  station  to  the  central 
office;  but  the  student  must  avoid  interpreting  them  as  actual  rep- 
resentations of  the  subscriber's  station  calling  apparatus  or  the  sub- 
scriber's line  since  their  counterparts  are  not  to  be  found  in  the 
system  as  it  really  exists.  Here  again  accuracy  has  been  sacrificed 
for  ease  in  setting  forth  a  feature  of  operation. 

Still  referring  to  Fig.  380,  it  will  be  seen  that  the  bank  contacts 
consist  of  ten  rows,  each  having  ten  pairs  of  contacts.  Assume 
again,  for  the  sake  of  simplicity,  that  the  exchange  under  considera- 
tion has  one  hundred  subscribers  and  that  each,  pair  of  bank  con- 
tacts represents  the  terminals  of  one  subscriber's  line.  Assume 
further  that  the  key  switches  in  the  lower  left-hand  corner  of  the 
figure  are  being  manipulated  by  a  subscriber  at  that  station  and 
that  he  wishes  to  obtain  a  connection  with  line  No.  67.  By  pressing 
and  releasing  the  left-hand  key  six  times,  he  will  cause  six  separate 
impulses  of  current  to  flow  through  the  upper  left-hand  magnet  and 
this  will  cause  the  switch  shaft  to  move  up  six  steps  and  bring  the 
wiper  arm  opposite  the  sixth  row  of  bank  contacts.  If  he  now 
presses  and  releases  his  right-hand  key  seven  times,  he  will,  through 
the  action  of  the  right-hand  magnet,  rotate  the  shaft  seven  steps, 
thus  bringing  the  wipers  into  contact  with  the  seventh  contact  of  the 
sixth  row  and  thus  into  contact  with  the  desired  line.  As  the  wiper 
contacts  on  the  switch  arm  form  the  terminals  of  the  calling  subscrib- 
er's line,  it  will  be  apparent  that  the  calling  subscriber  is  now  con- 
nected through  his  switch  with  the  line  of  subscriber  No.  67. 

As  stated,  each  of  the  pairs  of  bank  contacts  are  connected  with 
the  line  of  a  subscriber;  the  line,  Fig.  380,  is  shown  so  connected  to 
the  forty-first  pair  of  contacts,  that  is  to  the  first  contact  in  the  fourth 
row.  The  selecting  switch  shown  in  Fig.  380  would  be  for  the  sole 
use  of  the  subscriber  on  the  line  No.  41.  Each  of  the  other  subscribers 
would  have  a  similar  switch  for  his  own  exclusive  use.  Since  any 
of  the  switches  must  be  capable  of  reaching  line  No.  67.  for  instance, 
when  moved  up  six  rows  and  around  seven,  it  follows  that  the  sixty- 
seventh  pair  of  contacts  in  each  bank  of  the  entire  one  hundred  switches 
must  also  be  connected  together  and  to  line  No.  67.  The  same  is. 


518  TELEPHONY 

of  course,  true  of  all  the  contacts  corresponding  to  any  other  number. 
Multiple  connections  are  thus  involved  between  the  corresponding 
contacts  of  the  banks,  in  much  the  same  way  as  in  the  corresponding 
jacks  in  the  multiple  of  a  manual  switchboard.  As  a  result  of  this 
.  multiple  connection  of  the  bank  contacts,  any  subscriber  may  move 
the  wiper  arm  of  his  selecting  switch  into  connection  with  the  line  of 
any  other  subscriber. 

The  "Up-and- Around"  Movement.  The  elemental  idea  to  be 
grasped  by  the  discussion  so  far,  is  the  so-called  "up-and-around" 
method  of  action  of  the  selecting  switches  employed  in  this  sys- 
tem. This  preliminary  discussion  may  be  carried  a  step  further 
by  saying  that  the  arrangement  is  such  that  when  a  subscriber 
presses  both  his  keys  and  grounds  both  of  the  limbs  of  his  line, 
such  a  condition  is  brought  about  as  will  cause  all  holding  pawls 
to  be  withdrawn  from  the  shaft,  and  thus  allow  it  to  return  to  its 
normal  position  with  respect  to  both  its  vertical  and  rotary  move- 
ments. No  attempt  has  been  made  in  Fig.  380  to  show  how  this  is 
accomplished. 

Function  of  Line  Switch.  Such  a  system  as  has  been  briefly 
outlined  in  the  foregoing  would  require  a  separate  selecting  switch 
for  each  subscriber's  line  and  would  be  limited  to  use  in  exchanges 
having  not  more  than  one  hundred  lines.  In  the  modern  system  of 
the  Automatic  Electric  Company,  the  requirement  that  each  sub- 
scriber shall  have  a  selective  switch,  individual  to  his  own  line, 
has  been  eliminated  by  introducing  what  is  called  an  individual 
line  switch  by  means  of  which  any  one  of  a  group  of  subscribers' 
lines,  making  a  call,  automatically  appropriates  one  of  a  smaller 
group  of  selecting  switches  and  makes  it  its  own  only  while  the  con- 
nection exists. 

Subdivision  of  Subscribers'  Lines.  The  limitation  as  to  the 
size  of  the  exchange  has  been  overcome,  without  increasing  the  num- 
ber of  bank  contacts  in  any  selecting  switch,  by  dividing  the  sub- 
scribers' lines  into  groups  of  one  hundred  and  causing  selecting 
switches  automatically  to  extend  the  calling  subscriber's  line  first  into 
a  group  of  groups  corresponding,  for  instance,  to  the  thousand 
containing  the  called  subscriber's  line,  and  then  into  the  particular 
group  containing  the  line,  and  lastly,  to  connect  with  the  individual 
line  in  that  group. 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       519 

Underlying  Feature  of  Trunking  System.  It  will  be  remembered 
that  in  the  chapter  on  fundamental  principles  of  automatic  systems, 
it  was  stated  that  the  subscriber,  by  means  of  the  signal  transmitter 
at  his  station,  was  made  to  govern  the  action  of  the  central-office 
apparatus  in  the  selection  of  a  proper  group  of  trunks ;  and  the  group 
being  selected,  the  central-office  apparatus  was  made  to.  act  auto- 
matically to  pick  out  and  connect  with  the  first  idle  trunk  of  such 
group.  This  selection  by  the  subscriber  of  a  group  followed  by 
the  automatic  selection  from  among  that  group  forms  the  basis  of 
the  trunking  system.  It  is  impossible,  by  means  of  any  simple 
diagram,  to  show  a  complete  scheme  of  trunking  employed,  but 
Fig.  381  will  give  a  fundamental  conception  of  it.  This  figure 
shows  how  a  single  calling  line,  indicated  at  the  bottom,  may  find 
access  into  any  particular  line  in  an  office  having  a  capacity  for  ten 
thousand. 

Names  of  Selecting  Switches.  Selecting  switches  of  the  "up- 
and-around"  type  are  the  means  by  which  the  calling  line  selects 
and  connects  with  the  trunk  lines  required  in  building  up  the  con- 
nection, and  finally  selects  and  connects  with  the  line  of  the  called 
subscriber.  Where  such  a  switch  is  employed  for  the  purpose  of 
selecting  a  trunk,  it  is  called  a  selector  switch.  It  is  a  first  selector 
when  it  serves  to  pick  out  a  major  group  of  lines,  i.  e.,  a  group  con- 
taining a  particular  thousand  lines  or,  in  a  multi-office  system,  a 
group  represented  by  a  complete  central  office.  It  is  &  second  se- 
lector when  it  serves  to  make  the  next  subdivision  of  groups;  a  third 
selector  if  further  subdivision  of  groups  is  necessary;  and  finally  it  is 
a  connector  when  it  is  employed  to  pick  out  and  connect  with  the 
particular  line  in  the  final  group  of  one  hundred  lines  to  which  the 
connection  has  been  brought  by  the  selectors.  In  a  single  office  of 
10,000-line  capacity,  therefore,  we  would  have  first  and  second  se- 
lectors and  connectors,  the  first  selectors  picking  out  the  thousands, 
the  second  selectors  the  hundreds,  and  the  connectors  the  individual 
line.  In  a  multi-office  system  we  may  have  first,  second,  and  third 
selectors  and  connectors,  the  first  selector  picking  out  the  office,  the 
second  selector  the  thousands  in  that  office,  the  third  selector  the 
hundreds,  and  the  connector  the  individual  lines. 

The  Line  Switch.  In  addition  to  the  selectors  and  connectors 
there  are  line  switches,  which  are  comparatively  simple,  one  individual 


520  TELEPHONY 

to  each  line.  Each  of  these  has  the  function,  purely  automatic, 
of  always  connecting  a  line,  as  soon  as  a  call  is  originated  on  it,  to 
some  one  of  a  smaller  group  of  first  selectors  available  to  that  line. 
This  idea  may  be  better  grasped  when  it  is  understood  that,  in  the 
earlier  systems  of  the  Automatic  Electric  Company,  there  was  a 
first  selector  permanently  associated  with  each  line.  By  the  addi- 
tion of  the  comparatively  simple  line  switch,  a  saving  of  about  ninety 
per  cent  of  the  first  selectors  was  effected,  since  the  number  of  first 
selectors  was  thereby  reduced  from  a  number  equal  to  the  number 
of  lines  in  a  group  to  a  number  equal  to  the  number  of  simultaneous 
connections  resulting  from  calls  originating  in  that  group.  In  other 
words,  by  the  line  switch,  the  number  of  first  selectors  is  determined 
by  the  traffic  rather  than  by  the  number  of  lines. 

Scheme  of  Trunking.  With  this  understanding  as  to  the  names  and 
broader  functions  of  the  things  involved,  Fig.  381  may  now  be  under- 
stood. The  line  switch  of  the  single  line,  as  indicated  here,  has  only 
the  power  of  selection  among  three  trunks,  but  it  is  to  be  understood 
that  in  actual  practice,  it  would  have  access  to  a  greater  number, 
usually  ten.  So,  also,  throughout  this  diagram  we  have  shown  the 
apparatus  and  trunks  arranged  in  groups  of  three  instead  of  in  groups 
of  ten,  only  the  first  three  thousands  groups  being  indicated  and  the 
first  three  hundreds  groups  in  each  thousand.  Again  only  three 
levels  instead  of  ten  are  indicated  for  each  selecting  switch,  it  being 
understood  that  in  the  diagram  the  various  levels  are  represented  by 
concentric  arcs  of  circles,  and  the  trunk  contacts  by  dots  on  these 
arcs. 

Line-Switch  Action.  When  the  subscriber,  whose  line  is  shown 
at  the  bottom  of  the  figure,  begins  to  make  a  call,  the  line  switch 
acts  to  connect  his  line  with  one  of  the  first  selector  trunks  available 
to  it.  This  selection  is  entirely  preliminary  and,  except  to  start 
it,  is  in  no  way  under  the  control  of  the  calling  subscriber.  The 
calling  line  now  has  under  its  control  a  first  selector  which,  for 
the  time  being,  becomes  individual  to  it.  Let  it  be  assumed  that 
the  line  switch  found  the  first  of  the  first  selector  trunks  already  ap- 
propriated by  some  other  switch,  but  that  the  second  one  of  these 
trunks  was  found  idle.  This  trunk  being  appropriated  by  the  line 
switch  places  the  center  one  of  the  first  selectors  shown  under  the 
control  of  the  subscriber's  line.  This  first  selector  then  acts  in  re- 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       521 


522  TELEPHONY 

sponse  to  the  first  set  of  selective  impulses  sent  out  by  his  signal 
transmitter. 

First  Selector  Action.  We  will  assume  that  the  calling  subscriber 
desires  to  connect  with  No.  3213.  The  first  movement  of  the  sub- 
scriber's signal  transmitter  will  send,  therefore,  three  impulses  over 
the  line.  These  impulses  will  act  on  the  vertical  magnet  of  the  first 
selector  switch  to  move  it  up  three  steps.  On  this  "level"  of  the 
contact  bank  of  this  switch  all  of  the  contacts  will  represent  second 
selector  trunks  leading  to  the  third  thousand  group.  The  other 
ends  of  these  trunks  will  terminate  in  the  wipers  and  also  in  the  con- 
trolling magnets  of  second  selectors  serving  this  thousand.  This 
function  on  the  part  of  the  first  selector  controlled  by  the  act  of  the 
subscriber  will  have  thus  selected  a  group  of  trunks  leading  to  the 
third  thousand,  but  the  subscriber  has  nothing  to  do  with  which  one 
of  the  trunks  of  this  group  will  actually  be  used.  Immediately 
following  the  vertical  movement  of  the  first  selector  switch  the  rotary 
movement  of  this  switch  will  start  and  will  continue  until  the  wipers 
of  that  switch  have  found  contacts  of  an  idle  trunk  leading  to  a  sec- 
ond selector.  Assuming  that  the  first  trunk  was  the  one  found 
idle,  the  first  selector  wipers  would  pause  on  the  first  pair  of  contacts 
in  the  third  level  of  its  bank,  and  the  trunk  chosen  may  be  seen  leading 
from  that  contact  off  to  the  group  of  second  selectors  belonging  to 
the  third  thousand  For  clearness,  the  chosen  trunks  in  this  assumed 
connection  are  shown  heavier  than  the  others. 

Second  Selector  Action.  The  next  movement  of  the  dial  by  the 
subscriber  in  establishing  his  desired  connection  will  send  two  im- 
pulses, it  being  desired  to  choose  the  second  hundred  in  the  third 
thousand.  The  first  selector  will  have  become  inoperative  before 
this  second  series  of  impulses  is  sent  and,  therefore,  only  the  second 
selector  will  respond.  Its  vertical  magnet  acting  under  the  influence 
of  these  two  impulses  will  step  up  its  wiper  contacts  opposite  the 
second  row  of  bank  contacts,  and  the  subscriber  will  thus  have 
chosen  the  group  of  trunks  leading  to  the  second  hundred  in  the 
third  thousand.  Here,  again,  the  automatic  operation  of  picking 
out  the  first  idle  one  of  this  chosen  group  of  trunks  will  take  place 
without  the  volition  of  the  subscriber,  and  it  will  be  assumed  that 
the  first  two  trunks  on  this  level  of  the  second  selector  were  found 
already  engaged  and  that  the  third  was  therefore  chosen.  The 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       523 

connection  continues,  as  indicated  by  heavy  lines  in  Fig.  381,  to  the 
third  one  of  the  connectors  in  the  second  hundred  of  the  third  thou- 
sand. Any  one  of  these  connectors  would  have  accomplished  the 
purpose  but  this  is  assumed  to  be  the  first  one  found  idle  by  the 
second  selector. 

Connector  Action.  The  third  movement  of  the  subscriber's  dial 
will  send  but  one  impulse,  this  corresponding  to  the  first  group  of 
ten  in  the  second  hundred  in  the  third  thousand.  This  impulse  will 
move  the  connector  shaft  up  to  the  first  level  of  bank  contacts;  and 
from  now  on  the  action  of  the  connector  differs  radically  from  that 
of  the  selectors.  The  connector  is  not  searching  for  an  idle  trunk  in 
the  group  but  for  a  particular  line  and,  therefore,  having  chosen  the 
group  of  ten  lines  in  the  desired  hundred,  the  connector  switch  waits 
for  further  guidance  from  the  subscriber.  This  comes  in  the  form 
of  the  final  set  of  impulses  sent  by  the  subscriber's  signal  trans- 
mitter which,  in  this  case,  will  be  three  in  number,  corresponding 
to  the  final  digit  in  the  number  of  the  called  subscriber.  This  series 
of  impulses  will  control  the  rotary  movement  of  the  connector  wipers 
which  will  move  along  the  first  level  and  stop  on  the  third  one.  The 
process  is  seen  to  be  one  of  successive  selection,  first  of  a  large  group, 
then  of  a  smaller,  agrjn  of  a  smaller,  and  finally  of  an  individual. 

If  the  line  is  found  not  busy,  the  connection  between  the  two 
subscribers  is  complete  and  the  called  subscriber's  bell  will  be  rung. 
If  it  is  found  busy,  however,  the  connector  will  refuse  to  connect  and 
will  drop  back  to  its  normal  position,  sending  a  busy  signal  back 
to  the  calling  subscriber.  The  details  of  ringing  and  the  busy-back 
operation  may  only  be  understood  by  a  discussion  of  drawings,  sub- 
sequently to  be  referred  to. 

Two=Wire  and  Three=Wire  Systems.  In  most  of  the  systems 
of  the  Automatic  Electric  Company  in  use  today  the  impulses  by 
which  the  subscriber  controls  the  central-office  apparatus  flow  over 
one  side  of  the  line  or  the  other  and  return  by  ground.  The  metallic 
circuit  is  used  for  talking  and  for  ringing  the  called  subscriber's 
bell,  while  ground  return  circuits,  on  one  side  of  the  line  or  the  other, 
are  used  for  sending  all  the  switch  controlling  impulses. 

Recently  this  company  has  perfected  a  system  wherein  no 
ground  is  required  at  the  subscriber's  station  and  no  ground  return 
path  is  used  for  any  purpose  between  the  subscriber  and  the  central 


524 


TELEPHONY 


office.  This  later  system  is  known  as  the  "two-wire"  system,  and  in 
contra-distinction  to  it,  the  earlier  and  most  used  system  has  been 
referred  to  as  the  "three-wire."  It  is  not  meant  by  this  that  the  line 
circuits  actually  have  three  wires  but  that  each  line  employs  three 
conductors,  the  two  wires  of  the  line  and  the  earth.  The  three- 
wire  system  will  be  referred  to  and  described  in  detail,  and  from  it 
the  principles  of  the  two-wire  system  will  be  readily  understood. 

Subscriber's  Station  Apparatus.  The  detailed  operation  of  the 
three-wire  system  may  be  best  understood  by  considering  the  sub- 
scriber's station  apparatus  first.  The  general  appearance  of  the 


Fig.  382.     Automatic  Wall  Set 


Pig.  383.     Automatic  Desk  Stand 


wall  set  is  shown  in  Fig.  382,  and  of  the  desk  set  in  Fig.  383.  These 
instruments  embody  the  usual  talking 'and  call-receiving  apparatus 
of  a  common-battery  telephone  and  in  addition  to  this,  the  signal 
transmitter,  which  is  the  thing  especially  to  be  considered  now.  The 
diagrammatic  illustration  of  the  signal  transmitter  and  of  the  relation 
that  its  parts  bear  to  the  other  elements  of  the  telephone  set  is  shown 
in  Fig.  384.  It  has  already  been  stated  that  the  subscriber  manipu- 
lates the  signal  transmitter  by  rotating  the  dial  on  the  'face  of  the 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       525 

instrument.     A  clearer  idea  of  this  dial  and  of  the  finger  stop  for  it 
may  be  obtained  from  Figs.  382  and  383. 

Operation.  To  make  a  call  for  a  given  number  the  subscriber 
removes  his  receiver  from  its  hook,  then  places  his  forefinger  in  the 
hole  opposite  the  number  corresponding  to  the  first  digit  of  the  de- 
sired number.  By  means  of  the  grip  thus  secured,  he  rotates  the 


Fig.  384.     Circuits  of  Telephone  Set 

dial  until  its  movement  is  stopped  by  the  impact  of  the  finger  against 
the  stop.  The  dial  is  then  released  and  in  its  return  movement  it 
sends  the  number  of  impulses  corresponding  to  the  first  digit  in  the 
called  number.  A  similar  movement  is  made  for  each  digit. 

In  Fig.  384  is  given  a  phantom  view  of  the  dial,  in  order  to  show 
more  clearly  the  relation  of  the  mechanical  parts  and  contacts  con- 


526  TELEPHONY 

trolled  by  it.  For  a  correct  idea  of  its  mechanical  action  it  must  be 
understood  that  the  shaft  1,  the  lever  2,  and  the  interrupter  segment 
S  are  all  rigidly  fastened  to  the  dial  and  move  with  it.  A  coiled 
spring  always  tends  to  move  the  dial  and  these  associated  parts 
back  to  their  normal  positions  when  released  by  the  subscriber,  and 
a  centrifugal  governor,  riot  shown,  limits  the  speed  of  the  return 
movement. 

The  subscriber's  hook  switch  is  mechanically  interlocked  with 
the  dial  so  as  to  prevent  the  dial  being  moved  from  its  normal  posi- 
tion until  the  hook  is  in  its  raised  position.  This  interlocking  func- 
tion involves  also  the  pivoted  dog  4-  Normally  the  lower  end  of  this 
dog  lies  in  the  path  of  the  pin  5  cariied  on  the  lever  2,  and  thus  the 
shaft,  dial,  and  segment  are  prevented  from  any  considerable  move- 
ment when  the  receiver  is  on  the  hook.  However,  when  the  receiver 
is  removed  from  its  hook,  the  upwardly  projecting  arm  from  the 
hook  engages  a  projection  on  the  dog  4  and  moves  the  dog  out  of  the 
path  of  the  pin  5.  Thus  the  dial  is  free  to  be  rotated  by  the  sub- 
scriber. The  pin  6  is  mounted  in  a  stationary  position  and  serves 
to  limit  the  backward  movement  of  the  dial  by  the  lever  2  striking 
against  it. 

Ground  Springs: — Five  groups  of  contact  springs  must  be 
considered,  some  of  which  are  controlled  wholly  by  the  position 
of  the  switch  hook,  others  jointly  by  the  position  of  the  switch  hook 
and  the  dial,  others  by  the  movement  of  the  dial  itself,  and  still  others 
by  the  pressure  of  the  subscriber's  finger  on  a  button.  The  first 
of  these  groups  consists  of  the  springs  7  and  8,  the  function  of  which 
is  to  control  the  continuity  of  the  ground  connection  at  the  subscrib- 
er's station.  The  arrangement  of  these  two  springs  is  such  that  the 
ground  connection  will  be  broken  until  the  subscriber's  receiver  is 
removed  from  its  hook.  As  soon  as  the  receiver  is  raised,  these 
springs  come  together  in  an  obvious  manner,  the  dog  4  being  lifted 
out  of  the  way  by  the  action  of  the  hook.  The  ledge  on  the  lower 
portion  of  the  spring  7  serves  as  a  rest  for  the  insulated  arm  of  the 
dog  4  to  prevent  this  dog,  which  is  spring  actuated,  from  returning 
and  locking  the  dial  until  after  the  receiver  has  been  hung  up. 

Bell  and  Transmitter  Springs : — The  second  group  is  that  em- 
bracing the  springs  9,  10,  11,  and  12.  The  springs  10  and  JJ 
are  controlled  by  the  lower  projection  from  the  switch  book,  the 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       527 

spring  11  engaging  the  spring  12  only  when  the  hook  is  down. 
The  spring  10  engages  the  spring  9  only  when  the  hook  lever 
is  up  and  not  then  unless  the  dial  is  in  its  normal  position.  While 
the  hook  is  raised,  therefore,  the  springs  9  and  10  break  contact 
whenever  the  dial  is  moved  and  make  contact  again  when  it  returns 
to  its  normal  position.  The  springs  11  and  12  control  the  circuit 
through  the  subscriber's  bell  while  the  springs  9  and  10  control  the 
continuity  of  the  circuit  from  one  side  of  the  line  to  the  other  so  as 
to  isolate  the  limbs  from  each  other  while  the  signal  transmitter 
is  sending  its  impulses  to  the  central  office. 

Impulse  Springs: — The  third  group  embraces  springs  13,  14, 
and  15  and  these  are  the  ones  by  which  the  central-office  switches 
are  controlled  in  building  up  a  connection. 

Something  of  the  prevailing  nomenclature  which  has  grown 
up  about  the  automatic  system  may  be  introduced  at  this  point. 
The  movements  of  the  selecting  switches  at  the  central  office  are 
referred  to  as  vertical  and  rotary  for  obvious  reasons.  On  account 
of  this  the  magnet  which  causes  the  vertical  movement  is  referred  to  as 
the  vertical  magnet  and  that  which  accomplishes  the  rotary  move- 
ment as  the  rotary  magnet.  It  happens  that  in  all  cases  the  selecting 
impulses  sent  by  the  subscriber's  station,  corresponding  respectively 
to  the  number  of  digits  in  the  called  subscriber's  number,  are  sent 
over  one  side  of  the  line  and  in  nearly  all  cases  these  selecting  im- 
pulses actuate  the  vertical  movements  of  the  selecting  switches. 
For  this  reason  the  particular  limb  of  the  line  over  which  the  selecting 
impulses  are  sent  is  called  the  vertical  limb.  The  other  limb  of  the 
line  is  the  one  over  which  the  single  impulse  is  sent  after  each  group 
of  selecting  impulses,  and  it  is  this  impulse  in  every  case  which  causes 
the  selector  switch  to  start  rotating  in  its  hunt  for  an  idle  trunk. 
This  side  of  the  line  is,  therefore,  called  rotary.  For  the  same  rea- 
sons the  impulses  over  the  vertical  side  of  the  line  are  called  vertical 
impulses  and  those  over  the  rotary  side,  rotary  impulses.  The  nam- 
ing of  the  limbs  of  the  line  and  of  the  current  impulses  vertical  and 
rotary  may  appear  odd  but  it  is,  to  say  the  least,  convenient  and  ex- 
pressive. 

Coming  back  to  the  functions  of  the  third  group  of  springs, 
13,  14,  and  15,  15  may  be  called  the  vertical  spring  since  it  sends 
vertical  impulses;  13,  the  rotary  spring  since  it  sends  rotary  impulses; 


528  TELEPHONY 

and  14,  the  ground  spring  since,  when  the  hook  is  up,  it  is  connected 
with  the  ground. 

On  the  segment  3  there  are  ten  projections  or  cams  16  which, 
when  the  dial  is  moved,  engage  a  projection  of  the  spring  15.  When 
the  dial  is  being  pulled  by  the  subscriber's  finger,  these  cams  engage 
the  spring  15  in  such  a  way  as  to  move  it  away  from  the  ground 
spring  and  no  electrical  contact  is  made.  On  the  return  of  the  dial, 
however,  these  cams  engage  the  projection  on  the  spring  15  in  the 
opposite  wray  and  the  passing  of  each  cam  forces  this  vertical  spring 
into  engagement  with  the  ground  spring.  It  will  readily  be  seen, 
therefore,  by  a  consideration  of  the  spacing  of  these  cams  on  the 
segment  and  the  finger  holes  in  the  dial  that  the  number  of  cams 
which  pass  the  vertical  spring  15  will  correspond  to  the  number  on 
the  hole  used  by  the  subscriber  in  moving  the  dial. 

Near  the  upper  right-hand  corner  of  the  segment  3,  as  shown 
in  Fig.  384,  there  is  another  projection  or  cam  17,  the  function  of 
which  is  to  engage  the  rotary  spring  13  and  press  it  into  contact  with 
the  ground  spring.  Thus,  the  first  thing  that  happens  in  the  move- 
ment of  the  dial  is  for  the  projection  17  to  ride  over  the  hump  on  the 
rotary  spring  and  press  the  contact  once  into  engagement  with  the 
ground  spring;  and  likewise,  the  last  thing  that  happens  on  the  re- 
turn movement  of  the  dial  is  for  the  rotary  spring  to  be  connected 
once  to  the  ground  spring  after  the  last  vertical  impulse  has  been 
sent. 

If  both  the  rotary  and  vertical  sides  of  the  line  are  connected 
with  the  live  side  of  the  central-office  battery,  it  follows  that  every 
contact  between  the  vertical  and  the  ground  spring  or  between  the 
rotary  and  the  ground  spring  will  allow  an  impulse  of  current  to  flow 
over  the  vertical  or  the  rotary  side  of  the  line. 

We  may  summarize  the  action  of  these  impulse  springs  by 
saying  that  whenever  the  dial  is  moved  from  its  normal  posi- 
tion, there  is,  at  the  beginning  of  this  movement,  a  single  rotary  im- 
pulse over  the  rotary  side  of  the  line;  and  that  while  the  dial  returns, 
there  is  a  series  of  vertical  impulses  over  the  vertical  side  of  the  line; 
and  just  before  the  dial  reaches  its  normal  position,  after  the  sending 
of  the  last  vertical  impulse,  there  is  another  impulse  over  the  rotary 
side  of  the  line. 

The  mechanical  arrangements  of  the  interrupter  segment  3  and 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       529 

its  associated  parts  have  been  greatly  distorted  in  Fig.  384  in  order 
to  make  clear  their  mode  of  operation.  This  drawing  has  been 
worked  out  with  great  care,  with  this  in  mind,  at  a  sacrifice  of  ac- 
curacy in  regard  to  the  actual  structural  details. 

Ringing  Springs: — The  fourth  group  of  springs  in  the  sub- 
scriber's telephone  is  the  ringing  group  and  embraces  the  springs 
18,  19,  and  20.  The  springs  19  and  20  are  normally  closed  and 
maintain  the  continuity  of  the  talking  circuit.  When,  however, 
the  button  attached  to  the  spring  19 — which  button  may  be  seen 
projecting  from  the  instrument  shown  in  Fig.  382,  and  from  the  base 
of  the  one  shown  in  Fig.  383 — is  pressed,  the  continuity  of  the  talking 
circuit  is  interrupted  and  the  vertical  side  of  the  line  is  connected 
with  the  ground.  It  is  by  this  operation,  after  the  connection  has 
been  made  with  the  desired  subscriber's  line,  that  the  central-office 
apparatus  acts  to  send  ringing  current  out  on  that  line. 

Release  Springs: — The  fifth  set  of  springs  is  the  one  shown  at 
the  left-hand  side  of  Fig.  384,  embracing  springs  21, 22,  and  23.  The 
long  curved  spring  21  is  engaged  by  the  projecting  lug  on  the  switch 
hook  when  it  rises  so  as  to  press  this  spring  away  from  the  other 
two.  On  the  return  movement  of  the  hook,  however,  this  spring 
is  pressed  to  the  left  so  as  to  bring  all  three  of  them  into  contact, 
and  this,  it  will  be  seen,  grounds  both  limbs  of  the  line  at  the  sub- 
scriber's station.  This  combination  cannot  be  effected  by  any  of 
the  other  springs  at  any  stage  of  their  operation,  and  it  is  the  one 
which  results  in  the  energization  of  such  a  combination  of  relays 
and  magnets  at  the  central  office  as  will  release  all  parts  involved  in 
the  connection  and  allow  them  to  return  to  their  normal  positions 
ready  for  another  call. 

Salient  Points.  If  the  following  things  are  borne  in  mind  about 
the  operation  of  the  subscriber's  station  apparatus,  an  understanding 
of  the  central-office  operations  will  be  facilitated.  First,  the  selec- 
tive impulses  always  flow  over  the  vertical  side  of  the  line;  they  are 
always  preceded  and  always  followed  by  a  single  impulse  over  the 
rotary  side  of  the  line.  The  ringing  button  grounds  the  vertical  side 
of  the  line  and  the  release  springs  ground  both  sides  of  the  line 
simultaneously. 

The  Line  Switch.  The  first  thing  to  be  considered  in  connection 
with  the  central-office  apparatus  is  the  line  switch.  This,  it  will 


530  TELEPHONY 

be  remembered,  is  the  device  introduced  into  each  subscriber's  line 
at  the  central  office  for  the  purpose  of  effecting  a  reduction  of  the 
number  of  first  selectors  required  at  the  central  office,  and  also  for 
bringing  about  certain  important  functional  results  in  connection 
with  trunking  between  central  and  sub-offices.  The  function  of  the 
line  switch  in  connection  with  the  subscriber's  line,  however,  is 
purely  that  of  reducing  the  number  of  first  selectors. 

The  line  switches  of  one  hundred  lines  are  all  associated  to 
form  a  single  unit  of  apparatus,  which,  besides  the  individual  line 
switches,  includes  certain  other  apparatus  common  to  those  lines. 
Such  a  group  of  one  hundred  line  switches  and  associated  common 
apparatus  is  called  a  line-switch  unit,  or  frequently,  a  Keith  unit. 
Confusion  is  likely  to  arise  in  the  mind  of  the  reader  between  the 
individual  line  switch  and  the  line-switch  unit,  and  to  avoid  this  we 
will  refer  to  the  piece  of  apparatus  individual  to  the  line  as  the  line 
switch,  and  to  the  complete  unit  formed  of  one  hundred  of  these  de- 
vices as  a  line-switch  unit. 

Line  and  Trunk  Contacts.  Each  line  switch  has  its  own  bank 
of  contacts  arranged  in  the  arc  of  a  circle,  and  in  this  same  arc  are 
also  placed  the  contacts  of  each  of  the  ten  individual  trunks  which 
it  is  possible  for  that  line  to  appropriate.  The  contacts  individual  to 
the  subscriber's  line  in  the  line  switch  are  all  multipled  together, 
the  arrangement  being  such  that  if  a  wedge  or  plunger  is  inserted  at 
any  point,  the  line  contacts  will  be  squeezed  out  of  their  normal  posi- 
tion so  as  to  engage  the  contacts  of  the  trunk  corresponding  to  the 
particular  position  in  the  arc  at  which  the  wedge  or  plunger  is  in- 
serted. A  small  plunger  individual  to  each  line  is  so  arranged  that 
it  may  be  thrust  in  between  the  contact  springs  in  the  line-switch 
bank  in  such  manner  as  to  connect  any  one  of  the  trunks  with  the 
h'ne  terminals  represented  in  that  rowr,  the  particular  trunk  so  con- 
nected depending  on  the  portion  of  the  arc  toward  which  the 
plunger  is  pointed  at  the  time  it  is  thrust  in  the  contacts. 

These  banks  of  lines  and  trunk  contacts  are  horizontally  arranged, 
and  piled  in  vertical  columns  of  twenty-five  line  switches  each.  The 
ten  trunk  contacts  are  multipled  vertically  through  the  line-switch 
banks,  so  that  the  same  ten  trunks  are  available  to  each  of  the 
twenty-five  lines.  We  thus  have,  in  effect,  an  old  style,  Western 
Union,  cross-bar  switchboard,  the  line  contacts  being  represented 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       531 

to 

in  horizontal  rows  and  the  trunk  contacts  in  vertical  rows,  the  con- 
nection between  any  line  and  any  trunk  being  completed  by  inserting 
a  plunger  at  the  point  of  intersection  of  the  horizontal  and  the  ver- 
tical rows  corresponding  to  that  line  and  trunk. 

Trunk  Selection.  The  plungers  by  which  the  lines  and  trunks 
are  connected  are,  as  has  been  said,  individual  to  the  line,  and  all 
of  the  twenty-five  plungers  in  a  vertical  row  are  mounted  in  such 
manner  as  to  be  normally  held  in  the  same  vertical  plane,  and  this 
vertical  plane  is  made  to  oscillate  back  and  forth  by  an  oscillating 
shaft  so  as  always  to  point  the  plungers  toward  a  vertical  row  of 
trunk  contacts  that  represent  a  trunk  that  is  not  in  use  at  the  time. 
The  to-and-fro  movement  of  this  oscillating  shaft,  called  the  master 
bar,  is  controlled  by  a  master  switch  and  the  function  of  this  master 
switch  is  always  to  keep  the  plungers  pointed  toward  the  row  of 
contacts  of  an  idle  trunk.  The  thrusting  movement  of  the  indi- 
vidual plungers  into  the  contact  bank  is  controlled  by  magnets 
individual  to  the  line  and  under  control  of  the  subscriber  in 
initiating  a  call.  As  soon  as  the  plunger  of  a  line  has  been  thus 
thrust  into  the  contact  bank  so  as  to  connect  the  terminals  of  that 
line  with  a  given  trunk,  the  plunger  is  no  longer  controlled  by  the 
master  bar  and  remains  stationary.  The  master  bar  then  at  once 
moves  all  of  the  other  plungers  that  are  not  in  use  so  that  they  will 
point  to  the  terminals  of  another  trunk  that  is  not  in  use.  The 
plungers  of  all  the  line  switches  in  a  group  of  twenty-five  are,  there- 
fore, subject  to  the  oscillating  movements  of  the  master  bar  when 
the  line  is  not  connected  to  a  first  selector  trunk.'  As  soon  as  a  call 
is  originated  on  a  line,  the  corresponding  plunger  is  forced  into  the 
bank  and  is  held  stationary  in  maintaining  the  connection  to  a  first 
selector  trunk,  and  all  of  the  other  plungers  not  so  engaged,  move  on 
so  as  to  be  ready  to  engage  another  idle  trunk. 

Trunk  Ratio.  The  assignment  of  ten  trunks  to  twenty-five 
lines  would  be  a  greater  ratio  of  trunks  than  ordinary  traffic  con- 
ditions require.  This  ratio  of  trunks  to  lines  is,  however,  readily 
varied  by  multipling  the  trunk  contacts  of  several  twenty-five  line 
groups  together.  Thus,  ten  trunks  may  be  made  available  to  one 
hundred  subscribers'  lines  by  multipling  the  trunks  of  four  twenty- 
five  line  switch  groups  together.  In  this  case  the  four  master  bars 
corresponding  to  the  four  groups  of  twenty-five  line  switches  are  all 


532 


TELEPHONY 


mechanically  connected  together  so  as  to  move  in  unison  under  the 
control  of  a  single  master  switch.  If  more  than  ten  and  less  than 
twenty-one  trunks  are  assigned  to  one  hundred  lines,  then  each  set 
of  ten  trunks  is  multipled  to  the  trunk  contacts  of  fifty  line  switches, 
the  two  master  bars  of  these  switches  being  connected  together  and 
controlled  by  a  common  master  switch. 

Structure  of  Line  Switch.  The  details  of  the  parts  of  a  line 
switch  that  are  individual  to  the  line  are  shown  in  Fig.  385,  the  line 
and  trunk  contact  bank  being  shown  in  the  lower  portion  of  this 
figure  and  also  in  a  separate  view  in  the  detached  figure  at  the  right. 
A  detailed  group  of  several  such  line  switches  with  the  oscillating 


Fig.  385.     Line  Switch 


master  bar  is  shown  in  Fig.  386.  This  figure  shows  quite  clearly 
the  relative  arrangement  of  the  line  and  trunk  contact  banks,  the 
plungers  for  each  bank,  and  the  master  bar. 

In  practice,  four  groups  of  twenty-five  line  switches  each  are 
mounted  on  a  single  framework  and  the  group  of  one  hundred  line 
switches,  together  with  certain  other  portions  of  the  apparatus  that 
will  be  referred  to  later,  form  a  line-switch  unit.  A  front  view  of 
such  a  unit  is  shown  in  Fig.  387.  In  order  to  give  access  to  all  por- 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM        533 

tions  of  the  wiring  and  apparatus,  the  framework  supporting  each 
column  of  fifty  line  switches  is  hinged  so  as  to  open  up  the  interior 
of  the  device  as  a  whole.  A  line-switch  unit  thus  opened  out  is  shown 
in  Fig.  388. 

Circuit  Operation.     The  mode  of  operation  of  the  line  switch 
may  be  best  understood  in  connection  with  Fig.  389,  which  shows 


Fig.  386.     Portion  of  Line- Switch  Unit 


in  a  schematic  way  the  parts  of  a  line  switch  that  are  individual  to  a 
subscriber's  line,  and  also  those  that  are  common  to  a  group  of  fifty 


Fig.  387.    Line-Switch  Unit 


Fig.  388.     Line- Switch  Unit 


536 


TELEPHONY 


or  one  hundred  lines.  Those  portions  of  Fig.  389  which  are  indi- 
vidual to  the  line  are  shown  below  the  dotted  line  extending  across 
the  page.  The  task  of  understanding  the  line  switch  will  be  made 
somewhat  easier  if  Figs.  385  and  389  are  considered  together.  The 
individual  parts  of  the  line  switch  are  shown  in  the  same  relation  to 
each  other  in  these  two  figures  with  the  exception  that  the  bank  of 
line  and  trunk  springs  in  the  lower  right-hand  corner  of  Fig.  389 
have  been  turned  around  edgewise  so  as  to  make  an  understanding 
of  their  circuit  connections  possible. 

The  vertical  and  rotary  sides  of  the  subscriber's  line  are  shown 
entering  at  the  lower  left-hand  corner  of  this  figure,  and  they  pass 


Fig.  389.     Circuits  of  Line-Switch  Unit 

to  the  springs  of  the  contact  bank.  Immediately  adjacent  to  these 
springs  are  the  trunk  contacts  from  which  the  vertical  and  the  ro- 
tary limbs  of  the  first  selector  trunk  proceed.  The  plunger  is  in- 
dicated at  1,  it  being  in  the  form  of  a  wheel  of  insulating  material. 
It  is  carried  on  the  rod  2  pivoted  on  a  lever  3,  which,  in  turn,  is  pivoted 
at  4  in  a  stationary  portion  of  the  framework.  A  spring  5,  secured 
to  the  underside  of  the  lever  3  and  projecting  to  the  left  beyond  the 
pivot  4  °f  this  lever,  serves  always  to  press  the  right-hand  portion  of 
the  lever  3  forward  in  such  direction  as  to  tend  to  thrust  it  into  the 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM        537 

contact  bank.  The  plunger  is  normally  held  out  of  the  contact 
bank  by  means  of  the  latch  6  carried  on  the  armature  7  of  the  trip 
magnet.  When  the  trip  magnet  is  energized  it  pulls  the  armature 
7  to  the  left  and  thus  releases  the  plunger  and  allows  it  to  enter  the 
contact  bank. 

The  master  bar  is  shown  at  8,  and  a  feather  on  this  bar  engages 
a  notch  in  the  segment  attached  to  the  rear  end  of  the  plunger  rod  2. 
This  master  bar  is  common  to  all  of  the  plunger  rods  and  by  its  os- 
cillatory movement,  under  the  influence  of  the  master  switch,  it  al- 
ways keeps  all  of  the  idle  plunger  bars  pointed  toward  the  contacts 
of  an  idle  trunk.  As  soon,  however,  as  the  trip  magnet  is  operated 
to  cause  the  insertion  of  a  plunger  into  the  contact  bank,  the  feather 
on  the  master  bar  is  disengaged  by  the  notch  in  the  segment  of  the 
plunger  rod,  and  the  plunger  rod  is,  therefore,  no  longer  subject  to 
the  oscillating  movement  of  the  master  bar. 

When  the  release  magnet  is  energized,  it  attracts  its  armature 
9  and  this  lifts  the  armature  7  of  the  trip  magnet  so  that  the  latch  6 
rides  on  top  of  the  left-hand  end  of  the  lever  3.  Then,  when  the  re- 
lease magnet  is  de-energized,  the  spring  5,  which  was  put  under 
tension  by  the  latch,  moves  the  entire  structure  of  levers  back 
to  its  normal  position,  withdrawing  the  plunger  from  the  bank  of 
contacts.  The  notch  on  the  edge  of  the  segment  of  the  plunger 
rod,  when  thus  released,  will  probably  not  strike  the  feather  on  the 
master  bar,  and  the  plunger  rod  will  thus  not  come  under  the  con- 
trol of  the  master  bar  until  the  master  bar  has  moved,  in  its  oscilla- 
tion, so  that  the  feather  registers  with  the  notch,  after  which  this  bar 
will  move  with  all  the  others. 

If,  while  the  plunger  is  waiting  to  be  picked  up  by  the  master 
bar,  the  same  subscriber  should  call  again,  his  line  will  be  connected 
with  the  same  trunk  as  before.  There  is  no  danger  in  this,  however, 
that  the  trunk  will  be  found  busy,  because  the  master  bar  will  not 
have  occupied  a  position  which  would  make  it  possible  for  any  of 
the  lines  to  appropriate  this  trunk  during  the  intervening  time. 

Master  Switch.  Associated  with  each  master  bar  there  is  a 
master  switch  which  determines  the  position  in  which  the  master 
bar  shall  stop  in  order  that  the  idle  plungers  may  be  pointed 
always  to  the  contacts  of  an  idle  trunk.  The  arm  10  of  this 
switch  is  attached  to  the  master  bar  and  oscillates  with  it  and 


538  TELEPHONY 

serves  to  connect  the  segment  11  successively  with  the  contacts  12, 
which  are  connected  respectively  to  the  third,  or  release  wire  of 
each  first  selector  trunk.  In  the  figure  the  arm  10  is  shown  resting 
on  the  sixth  contact  of  the  switch  arid  this  sixth  contact  is  connected 
to  a  spring  13  in  the  line-switch  contact  bank  that  has  not  yet  been 
referred  to.  As  soon  as  the  plunger  is  inserted  into  the  contact 
bank,  the  spring  14  will  be  pressed  into  engagement  with  the  spring 
18,  and  this  spring  14  is  connected  with  the  live  side  of  the  battery 
through  the  release  magnet  winding. 

The  contact  strip  11  on  the  master  switch  is  thus  connected 
through  the  release  magnet  to  the  battery  and  from  this  current 
flows  through  the  left-hand  winding  of  the  master-switch  relay. 
This  energizes  this  relay  and  causes  the  closure  of  the  circuit  of 
the  locking  magnet  which  magnet  unlocks  the  master  bar  to  permit 
its  further  rotation.  The  unlocking  of  the  master  bar  brings  the 
spring  15  into  engagement  with  16  and  thus  energizes  the  master 
magnet,  the  armature  of  which  vibrates  back  and  forth  after  the 
manner  of  an  electric-bell  armature,  and  steps  the  wheel  17  around. 
The  wheel  17  is  mechanically  connected  to  the  master  bar  so  that 
each  complete  revolution  of  the  wheel  will  cause  one  complete  oscilla- 
tion of  the  master  bar.  The  master  bar  will  thus  be  moved  so  as  to 
cause  all  the  idle  plungers  to  sweep  through  an  arc  and  this  move- 
ment will  stop  as  soon  as  the  master-switch  arm  10  connects  the 
arc  11  with  one  of  the  contacts  12  that  is  not  connected  to  the  live 
side  of  the  battery  through  the  springs  13  and  14  of  some  other  line 
switch.  It  is  by  this  means  that  the  plungers  of  the  line  switches 
are  always  kept  pointing  at  the  contacts  of  an  idle  trunk.  The  way 
in  which  this  feature  has  been  worked  out  must  demand  admiration 
and  accounts  for  the  marvelous  quickness  of  this  line  switch.  The 
fact  that  the  plungers  are  pointed  in  the  right  direction  before  the 
time  comes  for  their  use,  leaves  only  the  simple  thrusting  motion  of 
the  plunger  to  accomplish  the  desired  connection  immediately  upon 
the  initiation  of  a  call  by  the  subscriber. 

Locking  Segment.  It  will  be  understood  that  the  locking  seg- 
ment 18  and  the  master-switch  contact  finger  10  are  both  rigidly  con- 
nected with  the  master  bar  8  and  move  with  it,  the  locking  segment 
18  serving  always  to  determine  accurately  the  angular  position  at 
which  the  master  bar  and  the  master-switch  arm  are  brought  to  rest. 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       539. 

Bridge  Cut-Off.  One  important  feature  of  automatic  switch- 
ing, particularly  as  exemplified  in  the  system  of  the  Automatic  Elec- 
tric Company,  is  the  disconnection,  after  its  use,  of  each  operating 
magnet  of  each  piece  of  apparatus  involved  in  making  a  connection. 
Since  these  operating  magnets  are  always  bridged  across  the  line  at 
the  time  of  their  operation  and  then  cut  off  after  they  have  performed 
their  function,  this  feature  may  be  referred  to  as  the  bridge  cut-off 

Guarding  Functions.  Still  another  feature  of  importance  is 
the  means  for  guarding  a  line  or  a  piece  of  apparatus  that  has  already 
been  appropriated  or  made  busy,  so  that  it  will  not  be  appropriated 
or  connected  with  for  use  in  some  other  connection.  For  this  latter 
purpose  contacts  and  wires  are  associated  with  each  piece  of  appara- 
tus, which  are  multipled  to  similar  contacts  on  other  pieces  of  ap- 
paratus in  much  the  same  way  and  for  a  similar  purpose  that  the 
test  thimbles  in  a  multiple  switchboard  are  multipled  together. 
Such  wires  and  contacts  in  the  Automatic  Electric  Company's  ap- 
paratus are  called  private  wires  and  contacts. 

The  bridge  cut-off  and  guarding  functions  are  provided  for  in 
the  line  switch  by  a  bridge  cut-off  relay  shown  in  Fig  389  and  also 
in  Fig.  385,  it  being  the  upper  one  of  the  individual  line  relays  in 
each  of  those  figures.  This  bridge  cut-off  relay  is  operated  as  soon 
as  the  plunger  of  the  line  is  thrust  into  the  bank;  the  contacts  19  and 
20,  closed  by  the  plunger,  serving  to  complete  the  circuit  of  this  relay. 
To  make  clear  the  bridge  cut-off  feature  it  will  be  noted  that  the  trip 
magnet  of  a  line  switch  is  connected  in  a  circuit  traced  from  the 
rotary  side  of  the  line  through  the  contacts  21  and  22  of  the  bridge 
cut-off  relay,  thence  through  the  coil  of  the  trip  magnet  to  the  com- 
mon wire  leading  to  the  spring  23  of  the  master-bar  locking  device 
and  thence  to  the  live  side  of  the  battery.  Obviously,  therefore,  as 
soon  as  the  bridge  cut-off  relay  operates,  the  trip  magnet  becomes 
inoperative  and  can  cause  no  further  action  of  the  line  switch  be- 
cause its  circuit  is  broken  between  the  springs  21  and  22. 

The  private  or  guarding  feature  is  taken  care  of  by  the  action 
of  the  plunger  in  closing  contacts  19  and  20,  since  the  private  wire 
leading  to  the  bridge  cut-off  relay  is,  as  has  already  been  stated,  con- 
nected to  ground  when  these  contacts  are  closed.  This  private  wire 
leads  off  and  is  multipled  to  the  private  contacts  on  all  the  connectors 
that  have  the  ability  to  reach  this  line,  and  the  fact  that  this  wire 


540  TELEPHONY 

is  grounded  by  the  line  switch  as  soon  as  it  becomes  busy,  estab- 
lishes such  conditions  at  all  of  the  connectors  that  they  will  refuse 
to  connect  with  this  line  as  long  as  it  is  busy,  in  a  way  that  will  be 
pointed  out  later  on. 

Relation  of  Line  Switch  and  Connectors.  The  vertical  and 
rotary  wires  of  the  subscriber's  line  are  shown  leading  off  to  the 
connector  banks  at  the  left-hand  side  of  Fig.  389,  and  one  side  of  this 
connection  passes  through  the  contacts  24  and  25  of  the  bridge  cut- 
off relay  on  the  line  switch.  It  is  through  this  path  that  a  connection 
from  some  other  line  through  a  connector  to  this  line  is  established 
and  it  is  seen  that  this  path  is  held  open  until  the  bridge  cut-off  relay 
of  the  line  switch  is  operated.  For  such  a  connection  to  this  line  the 
bridge  cut-off  relay  of  the  line  switch  is  operated  over  the  private 
wire  leading  from  the  connector,  and  the  operation  of  the  bridge  cut- 
off relay  at  this  time  serves  to  render  inoperative  the  line  switch,  so 
that  it  will  not  perform  its  usual  functions  should  the  called  subscriber 
start  to  make  a  call  after  his  line  had  been  seized. 

Summary  of  Line-Switch  Operation.  To  summarize  the  oper- 
ation of  a  line  switch  when  a  call  is  originated  on  its  line,  the 
first  movement  of  the  calling  subscriber's  dial  will  ground  the  ro- 
tary side  of  the  line  and  operate  the  trip  magnet.  This  will  cause 
the  plunger  to  be  inserted  into  the  bank,  and  thus  extend  the  line  to 
the  first  selector  trunk  through  the  closing  of  the  right-hand  set  of 
springs  shown  in  the  lower  right-hand  corner  of  Fig.  389.  The 
insertion  of  the  plunger  will  also  connect  the  battery  through  the 
left-hand  winding  of  the  master-switch  relay  and,  by  the  sequence  of 
operations  which  follows,  cause  the  master  bar  to  move  all  of  the  idle 
plungers  so  as  to  again  point  them  to  an  idle  trunk.  The  closure  of 
contacts  19  and  20  by  the  plunger  causes  the  operation  of  the  bridge 
cut-off  relay  which  opens  the  circuit  of  the  trip  magnet,  rendering  it 
inoperative;  and  also  establishes  ground  potential  on  all  the  private 
wire  contacts  of  that  line  in  the  banks  of  the  connectors,  so  as 
to  guard  the  line  and  its  associated  apparatus  against  intrusion  by 
others.  The  line  is  cut  through,  therefore,  to  a  first  selector  and  all 
of  the  line-switch  apparatus  is  completely  cut  off  from  the  talking 
circuit. 

It  must  be  remembered  that  all  of  the  actions  of  the  line  switch, 
which  it  has  taken  so  long  to  describe,  occur  practically  instantane- 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       541 

ously  and  as  a  result  of  the  first  part  of  the  first  movement  of  the  sub- 
scriber's dial.  The  line  switch  has  done  its  work  and  "gone  out  of 
business"  before  the  selective  impulses  of  the  first  digit  begin  to 
take  place. 

Selecting  Switches.  The  first  selector  is  now  in  control  of  the 
calling  subscriber.  The  circuits  and  elements  of  the  first  selector 
switch  are  shown  in  Fig.  390.  The  general  mechanical  structure 
of  the  first  selectors,  second  selectors,  and  connectors,  is  the  same 
and  may  be  referred  to  briefly  here.  Fig.  391  shows  a  rear  view  of  a 
first  selector;  Fig.  392,  a  side  view  of  a  second  selector;  and  Fig. 
393,  a  front  view  of  a  connector.  The  arrangement  of  the  vertical 
and  rotary  magnets,  of  the  selector  shafts,  and  of  the  contact  banks 
are  identical  in  all  three  of  these  pieces  of  apparatus  and  all  these 
switches  work  on  the  "up-and-around  principle"  referred  to  in  con- 
nection with  Fig.  380.  It  is  thought  that  with  the  general  structure 
shown  in  Figs.  391,  392,  and  393  in  mind,  the  actual  operation  may 
be  understood  much  more  readily  from  Fig.  390. 

Four  magnets — the  vertical,  the  rotary,  the  private,  and  the  re- 
lease— produce  the  switching  movements  of  the  machine.  These 
magnets  are  controlled  by  various  combinations  brought  upon  the 
circuits  by  three  relays — the  vertical,  the  rotary,  and  the  back  re- 
lease. The  fourth  relay  shown,  called  the  off-normal,  is  purely  for 
signaling  purposes,  as  will  be  described. 

Side  Switch.  Another  important  element  of  the  selecting 
switches  is  the  so-called  side  switch  which  might  better  be  called  a 
pilot  switch — but  we  are  not  responsible  for  its  name.  This  side 
switch  has  for  its  function  the  changing  of  the  control  of  the  sub- 
scriber's line  to  successive  portions  of  the  selector  mechanism,  ren- 
dering inoperative  those  portions  that  have  already  performed  their 
functions  and  that,  therefore,  are  no  longer  needed.  This  switch 
may  be  seen  best  in  Fig.  392  just  above  the  upper  bank  of  contacts. 
It  is  shown  in  Fig.  390  greatly  distorted  mechanically  so  as  to  better 
illustrate  its  electrical  functions. 

The  contact  levers  1,  2,  3,  and  4  of  the  side  switch  are  carried 
upon  the  arm  5  which  is  pivoted  at  6.  All  of  these  contact  levers, 
therefore,  move  about  6  as  an  axis.  The  side  switch  has  three  po- 
sitions and  it  is  shown,  in  Fig.  390,  in  the  first  one  of  these.  When 
the  private  magnet  armature  is  attracted  and  released  once,  the  es- 


Fig.   390.     Circuits  of  First  Selector 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       543 


capement  carried  by  it  permits  the  spring  7  to  move  the  arm  5  so  as 
to  bring  the  wipers  of  the  side  switch  into  its  second  position;  the 
second  pulling  up  and  release  of  the  private  magnet  armature  will 
cause  the  movement  of  the  side  switch  wipers  into  the  thira  position. 
It  is  to  be  noted  that  the  escape- 
ment which  releases  the  side 
switch  arm  may  be  moved  either 
by  the  private  or  by  the  rotary 
magnet,  since  the  armature  of  the 
latter  has  a  finger  which  engages 
the  private  magnet  armature. 

Functions  of  Side  Switch. 
The  functions  of  the  side  switch 
may  be  briefly  outlined  in  con- 
nection with  the  first  selector,  as 
an  example.  In  the  first  position 
it  extends  the  control  of  the  sub- 
scriber's signal  transmitter  through 
the  first  selector  trunk  and  line 
relays  to  the  vertical  and  private 
magnets  so  that  these  magnets 
will  be  responsive  to  the  selecting 
impulses  corresponding  to  the  first 
digit.  In  its  second  position  it 
brings  about  such  a  condition  of 
aft'airs  that  the  rotary  magnet  will 
be  brought  into  play  arid  auto- 
matically move  the  wipers  over 
the  bank  contacts  in  search  of  an 
idle  trunk.  In  its  third  position, 
both  the  vertical  arid  rotary  re- 
lays are  cut  off  and  the  line  is  cut 
straight  through  to  the  second  se- 
lector trunk,  and  only  those  parts 
of  the  first  selector  apparatus  are  left  in  an  operative  state  which 
have  to  do  with  the  private  or  guarding  circuits  and  with  the  release. 
Similar  functions  are  performed  by  the  side  switch  in  connection 
with  the  other  selecting  switches. 


Fig.  391.     Rear  View  of  First  Selector 


544 


TELEPHONY 


Release  Mechanism.  Another  one  of  the  features  of  the  switch 
that  needs  to  be  considered  before  a  detailed  understanding  of  its 
operation  may  be  had,  is  the  mechanical  relation  of  the  holding  and 
the  release  dog.  This  dog  is  shown  at  8  and,  in  the  language  of 

the  art,  is  called  the  double  dog. 
As  will  be  seen,  it  has  two  retain- 
ing fingers,  one  adapted  to  en- 
gage the  vertical  ratchet  and  the 
other,  the  rotary  ratchet  on  the 
selector  shaft.  This  double  dog 
is  pivoted  at  9  and  is  interlinked 
in  a  peculiar  way  with  the  arma- 
ture of  the  vertical  magnet,  the 
armature  of  the  release  magnet, 
and  the  arm  of  the  side  switch. 
The  function  of  this  double  dog 
is  to  hold  the  shaft  in  whatever 
vertical  position  it  is  moved  by 
the  vertical  magnet  and  then, 
when  the  rotary  magnet  begins 
to  operate,  to  hold  the  shaft  in 
its  proper  angular  position.  It 
will  be  noted  that  the  fixed  dog 
10  is  ineffective  when  the  shaft 
is  in  its  normal  angular  position. 
But  as  soon  as  the  shaft  is  rotat- 
ed, this  fixed  dog  10  becomes  the 
real  holding  pawl  so  far  as  the 
vertical  movement  is  concerned. 
The  double  dog  8  is  normally 
held  out  of  engagement  with  the 
vertical  and  the  rotary  ratchets 
by  virtue  of  the  link  connection, 
shown  Sit  11,  between  the  release 
magnet  armature  and  the  rear 

end  of-  the  double  dog.  On  the  previous  release  of  the  switch  the 
attraction  of  the  release  magnet  armature  permitted  the  link  11  to 
hook  over  the  end  of  the  dog  8  and  thus,  on  its  return  movement, 


Pig.  392.     Side  View,  of  Second  Selector 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM        545 


to  pull  this  dog  out  of  engagement  with  its  ratchets.  This  move- 
ment also  resulted  in  pushing  on  the  link  12  which  is  pivoted  to  the 
side  switch  arm  5,  and  thus  the  return  movement  of  the  release 
magnet  is  made  to  restore  the  side  switch  to  its  normal  position. 
In  order  that  the  double  dog  may 
be  made  effective  when  it  is  re- 
quired, and  in  order  that  the  side 
switch  may  be  free  to  move  under 
the  influence  of  the  private  mag- 
net, the  double  dog  is  released 
from  its  connection  with  the  re- 
lease magnet  armature  by  the  first 
movement  of  the  vertical  magnet 
in  a  manner  which  is  clear  from 
the  drawing. 

First  Selector  Operation.  In 
discussing  the  details  of  operation 
of  the  various  selectors  it  will  be 
found  convenient  to  divide  the 
discussion  according  to  the  posi- 
tion of  the  side  switch.  This  will 
bring  about  a  logical  arrangement 
because  it  is  really  the  side  switch 
which  determines  by  its  position 
the  sequence  of  operation. 

First  Position  of  Side  Switch. 
This  is  the  position  shown  in  Fig. 
390,  and  is   the   normal   position. 
The  vertical  and  the  rotary  lines 
extending    from   the   calling   sub- 
scriber are  continued  by  the  levers 
1  and  2  of  the  side  switch  through 
the  vertical   and   the  rotary  relay 
coils,  respectively,  to  the  live  side 
of   battery.      The   lever  4  of   the        Fi«-  393-    Front  View  of  connector 
side   switch   in   this   position   connects  to  ground  the  circuit  lead- 
ing from  the  line  switch  through  the  release  trunk,  and  the  wind- 
ing of    the  off-normal  relay.     This  winding  is  thus  put  in  series 


546  TELEPHONY 

with  the  release  magnet  of  the  line  switch,  but  on  account  of  high 
resistance  of  the  off-normal  relay  no  operation  of  the  release  magnet 
is  caused.  This  will,  however,  permit  such  current  to  flow  through 
the  release  circuit  as  will  energize  the  sensitive  off-normal  relay 
and  cause  it  to  attract  its  armature  and  light  the  off-normal  lamp. 
If  this  lamp  remains  lighted  more  than  a  brief  period  of  time,  it 
will  attract  notice  and  will  indicate  that  the  corresponding  selector 
has  been  appropriated  by  a  line  switch  and  that  for  some  reason  the 
selector  has  gone  no  further.  This  lamp,  therefore,  is  an  aid  in  pre- 
venting the  continuance  of  this  abnormal  condition. 

The  first  thing  that  happens  after  the  line  switch  has  connected 
the  calling  subscriber  with  the  first  selector  is  a  succession  of  im- 
pulses over  the  vertical  side  of  the  line,  this  being  the  set  of  impulses 
corresponding  in  number  to  the  thousands  digit  or  to  the  office,  if 
there  is  more  than  one.  It  will  be  understood  that  here  we  are  con- 
sidering a  single  office  of  ten-thousand-line  capacity  or  thereabouts, 
and  that,  therefore,  this  first  set  of  impulses  corresponds  to  the 
thousands  digit  in  the  called  subscriber's  line.  Each  one  of  these  im- 
pulses will  flow  from  the  battery  through  the  vertical  relay  and  each 
movement  of  this  relay  armature  will  close  the  circuit  of  the  vertical 
magnet  and  cause  the  shaft  of  the  selector  to  be  stepped  up  to  the 
proper  level.  Immediately  following  the  first  series  of  selecting 
impulses  from  the  subscriber's  station,  a  single  impulse  follows 
over  the  rotary  side  of  the  line.  This  gives  the  rotary  relay  arma- 
ture one  impulse  and  this  in  turn  closes  the  circuit  of  the  private 
magnet  once.  The  single  movement  of  the  private  magnet  arma- 
ture allows  the  escapement  finger  on  the  arm  5  to  move  one  step  and 
this  brings  the  side  switch  contacts  into  the  second  position. 

Second  Position  of  Side  Switch.  In  this  position  lever  4  of  the 
side  switch  places  a  ground  on  the  wire  leading  through  the  rotary 
magnet  to  a  source  of  interrupted  battery  current.  The  impulses 
which  thus  flow  through  the  rotary  magnet  occur  at  a  frequency 
dependent  upon  the  battery  interrupter  and  this  is  at  a  rate  of  ap- 
proximately fifteen  impulses  per  second.  The  rotary  magnet  will 
step  the  selector  shaft  rapidly  around  until  something  occurs  to  stop 
these  impulses.  This  something  is  the  finding  by  the  private  wiper 
of  an  ungrounded  private  contact  in  the  bank,  since  all  of  £he  con- 
tacts corresponding  to  busy  trunks  are  grounded,  as  will  be  explained. 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       547 

The  action  of  the  private  magnet  enters  into  this  operation  in 
the  following  way :  A  circuit  may  be  traced  from  the  battery  through 
the  private  magnet  to  the  third  side  switch  wiper  when  in  its  second 
position,  thence  through  the  back  release  relay  to  the  private 
wiper.  If  the  wiper  is  at  the  time  on  the  private  bank  contact  of  a 
busy  trunk,  it  will  find  that  contact  grounded  and  the  private  mag- 
net will  be  energized.  The  energizing  of  this  magnet  will  not,  how- 
ever, cause  the  release  of  the  side  switch.  It  must  be  energized  and 
de-energized.  The  private  magnet  armature  will,  therefore,  be  oper- 
ated by  the  finger  of  the  rotary  magnet  armature  on  the  first  rotary 
step.  The  private  magnet  will  be  energized  and  hold  its  armature 
operated  if  the  private  wiper  finds  a  ground  on  the  first  bank  con- 
tact and  will  stay  energized  as  long  as  the  private  wiper  is  passing 
over  private  contacts  of  busy  trunks.  Its  armature  will  not  be 
allowed  to  fall  back  during  the  passage  of  the  wiper  from  one  trunk 
to  another,  because  during  that  interval  the  finger  of  the  rotary 
magnet  will  hold  it  operated/  As  soon,  however,  as  the  private 
wiper  reaches  the  private  bank  contact  of  an  idle  trunk,  no  ground 
will  be  found  and  the  circuit  of  the  private  magnet  will  be  left  open. 
When  the  impulse  through  the  rotary  magnet  ceases,  the  private 
magnet  armature  will  fall  back  and  the  side  switch  will  be  released  to 
its  third  position. 

Third  Position  of  Side  Switch.  The  first  thing  to  be  noted  in  this 
position  is  that  the  calling  line  is  cut  straight  through  to  the  second 
selector  trunk,  the  connection  being  clean  with  no  magnets  bridged 
across  or  tapped  off.  The  third  wiper  of  the  side  switch,  when  in  its 
third  position,  is  grounded  and  this  connects  the  release  wire  of  the 
second  selector  trunk,  on  which  the  switch  wipers  rest,  through  the 
private  wiper,  the  winding  of  the  back  release  magnet,  and  the  third 
wiper  of  the  side  switch  to  ground.  This  establishes  a  path  for  the 
subsequent  release  current  through  the  back  release  magnet;  and,  of 
equal  importance,  it  places  a  ground  on  the  private  bank  contact  of 
that  trunk  so  that  the  private  wiper  of  any  other  switch  will  be  pre- 
vented from  stopping  on  the  contacts  of  this  trunk  in  the  same  man- 
ner that  the  wiper  of  this  switch  was  prevented  from  stopping  on  other 
trunks  that  were  already  in  use. 

The  fourth  lever  on  the  side  switch,  when  in  its  third  position, 
serves  merely  to  close  the  circuit  of  the  rotary  off-normal  lamp.  This 


548  TELEPHONY 

lamp  is  for  the  purpose  of  calling  attention  to  any  first  selector  switch 
that  has  been  brought  into  connection  with  some  second  selector 
trunk  and  which,  for  some  reason,  has  failed  in  its  release.  These 
off-normal  lamps  are  so  arranged  that  they  may  be  switched  off 
manually  to  avoid  burning  them  during  the  hours  of  heaviest 
traffic.  At  night  they  afford  a  ready  means  of  testing  for  switches 
that  have  been  left  off-normal,  since  the  manual  switches  controlling 
these  lamps  may  then  be  closed,  and  any  lamps  which  burn  will  show 
that  the  switches  corresponding  to  them  are  off-normal.  Simple 
tests  then  suffice  to  show  whether  they  are  properly  or  improperly 
in  their  off-normal  position. 

Release  of  the  First  Selector.  As  will  be  shown  later,  the  normal 
way  of  releasing  the  switches  is  from  the  connector  back  over  the  re- 
lease wire.  It  is  sufficient  to  say  at  this  point  that  when  the  proper 
time  for  release  comes,  an  impulse  of  current  will  come  back  over 
the  second  selector  trunk  release  wire  through  the  private  wiper, 
to  the  back  release  relay  magnet,  and  thence  to  ground  through  the 
third  wiper  of  the  side  switch  which  is  in  its  third  position.  It  may 
be  asked  why  the  back  release  magnet  was  not  energized  during  the 
previous  operations  described,  when  current  passed  through  it. 
The  reason  for  this  is  that  in  those  previous  operations  the  private 
magnet  was  always  included  in  series  in  the  circuit  and  on  account  of 
the  high  resistance  of  the  private  magnet,  sufficient  current  did  not 
pass  through  the  back  release  magnet  to  energize  it. 

When  the  back  release  relay  is  energized,  it  closes  the  circuit 
of  the  release  magnet  and  thus,  through  the  link  11,  draws  the  double 
dog  away  from  its  engagement  with  the  shaft  ratchets  and  at  the  same 
time,  through  the  link  12,  restores  the  side  switch  to  its  normal  position. 
Whenever  the  release  magnet  is  operated  it  acts  as  a  relay  to  close  a 
pair  of  contacts  associated  with  it  and  thus  to  momentarily  ground 
the  release  wire  of  the  first  selector  trunk  extending  back  to  the  line 
switch.  Referring  to  Fig.  389,  it  will  be  seen  that  this  path  leads 
through  the  contacts  13  and  14  and  the  release  magnet  to  the  bat- 
tery. It  is  by  this  means  that  the  line  switch  is  released,  the  re- 
lease impulse  being  relayed  back  from  the  first  selector. 

Second  Selector  Operation.  For  the  purpose  of  considering  the 
action  of  the  second  selector,  we  will  go  back  to  the  point  where  the 
first  selector  had  connected  with  a  second  selector  trunk  and  where 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       549 

its  side  switch  had  moved  into  its  third  position.  In  this  condi- 
tion, it  will  be  remembered,  the  trunk  line  was  cut  through  to  a  sec- 
ond selector  trunk  and  all  first  selector  apparatus  cleared  from  the 
talking  circuit. 

The  second  selector  chosen  is  one  corresponding  to  the  thou- 
sands group  as  determined  by  the  first  digit  of  the  called  subscriber's 
number.  The  circuits  of  a  second  selector  are  shown  in  Fig.  394 
and  it  must  be  borne  in  mind  that  the  mechanical  arrangements  for 
producing  the  vertical  and  the  rotary  movement  of  the  shaft  and 
for  operating  the  side  switch  are  practically  the  same  as  those  of  the 
first  selector.  As  in  the  first  selector,  the  sequence  of  operation 
is  controlled  by  the  successive  positions  of  the  side  switch,  the 
first  position  permitting  the  selection  of  the  hundreds  corresponding 
to  the  vertical  impulses,  the  second  position  allowing  the  selector 
to  search  for  an  idle  trunk  in  that  hundred,  and  the  third  position 
cutting  the  trunk  through  and  clearing  the  circuit  of  obstructing 
apparatus. 

First  Position  of  Side  Switch.  The  first  thing  that  happens 
when  the  subscriber  begins  to  move  his  dial  in  the  transmission  of 
the  second  series  of  selecting  impulses  is  the  sending  of  a  preliminary 
impulse  over  the  rotary  side  of  the  line.  This,  in  the  case  of  the 
second  selector,  energizes  the  rotary  relay  which,  in  turn,  energizes 
the  private  magnet;  but  the  private  magnet  in  the  case  of  the  second 
selector  can  do  nothing  toward  the  release  of  the  side  switch  because 
the  projection  5',  on  the  side  switch  arm  5,  meets  a  projection  on  the 
rear  of  the  selector  shaft  which  thus  prevents  the  movement  of  the 
side  switch  arm  5  until  the  selector  shaft  has  been  moved  out  of 
its  normal  position. 

Immediately  after  the  establishment  of  the  connection  to  the 
selector,  the  second  set  of  selecting  impulses  comes  in  over  the  ver- 
tical wire  from  the  subscriber's  station.  These  impulses,  corre- 
sponding in  number  to  the  hundreds  digit,  will  energize  the  vertical 
relay  and  cause  it,  in  turn,  to  energize  the  vertical  magnet,  stepping 
up  the  selector  shaft  to  the  level  corresponding  to  the  hundred  sought. 
The  single  rotary  impulse,  which  follows  just  before  the  subscriber's 
dial  reaches  its  normal  position,  will  energize  the  rotary  relay  of  the 
second  selector.  This,  in  turn,  energizes  the  private  magnet  which 
makes  a  single  movement  of  its  armature  and  allows  the  escapement 


COMMECTOR 
TftUMK v 

.    -J 


Fig.   394.     Circuits  of  Second  Selector 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       551 

finger  on  the  side  switch  arm  to  move  one  step  and  bring  the  side 
switch  contact?  into  the  second  position. 

Second  Position  of  Side  Switch.  No  detailed  discussion  of  this  is 
.necessary,  since,  with  the  side  switch  in  its  second  position,  the  actions 
which  occur  in  causing  the  wipers  of  the  second  selector  to  seek  and  con- 
nect with  an  idle  trunk  line,  are  exactly  the  same  as  in  the  case  of  the 
first  selector.  When  the  second  selector  wipers  finally  reach  a  rest- 
ing place  on  the  bank  contacts,  the  private  magnet  armature,  operated 
during  the  hunting  process,  is  released  and  the  side  switch  is  thus 
shifted  into  the  third  position. 

Third  Position  of  Side  Switch.  The  moving  of  the  side  switch 
into  its  final  position  brings  about  the  same  state  of  affairs  with  re- 
spect to  the  second  selector  that  already  exists  with  respect  to  the 
first  selector.  The  trunk  line  is  cut  straight  through  and  all  bridge 
circuits  or  bi-paths  from  it  are  cut  off.  The  same  guarding  condi- 
tions are  established  to  prevent  other  lines  or  other  pieces  of  apparatus 
from  making  connections  that  will  interfere  with  the  one  being  es- 
tablished, and  the  same  provisions  are  made  for  working  the  back 
release  when  the  proper  impulse  comes  from  the  connector,  and  for 
passing  this  back  release  impulse  on  to  the  first  selector  in  the  same 
way  that  the  first  selector  passes  it  on  to  the  line  switch.  The  line 
of  the  calling  subscriber  has  now  been  extended  to  a  connector,  and 
that  connector  is  one  of  a  group — usually  ten — which  alone  has  the 
ability  to  reach  the  particular  hundred  lines  containing  the  line  of 
the  desired  subscriber.  The  selection  has,  therefore,  been  narrowed 
down  from  one  in  ten  thousand  to  one  in  one  hundred. 

The  Connector — Its  Functions.  It  has  already  been  stated 
that  the  connector  is  of  the  same  general  type  of  apparatus  as  the 
first  and  the  second  selectors.  Unlike  the  first  and  the  second 
selectors,  however,  the  connector  is  required  to  make  a  double  selec- 
tion under  the  guidance  of  the  subscriber.  The  first  selector  makes 
a  single  selection  of  a  group  under  the  guidance  of  the  subscriber 
and  then  an  automatic  selection  in  that  group  not  controlled  by  the 
subscriber.  So  it  is  with  the  second  selector.  The  connector,  how- 
ever, makes  a  selection  of  a  group  of  ten  under  the  guidance  of  the 
subscriber  and  then,  again  under  the  guidance  of  the  subscriber, 
it  picks  out  a  particular  one  of  that  group. 

The  connector  also  has  other  functions  in  relation  to  the  ringing 


552  TELEPHONY 

of  the  called  subscriber  and  the  giving  of  a  busy  signal  to  the  calling 
subscriber  in  case  the  line  wanted  is  found  busy.  It  has  still  other 
functions  in  that  the  talking  current,  which  is  finally  supplied  to 
connected  subscribers,  is  supplied  through  paths  furnished  by  it. 

Location  of  the  Connectors.  Connectors  are  the  only  ones  of 
the  selecting  switches  that  are  in  any  sense  individual  to  the  subscrib- 
ers' lines.  None  of  them  is  individual  to  a  subscriber's  line,  but  it 
may  be  said  that  a  group  of  ten  connectors  is  individual  to  a  group  of 
one  hundred  subscribers'  lines.  Since  each  group  of  one  hundred 
lines  has  a  group  of  connectors  of  its  own  and  since  each  one  hundred 
lines  also  has  a  line-switch  unit  of  its  own,  and  since  the  lines  of 
this  group  must  be  multipled  through  the  bank  contacts  of  the 
connectors  of  this  individual  group  and  through  the  bank  contacts 
of  the  line  switches  of  this  particular  unit,  it  follows  that  on  ac- 
count of  the  wiring  problems  involved  there  is  good  reason  for  mount- 
ing the  connectors  in  close  proximity  to  the  line  switches  representing 
the  same  group  of  lines.  Some  help  in  the  grasping  of  this  thought 
may  result  if  it  be  remembered  that  the  line  switch  is,  so  to  speak, 
the  point  of  entry  of  a  call  and  that  the  connector  is  the  point  of  exit, 
and,  in  order  to  reduce  the  amount  of  wiring  and  to  economize  space, 
the  point  of  exit  and  the  point  of  entry  are  made  as  close  together 
as  possible. 

The  relative  locations  and  grouping  of  the  line  switches  and 
connectors  are  clearly  shown  in  Fig.  395,  which  is  a  rear  view  of  the 
same  line-switch  unit  that  was  illustrated  in  Figs.  387  and  388. 

Operation  of  the  Connector.  The  circuits  of  the  connector 
are  shown  in  Fig.  396.  In  addition  to  the  features  that  have  been 
pointed  out  in  the  first  and  the  second  selectors,  all  of  which  are 
to  be  found,  with  some  modifications,  perhaps,  in  the  connector, 
there  must  be  considered  the  features  in  the  connector  of  busy-signal 
operation,  of  ringing  the  called  subscriber,  of  battery  supply  to 
both  subscribers,  and  of  the  trunk  release  operation.  These  may 
be  best  understood  by  tracing  through  the  operations  of  the  connector 
from  the  time  it  is  picked  up  by  a  second  selector  until  the  connection 
is  finally  completed,  or  until  the  busy  signal  has  been  given  in  case 
completion  was  found  impossible.  As  in  the  first  and  the  second 
selectors,  the  sequence  of  operations  is  determined  by  the  position 
of  the  side  switch. 


Fig.  395.    Connector  Side  of  Line-Switch  Unit 


Tig.  396.     Circuits  of  Connector 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       555 

First  Position  of  Side  Switch.  The  connector  in  a  ten-thousand- 
line  system  is  the  recipient  of  the  impulses  resulting  from  the  third 
and  fourth  movements  of  the  subscriber's  dial.  Considering  the 
third  movement  of  the  subscriber's  dial,  the  first  impulse  resulting 
from  it  comes  over  the  rotary  side  of  the  line  and  results  in  the  rotary 
relay  attracting  its  armature  once.  This  results  in  a  single  impulse 
through  the  private  magnet  which,  however,  does  nothing  because  the 
projection  5'  strikes  against  a  projection  on  the  selector  shaft.  These 
two  projections  interfere  only  when  the  selector  shaft  is  in  its  normal 
position.  Then  follows  the  series  of  impulses  from  the  subscriber's 
station  corresponding  to  the  tens  digit  in  the  called  subscriber's 
number.  These  pass  over  the  vertical  side  of  the  line  and  through 
the  vertical  relay,  energizing  that  relay  a  corresponding  number 
of  times. 

The  vertical  magnet,  as  in  the  case  of  the  first  and  the  second 
selectors,  is  included  in  the  circuit  controlled  by  the  vertical  relay 
andthis  results  in  the  connector  shaft  being  stepped  up  to  the  level 
corresponding  to  the  particular  tens  group  containing  the  called  sub- 
scriber's number.  It  will  be  noted  that  the  impulses  from  the  verti- 
cal side  of  the  line,  which  cause  this  selection,  pass  through  one  wind- 
ing 13  of  the  calling  battery  supply  relay.  This  relay  is  operated  by 
these  vertical  selecting  impulses,  but  in  this  position  of  the  side  switch 
the  closure  of  its  local  circuits  accomplishes  nothing. 

Immediately  after  the  tens  group  of  selecting  impulses  over  the 
vertical  side  of  the  line,  there  follows  a  single  rotary  impulse  from  the 
subscriber's  station  which,  as  in  the  case  of  the  first  and  the  second 
selectors,  energizes  the  rotary  relay  and  causes  it  to  give  one  impulse 
to  the  private  magnet.  This  impulse  is  now  able,  since  the  shaft 
has  moved  from  its  normal  position,  to  release  the  side  switch  arm 
one  notch,  and  the  side  switch,  therefore,  moves  into  its  second  po- 
sition. 

Second  Position  of  Side  Switch.  It  is  principally  in  this  second 
position  of  the  side  switch  that  the  connector  selecting  function  differs 
from  that  of  the  first  and  the  second  selector.  There  is  no  trunk 
to  be  hunted,  but  rather  the  rotary  movement  of  the  connector  wip- 
ers must  be  made  in  response  to  the  impulses,  from  the  subscriber's 
station,  which  correspond  to  the  units  digit  in  the  selected  number. 
The  first  impulse  corresponding  to  the  fourth  movement  of  the  sub- 


556  TELEPHONY 

scriber's  dial  is  a  rotary  one,  and,  as  usual,  it  passes  through  the  ro- 
tary relay  winding  and  this,  in  turn,  gives  an  impulse  to  the  private 
magnet.  The  private  magnet  at  this  time  has  already  released  the 
side  switch  arm  to  its  second  position,  but  it  is  unable  to  release  it 
further  because  of  a  feather  on  the  wiper  shaft — which  projects  just 
far  enough  to  engage  the  lug  5',  when  the  shaft  is  in  its  normal  angu- 
lar position — thus  preventing  the  side  switch  arm  from  moving  far- 
ther than  its  second  position. 

Then  follows  over  the  vertical  side  of  the  line  the  last  set  of 
selecting  impulses  corresponding  to  the  units  digit.  This,  as  before, 
energizes  the  vertical  relay,-  but  in  the  second  position  of  the  side 
switch,  it  is  to  be  noted,  that  the  vertical  relay  no  longer  controls  the 
vertical  magnet;  the  side  switch  has  shifted  the  control  of  the  vertical 
relay  to  the  rotary  magnet.  The  rotary  magnet  is,  therefore,  ener- 
gized a  number  of  times  corresponding  to  the  last  digit  in  the  called 
number  and  the  wipers  of  the  connectors  are  thus  brought  to  the  con- 
tacts of  the  line  sought — their  final  goal.  At  this  point  many  things 
may  happen,  and  the  things  that  do  happen  depend  on  whether 
the  called  subscriber's  line  is  idle  or  busy. 

Called-Line  Busy: — It  will  first  be  assumed  that  the  called 
line  is  busy.  The  testing  operation  at  the  connectors  occurs  in  the 
second  position  of  the  side  switch.  If  the  called  line  is  busy,  it  will 
be  either  because  it  is  connected  to  by  some  other  connector  or  be- 
cause it  has  itself  made  a  call.  In  the  former  case  the  private  con- 
tacts of  that  line  in  the  banks  of  all  the  connectors  serving  that 
hundreds  group  of  lines  will  be  grounded  through  the  private  wiper 
of  some  other  connector.  That  this  is  so,  may  be  seen  by  tracing 
the  circuit  from  the  private  wiper  on  the  shaft  to  the  third  side  switch 
wiper  which  is  grounded  in  the  third  position;  the  other  connector 
that  has  already  engaged  the  line  will,  of  course,  have  its  side  switch 
in  its  final,  or  third  position.  Again,  if  the  line  called  is  busy,  be- 
cause a  call  has  already  been  made  from  this  line  to  some  other 
line,  the  private  contacts  on  the  connectors  corresponding  to  the  line 
will  be  grounded,  as  will  be  seen  by  tracing  from  the  private  bank 
contacts,  which  are  shown  in  Fig.  396,  through  the  private  wire  to  the 
line  switch,  which  is  shown  in  Fig.  389,  and  from  thence  to  ground 
through  the  springs  19  and  20,  which  are  brought  together  when  the 
line  switch  is  operated. 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM        557 

In  any  event,  therefore,  the  determining  condition  of  a  busy 
line  is  that  its  private  bank  contacts  on  all  connectors  of  its  group 
shall  be  grounded.  Under  the  present  assumed  condition,  therefore, 
the  connector  wipers,  which  have  been  brought  to  the  bank  contacts 
of  the  desired  line,  will  find  a  ground  at  the  private  bank  contact. 
The  connector  shaft  stops  for  an  instant  on  the  contacts  of  this  busy 
line  and  immediately  there  follows  over  the  rotary  side  of  the  line 
the  inevitable  single  rotary  impulse.  This  energizes  the  rotary  relay 
and  this,  as  usual,  energizes  the  private  magnet.  Remembering  now 
that  the  connector  side  switch  is  in  its  second  position  and  that  the 
private  wiper  of  the  connector  has  found  a  ground,  we  may  trace 
back  from  the  private  wiper  through  the  third  side  switch  wiper  to 
its  second  contact;  thence  through  the  contact  springs  14  arid  15, 
closed  by  the  private  magnet;  thence  through  the  release  magnet; 
thence  through  the  contact  springs  16  and  17  of  the  calling  battery 
supply  relay  to  the  live  side  of  the  battery.  This  calling  battery 
supply  relay  will,  at  this  time,  have  its  core  energized  because  the 
coil  18  is  in  series  with  the  rotary  relay  coil  which,  as  just  stated, 
was  energized  by  the  last  rotary  impulse.  This  series  of  operations 
has  led  to  the  energizing  of  the  release  magnet,  and,  as  a  result,  the 
double  dog  of  the  connector  is  pulled  out  of  the  connector  shaft 
ratchets  and  the  shaft  and  the  side  switch  are  restored  to  their 
normal  position. 

Busy-Back  Signal: — The  connector  has  dropped  back  to 
normal  in  all  respects.  The  calling  subscriber,  not  knowing  this, 
presses  his  ringing  button.  This  grounds  the  vertical  side  of  the  line 
at  his  station  and  operates  the  vertical  relay  at  the  connector.  This 
steps  the  shaft  of  the  connector  up  one  step  and  causes  the  closure 
of  the  contacts.  19  and  20  at  the  top  of  the  connector  shaft.  This 
establishes  a  connection  to  a  circuit  carrying  periodically  interrupted 
battery  current  on  which  an  inductive  hum  is  placed.  This  circuit 
may  be  traced  from  this  source  through  the  springs  20  and  19  to 
the  first  wiper  of  the  side  switch,  thence  through  the  normally  closed 
contacts  of  the  ringing  relay  to  the  rotary  side  of  the  line,  and  the 
varying  potential  to  which  this  path  is  subjected  produces  an  in- 
ductive flow  back  to  the  calling  subscriber's  telephone,  and  gives 
him  the  necessary  signal  which  consists  of  a  hum  or  buzzing  noise 
with  which  all  users  of  automatic  systems  soon  become  familiar. 


558  TELEPHONY 

Release  on  Busy  Connection: — The  connector,  since  its  last 
release,  has  been  stepped  up  one  notch  and  must  again  be  released. 
When  the  subscriber  hangs  up  his  receiver  after  receiving  the  busy 
signal,  he  grounds  both  sides  of  his  line  momentarily  by  the 
action  of  the  springs  21,  22,  and  23  of  Fig.  384.  This  operates  the 
rotary  and  the  vertical  relays  on  the  connector  simultaneously  and 
brings  together  for  the  first  time  the  springs  21  and  22  of  Fig.  396. 
This  establishes  a  connection  from  the  battery  through  the  springs 
16  and  17  on  the  calling  battery  supply  relay,  thence  through  the 
release  magnet  of  the  connector,  thence  through  the  springs  22  and 
21  of  the  vertical  and  the  rotary  relay,  thence  through  the  release 
trunk  back  to  the  second  selector.  From  here  the  circuit  passes 
through  the  private  wiper  of  that  selector  and  the  back  release  relay 
to  ground  through  the  third  side  switch  wiper  which  is  in  the  third 
position.  Considering  this  circuit  in  respect  to  its  action  on  the  con- 
nector it  is  obvious  that  it  energizes  the  release  magnet  on  the  con- 
nector which  restores  the  connector  to  normal  as  before.  At  the 
second  selector  this  circuit  passed  through  the  back  release  relay,  which 
closed  a  circuit  through  the  release  magnet  and  through  the  back  re- 
lease relay  contacts,  thence  back  over  the  second  selector  release  trunk 
to  the  back  release  relay  of  the  first  selector,  and  through  the  third 
wiper  of  the  side  switch  on  that  selector  to  ground,  since  that  side 
switch  also  is  in  its  third  position.  The  current  through  this  circuit 
energizes  the  release  magnet  of  the  second  selector  and  restores  it  to 
its  normal  position  and  also  energizes  the  back  release  relay  of  the 
first  selector.  This,  in  turn,  closes  the  circuit  from  the  battery 
through  the  release  magnet  of  the  first  selector  and  contacts  of  the 
back  release  relay  to  ground.  This  works  the  release  magnet  of 
the  first  selector  and  restores  that  selector  to  normal.  The  contacts 
on  the  first  selector  release  magnet,  shown  in  Fig.  390,  are  closed  by 
the  action  of  the  release  magnet  and  this  closes  the  path  from  ground 
back  through  the  first  selector  release  wire,  and  through  the  contacts 
13  and  14  of  the  line  switch,  through  the  line  switch  release  magnet 
to  battery,  and  this  restores  the  line  switch  to  normal. 

The  reason  for  the  term  back  release  will  now  be  apparent.  The 
release  operation  at  the  connector  is  relayed  back  to  the  second  se- 
lector; that  of  the  second  selector  back  to  the  first  selector;  and  that 
of  the  first  selector  back  to  the  line  switch.  Until  this  plan  was 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       559 

adopted,  the  release  magnet  of  each  selector  and  connector  involved 
in  a  connection  was  left  bridged  across  the  talking  circuit  so  as  to 
be  available  for  release;  and  it  sometimes  occurred  that  a  first  selec- 
tor would  be  released  before  a  second  selector  or  connector,  which 
latter  switches  would  thus  be  left  off-normal  until  rescued  by  an 
attendant.  The  back  release  plan  makes  it  impossible  for  the  con- 
nection necessary  for  the  release  of  a  switch  to  be  torn  down  until 
the  release  is  actually  accomplished. 

Called  Line  Found  Idle: — It  will  be  remembered  that,  before 
the  digression  necessary  to  trace  through  the  operations  occurring 
upon  the  finding  of  a  busy  line,  the  connector  wipers  had  been 
brought,  by  the  influence  of  the  calling  subscriber's  impulses,  into 
engagement  with  the  contacts  of  the  desired  line;  that  the  connector 
side  switch  was  in  its  second  position;  and  that  the  final  rotary  im- 
pulse following  the  last  series  of  selecting  impulses  had  not  been 
sent.  The  condition  now  to  be  assumed  is  that  the  called  subscriber's 
line  is  free  and  the  private  wiper,  therefore,  has  found  and  rests  on  an 
ungrounded  private  bank  contact.  The  final  rotary  impulse  which 
immediately  follows  will  operate  the  rotary  relay  and  this,  in  turn, 
will  operate  the  private  magnet.  This  happened  under  the  assumed 
condition  that  the  line  was  busy,  but  in  that  case  the  release  magnet 
was  also  operated  at  the  same  time  and  restored  all  conditions  to 
normal.  Under  the  present  condition  the  operation  of  the  private 
magnet  will  perform  its  usual  function  and  move  the  side  switch 
of  the  connector  into  its  third  position. 

Third  Position  of  Sid,e  Switch.  When  the  side  switch  of  the 
connector  moves  to  its  third  position,  it,  as  usual,  cuts  the  talking 
circuit  straight  through  from  the  vertical  and  the  rotary  sides  of  the 
trunk  leading  from  the  previous  selector  to  the  outgoing  terminal 
of  the  subscriber's  line,  which  may  be  traced  upon  Fig.  396  back 
through  the  line  switch,  shown  in  Fig.  389.  Several  things  are  to  be 
noted  about  the  talking  circuit  so  established:  First,  the  inclusion 
of  the  condensers  in  the  vertical  and  the  rotary  sides  of  the  con- 
nector circuit.  The  purpose  of  this  will  be  referred  to  later.  Second, 
the  inclusion  in  this  circuit  at  the  connector  of  a  pair  of  normally 
closed  contacts  in  the  ringing  relay.  It  may  be  said  in  passing  that 
the  ringing  relay  corresponds  exactly  in  function  to  a  ringing  key 
in  a  manual  switchboard.  Third,  the  talking  circuit  leading  from 


560  TELEPHONY 

the  connector  to  the  called  subscriber's  line  passes  on  one  side  through 
the  springs  24  and  25  of  the  bridge  cut-off  relay  of  the  line  switch, 
which  is  shown  in  Fig.  389.  These  springs  are  normally  open  and 
would  prevent  the  completion  of  the  talking  circuit  but  for  the  fact 
that  the  bridge  cut-off  relay  of  the  line  switch  is  energized  over  the 
private  wire  leading  to  the  connector  bank  and  then  through  the 
connector  wiper  to  the  third  side  switch  wiper  which,  at  this  time,  is 
in  its  third  position.  The  talking  circuit  is  thus  complete.  The 
operation  of  this  bridge  cut-off  relay  on  the  line  switch  has  not  only 
completed  the  talking  circuit  but  it  has  also  opened  the  circuit  of 
the  trip  magnet  of  the  line  switch  so  as  to  prevent  the  operation  of 
the  trip  magnet  by  the  subscriber  on  that  line  in  case  he  should  at- 
tempt to  make  a  call  during  the  interval  between  the  time  when  his 
line  was  connected  with  by  the  connector  and  the  time  when  he  an- 
swers the  call. 

The  third  wiper  of  the  connector  side  switch  when  moved  into 
its  third  position,  puts  the  ground  on  all  of  the  private  bank  contacts 
of  the  line  chosen  and  thus  guards  that  line  against  connection  by 
others,  as  already  described.  It  also  operates  the  bridge  cut-off 
relay  of  the  line  switch  as  just  mentioned. 

The  fourth  wiper  of  the  side  switch,  when  moved  into  its  third 
position,  establishes  such  a  connection  as  will  place  the  ringing  relay 
under  the  control  of  the  vertical  relay.  This  may  be  seen  by 
tracing  from  ground  to  the  vertical  relay  springs  23  and  24, 
thence  through  the  normally  closed  upper  pair  of  contacts  on  the 
private  magnet,  thence  through  the  fourth  wiper  on  the  side  switch 
to  its  third  contact,  thence  through  the  ringing  relay  magnet,  and 
through  the  springs  16  and  17  of  the  calling  battery  supply  relay 
and  to  battery.  The  calling  battery  supply  relay  winding  being  in 
series  with  the  vertical  relay  winding,  the  two  operate  together  and 
close  the  two  normally  open  points  in  the  ringing  relay  circuit.  This 
ringing  relay  acts  as  an  ordinary  ringing  key  and  connects  the  gen- 
erator to  the  called  subscriber's  line  in  an  obvious  manner,  at  the 
same  time  opening  the  talking  circuit  back  of  the  ringing  relay  in 
order  to  prevent  the  ringing  current  chattering  the  relays  in  the  cir- 
cuit back  of  it.  All  that  remains  now  is  for  the  called  subscriber 
to  respond.  When  he  does  he  closes  the  metallic  circuit  of  the  line 
through  his  talking  apparatus. 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       561 

Battery  Supply  to  Connected  Subscriber.  Throughout  the 
whole  process  of  building  up  a  connection,  it  will  be  remembered 
that  both  sides  of  the  calling  line  are  connected  through  the  re- 
spective vertical  and  rotary  relays  involved  in  building  up  the  con- 
nection with  the  live  side  of  the  battery.  At  the  time  when  the  con- 
nection is  finally  established  and  the  called  subscriber  rung,  both 
sides  of  the  calling  line  are  connected  through  various  relay  windings 
to  the  live  side  of  the  battery.  Such  a  condition  leaves  both  sides  of 
the  line  at  the  same  potential  and,  therefore,  there  is  no  tendency 
for  current  to  flow  through  the  calling  subscriber's  talking  apparatus, 
even  though  it  is  connected  across  the  circuit  of  the  line.  It  re- 
mains, therefore,  to  be  seen  how  these  conditions  are  so  changed 
after  the  building  up  of  a  connection  as  to  supply  the  calling  sub- 
scriber with  talking  current. 

The  calling  subscriber  can  get  no  current  until  the  called  sub- 
scriber responds.  When  the  connection  is  first  made  with  the  called 
subscriber's  line,  battery  connection  10  his  line  is  made  from  the  live 
side  of  battery  through  the  normally  closed  contacts  of  the  calling 
battery  supply  relay,  thence  through  the  winding  25  of  the  called 
battery  supply  relay  to  the  vertical  side  of  the  called  line.  The 
grounded  side  of  the  battery  is  connected  to  the  rotary  side  of  his 
line  through  the  third  wiper  of  the  connector  and  the  coil  26  of  the 
called  battery  supply  relay.  As  a  result,  this  subscriber  receives 
proper  talking  current  through  the  coils  25  and  26,  and  this  relay 
is  operated  by  the  flow  of  this  current.  The  operation  of  this  called 
battery  supply  relay  merely  shifts  the  connection  of  the  rotary  side 
of  the  calling  subscriber's  line  from  its  normal  battery  connection, 
to  ground,  and  thus  the  battery  is  placed  straight  across  the  calling 
subscriber's  line  so  as  to  supply  talking  current.  This  supply  cir- 
cuit to  the  calling  subscriber  may  be  traced  from  the  live  side  of 
the  battery  through  the  winding  JS  of  the  calling  battery  supply  relay 
and  the  winding  of  the  vertical  relay  to  the  vertical  side  of  the  line, 
and  from  the  grounded  side  of  battery  through  the  third  side  switch 
wiper  in  its  third  position  to  the  now  closed  pair  of  contacts  in  the 
called  battery  supply  relay  through  the  coil  18  of  the  calling  battery 
supply  relay  and  the  coil  of  the  rotary  relay  to  the  rotary  side  of 
the  line. 

It  will  be  noted  that  the  system  of  battery  supply  is  that  of  the 


562  TELEPHONY 

standard  condenser  and  retardation  coil  scheme  largely  employed 
in  manual  practice;  and  that  aside  from  the  coils  through  which 
the  battery  current  is  supplied  to  the  connected  subscribers,  there  are 
no  taps  from,  or  bridges  across,  the  two  sides  of  the  talking  circuit. 

Release  after  Conversation.  It  remains  now  only  to  secure 
the  disconnection  of  the  subscribers  after  they  are  through  talk- 
ing. When  the  calling  subscriber  hangs  up,  the  whole  disconnec- 
tion is  brought  about,  all  of  the  apparatus,  including  connector, 
selectors,  and  line  switch,  returning  to  normal.  This  is  done  by 
the  back  release  system  and  is  accomplished  in  almost  the  same 
way  as  has  already  been  described  in  connection  with  the  discon- 
nect after  an  unsuccessful  call.  There  is  this  difference,  however: 
after  an  unsuccessful  call  when  the  line  called  for  was  found  busy, 
the  release  was  made  while  the  connector  side  switch  was  in  its 
normal  position.  In  the  present  case,  the  release  must  be  made 
with  the  connector  side  switch  in  its  third  position  and  with  the 
talking  battery  bridged  across  the  metallic  circuit  rather  than  con- 
nected between  each  limb  of  the  line  and  ground.  It  must  be 
remembered  that  the  calling  battery  supply  relay,  while  traversed 
by  current  during  the  conversation,  is  not  magnetically  energized 
because,  with  the  current  flowing  through  the  metallic  circuit  of  the 
line,  the  two  windings  exert  a  differential  effect.  As  soon,  how- 
ever, as  the  calling  subscriber  hangs  up  his  receiver,  this  differential 
action  ceases,  due  to  the  grounding  of  both  sides  of  the  line  at  the  sub- 
scriber's station.  This  relay,  therefore,  operates  and  cuts  off  bat- 
tery from  the  called  battery  supply  relay  and  this,  in  turn,  releases 
its  armature  and  thus  changes  the  connection  of  the  rotary  side  of  the 
calling  line  from  ground  to  live  side  of  the  battery.  The  normal 
condition  of  the  battery  connection  now  being  restored,  both  the 
vertical  and  the  rotary  relays  at  the  connector  become  operated,  due 
to  the  ground  on  both  sides  of  the  line  at  the  subscriber's  station,  and 
this,  as  we  have  seen,  is  the  condition  which  brings  about  the  opera- 
tion of  the  connector  release  magnet,  and  the  relaying  back  of  the 
disconnect  impulse  successively  through  the  selectors  to  the  line 
switch. 

Multi=0ffice  System.  In  exchanges  involving  more  than  one 
office,  the  same  general  principles  and  mode  of  operation  already 
outlined  apply.  If  the  total  number  of  subscribers  in  the  multi- 


office  exchange  is  to  be  less  than  ten  thousand,  then  four  digit  num- 
bers suffice,  and  the  first  movement  of  the  dial  may  be  made  to  select 
the  office  into  which  the  connection  is  to  go,  the  subscribers'  lines 
being  so  numbered  with  respect  to  the  offices  that  each  office  will 
contain  only  certain  thousands.  The  choosing  of  the  thousand  by 
the  calling  subscriber,  therefore,  takes  care  in  itself  of  the  choice  of 
offices.  Where,  however,  a  multi-office  exchange  is  to  provide  for 
connections  among  a  greater  number  of  lines  than  ten  thousand  and 
less  than  one  hundred  thousand,  then  it  will  take  five  movements  of 
the  dial  to  make  the  selection — the  five  movements  corresponding 
either  to  the  five  digits  in  a  number  or  to  the  name  of  an  office,  as 
indicated  on  the  dial,  and  the  four  digits  of  a  smaller  number.  The 
lines  may  all  carry  five  digit  numbers  or,  what  is  considered  better 
practice,  may  be  designated  by  an  office  name  followed  by  a  four 
digit  number.  In  this  latter  case  the  numbers  of  the  subscribers' 
lines  will  in  each  case  be  contained  in  one  or  more  of  the  tens  of  thou- 
sands groups,  no  number  having  more  than  four  digits.  And  the 
first  movement  of  the  dial,  whether  the  name  or  number  plan  be 
adopted,  will  select  an  office;  or,  looking  at  it  another  way,  will  se- 
lect a  group  of  ten  thousand  and  this  being  done,  the  next  four  suc- 
cessive movements  of  the  dial  will  select  the  numbers  in  that  ten 
thousand  in  just  the  same  way  that  has  been  already  described. 

Certain  difficulties  arise,  however,  in  multi-office  working 
due  to  the  fact  that  the  three-wire  trunks  between  offices  would 
in  most  cases  be  objectionable.  As  long  as  the  trunks  extend 
between  the  various  groups  of  apparatus  in  the  same  office,  it  is  cheap- 
er to  provide  three  wires  for  each  of  them  than  it  is  to  make  any  ad- 
ditional complication  in  the  apparatus.  Where  the  trunking  is  done 
between  offices,  however,  the  system  may  be  so  modified  as  to  work 
over  two  wire  inter-office  trunks. 

The  Trunk  Repeater.  The  purpose  of  the  trunk  repeater  is 
to  enable  the  inter-office  trunking  to  be  done  over  two  wires.  It 
may  be  said  that  the  trunk  repeater  is  a  device  placed  in  the  outgoing 
trunk  circuit  at  the  office  in  wnich  a  call  originates,  which  will  do 
over  the  two  wires  of  the  trunk  leading  from  it  to  the  distant  office 
just  the  same  thing  that  the  subscriber's  signal  transmitter  does 
over  the  two  wires  of  the  subscriber's  lines.  It  has  certain  other 
functions  in  regard  to  feeding  the  battery  for  talking  purposes  back 


564 


TELEPHONY 


to  the  calling  subscriber's  line,  taking  the  place  in  this  respect  of  the 
calling  battery  feed  relay  in  the  connector  in  a  single  office  exchange. 
The  circuits  of  a  trunk  repeater  are  shown  in  Fig.  397.  In  con- 
sidering it,  it  must  be  understood  that  the  three  wires  entering  the 
figure  at  the  left  are  the  vertical,  rotary,  and  release  wires  of  a  sec- 
ond selector  trunk  leading  from  the  first  selector  banks  in  the  same 
office.  The  two  wires  leading  from  the  right  of  the  figure  are  those 
extending  to  the  distant  office,  and  terminate  there  in  second  selec- 
tors. The  vertical  and  the  rotary  sides  of  this  trunk  as  shown  at 
the  left  will  receive  the  impulses  from  the  subscriber's  station  com- 


Fig.  397.     Circuits  of  Trunk  Kepeater 

ing  through  the  line  switch  and  the  first  selector,  as  usual.  The 
vertical  impulses  will  pass  through  the  winding  of  the  vertical  relay 
and  through  the  winding  1  of  the  calling  battery  supply  relay  and 
thence  to  battery,  the  same  as  on  a  connector.  These  impulses 
will  work"  the  armatures  of  both  of  these  relays  in  unison.  The 
movements  of  the  vertical  relay  armature  in  response  to  these  im- 
pulses will  cause  corresponding  impulses  to  flow  over  a  circuit  which 
may  be  traced  from  ground,  through  the  springs  3  and  2  of  the  verti- 
cal relay,  the  springs  4  and  5  of  the  bridged  relay  6  and  thence  to  the 
vertical  side  of  the  trunk  and  to  the  distant  office,  where  it  passes  into 
a  second  selector  and  through  its  vertical  relay  to  battery.  Thus 
the  vertical  impulses  are  passed  on  over  the  two-wire  trunk  to  the 
second  selector  at  the  distant  office.  It  becomes  necessary,  how- 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       565 

ever,  to  prevent  these  impulses  from  passing  back  through  the  wind- 
ing of  the  bridge  relay  6  and  this  is  done  by  means  of  the  sluggish 
relay  7.  This  relay  receives  local  battery  impulses  in  unison  with 
those  sent  over  the  trunk  by  the  vertical  relay,  these  being  supplied 
from  the  battery  at  the  local  office  through  the  contacts  8  and  9  of 
the  calling  battery  supply  relay,  which  works  in  unison  with  the 
vertical  relay.  These  rapidly  recurring  impulses  are  too  fast  for 
the  sluggish  relay  7  to  follow.  And  this  relay  merely  pulls  up  its 
armature  and  cuts  off  both  sides  of  the  trunk  leading  back  to  the  first 
selector.  The  rotary  impulses  are  repeated  to  the  rotary  side  of 
the  two-wire  trunk  in  a  similar  way. 

Considering  now  the  operation  of  the  trunk  repeater  in  the  re- 
verse direction,  the  action  of  the  bridging  relay  6  is  of  vital  impor- 
tance. Normally  both  sides  of  trunk  line  are  connected  to  the  live 
side  of  the  battery  and,  therefore,  there  is  no  difference  of  potential 
between  them  and  no  tendency  to  operate  the  bridged  relay.  When 
the  connection  has  been  fully  established  to  the  subscriber  at  the 
distant  office,  and  that  subscriber  has  responded,  the  action  of  his 
battery  supply  relay  will,  as  before  stated,  change  the  connection 
of  the  rotary  side  of  the  line  from  battery  to  ground,  and  thus  bridge 
the  battery  at  the  distant  exchange  across  the  trunk.  This  action 
will  pull  up  the  bridged  relay  6  at  the  trunk  repeater  and  will  perform 
exactly  the  same  function  with  respect  to  the  connection  of  the  bat- 
tery with  the  calling  subscriber's  line.  In  other  words,  it  will  change 
the  connection  of  the  rotary  side  of  the  calling  line  from  battery  to 
ground,  thus  establishing  the  necessary  difference  in  potential  to  give 
the  calling  subscriber  the  necessary  current  for  transmission  purposes. 
The  disconnect  feature  is  about  the  same  as  already  described. 
When  the  calling  subscriber  hangs  up  his  receiver  both  the  vertical 
and  rotary  relays  of  the  trunk  repeater  operate,  which  places  the 
ground  on  both  sides  of  the  two-wire  trunk  to  the  distant  office, 
which  is  the  condition  for  releasing  all  of  the  apparatus  there. 

For  the  purpose  of  convenience  the  simplified  diagram  of  Fig. 
398  has  been  prepared,  which  shows  the  complete  connection  from 
a  calling  subscriber  to  a  called  subscriber  in  a  multi-office  exchange, 
wherein  the  first  movement  of  the  dial  is  employed  to  establish  the 
connection  to  the  proper  office  and  the  four  succeeding  movements 
to  make  a  selection  among  ten  thousand  lines  in  that  office.  This 


566 


TELEPHONY 


•5t/0SC#/B£/!&    ,     STATtOH 


circuit,  therefore,  employs  at  the  first  office  the  line  switch,  the 
first  selector,  and  the  trunk  repeater;  and  at  the  second  office  the 
second  selector,  third  selector,  connector,  and  line  switch. 

The  third  selector  is  omitted  from  Fig.  398,  but  this  will  cause 
no  confusion,  since  it  is  exactly  like  the  second  selector.  The  cir- 
cuits shown  are  exactly  like  those  previously  described  but  in  draw- 
ing them  the  main  idea  has  been  to  simplify  the  connections  to  the 
greatest  possible  extent  at  a  sacrifice  in  the  clearness  with  which 
the  mechanical  inter-relation  of  parts  is  shown.  No  correct  under- 
standing of  the  circuits  of  an  automatic  system  is  possible  without 
a  clear  idea  of  the  mechanical  functions  performed  by  the  different 
parts,  and,  therefore,  we  have  described  what  are  apparently  the 

more  complex  circuit  drawings  first. 
It  is  believed  that  the  student,  in  at- 
tempting to  gain  an  understanding 
of  this  marvel  of  mechanical  and  elec- 
trical intricacy,  will  find  his  task  less 
burdensome  if  he  will  refer  freely  to 
both  the  simplified  circuit  drawing  of 
Fig.  398  and  the  more  complex  ones 
preceding  it.  By  doing  so  he  will 
often  be  enabled  to  clear  up  a  doubtful 
circuit  point  from  the  simpler  dia- 
gram and  a  doubtful  mechanical  point 
from  those  diagrams  which  represent 
more  clearly  the  mechanical  relation  of 
parts. 


Fig.  398.     Connection  between  a  Calling  and 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       567 


SUBSCRIBER'S      STAT/ON 


Automatic  Sub=0ffices.  Obviously,  the  system  of  irunking  em- 
ployed in  automatic  exchanges  lends  itself  with  great  facility  to  the 
subdivision  of  an  exchange  into  a  large  number  of  comparatively 
small  office  districts  and  the  establishment  of  branch  offices  or  sub- 
offices  at  the  centers  of  these  districts. 

The  trunkirig  between  large  offices  has  already  been  described. 
An  attractive  feature  of  the  automatic  system  is  the  establishment 
of  so-called  sub-stations  or  sub-offices.  Where  there  is,  in  an  out- 
lying district,  a  distinct  group  of  subscribers  whose  lines  may  readily 
be  centered  at  a  common  point  within  that  district  and  where  the 
number  of  such  subscribers  and  lines  is  insufficient  to  establish  a 
fully  equipped  office,  it  is  possible  to 
establish  a  so-called  sub-station  or  sub- 
office  connected  with  the  main  office 
of  that  district  by  trunk  lines.  At 
this  sub-office  there  are  placed  only 
line  switches  and  connectors.  When  a 
call  is  originated  on  one  of  these  sub- 
office  lines,  the  line  switch  acts  in- 
stantly to  connect  that  line  with  one 
of  the  trunks  leading  to  the  main 
office  of  that  district,  at  which  this 
trunk  terminates  in  a  first  selector. 
From  there  on,  the  connection  is  the 
same  as  that  in  a  system  in  which 
no  sub-offices  are  employed.  Calls 
coming  into  this  sub-office  over  trunk 


TH/ftO    ~S£L£CTOft 
SECOND    -S£L£CTOft  v      /H3ERT£O    HERE  ,  CONNECTOR 


a  Called  Subscriber  in  an  Automatic  System 


568  TELEPHONY 

lines  from  the  main  office  are  received  on  the  connectors  at  the 
sub-office  and  the  connection  is  made  with  the  sub-office  line  by 
the  connector  in  the  usual  manner.  This  arrangement,  it  is  seen, 
amounts  merely  to  a  stretching  of  the  connector  trunks  for  a  given 
group  of  lines  so  that  they  will  reach  out  from  a  main  office  to  a  sub- 
office,  it  being  more  economical  to  lengthen  the  smaller  number  of 
trunks  and  by  so  doing  to  decrease  in  length  the  larger  number  of 
subscribers'  lines. 

The  Rotary  Connector.  For  certain  purposes  it  becomes 
desirable  in  automatic  work  to  employ  a  special  form  of  connector 
which  will  have  in  itself  a  certain  ability  to  make  automatic  selec- 
tion of  one  of  a  group  of  previously  chosen  trunks  in  much  the  same 
manner  as  the  first  and  second  selectors  automatically  choose  the 
first  idle  one  of  a  group  of  trunks. 

Such  a  use  is  demanded  in  private  branch-exchange  working  where 
a  given  business  establishment,  for  instance,  has  a  plurality  of  lines 
connecting  its  own  private  switchboard  with  the  central  office.  The 
directory  number  of  all  these  lines  is,  for  convenience,  made  the  same, 
and  it  is  important,  therefore,  that  when  a  person  attempts  to  make 
a  connection  with  this  establishment,  he  will  not  fail  to  get  his  con- 
nection simply  because  the  first  one  of  these  lines  happens  to  be  busy. 
For  such  use  a  given  horizontal  row  of  connector  terminals  or  a  part 
of  such  a  row  is  assigned  to  the  lines  leading  to  the  private  branch 
exchange  and  the  connector  is  so  modified  as  to  have  a  certain  "dis- 
cretionary" power  of  its  own.  As  a  result,  when  the  common  num- 
ber of  all  these  lines  is*  called,  the  connector  will  choose  the  first  one, 
if  it  is  not  already  engaged  by  some  other  connector,  but  if  it  is,  it 
will  pass  on  to  the  next,  and  so  on  until  an  idle  one  is  found.  It  is 
only  when  the  connector  has  hunted  through  the  entire  group  of 
lines  and  found  them  all  busy  that  it  will  refuse  to  connect  and  will 
give  the  busy  signal  to  the  calling  subscriber. 

Party  Lines.  The  description  of  this  system  as  given  above 
has  been  confined  entirely  to  direct  line  working;  however,  party 
lines  may  be  and  are  frequently  employed. 

The  circuits  and  apparatus  used  with  direct  lines  are,  with  slight 
modifications,  applicable  to  use  with  party  lines. 

The  harmonic  method  of  ringing  is  employed  and  the  stations 
are  so  arranged  with  respect  to  the  connectors  that  those  requiring 


AUTOMATIC  ELECTRIC  COMPANY'S  SYSTEM       569 

the  same  frequency  for  ringing  the  bells  are  in  groups  served  by 
the  same  set  of  connectors. 

The  party  lines  are  operated  on  the  principle  commonly 
known  in  manual  practice  as  the  jack  per  station  arrangement. 
Each  party  line  will,  therefore,  have  sets  of  terminals  appearing  in 
separate  hundreds;  the  connectors  associated  with  each  of  these 
hundreds  being  so  arranged  as  to  impress  the  proper  frequency  of 
ringing  current  on  the  line. 

From  the  subscribers'  standpoint  the  operation  is  the  same  as 
for  direct  lines,  as  the  particular  hundreds  digit  of  a  number  serves 
to  select  one  of  a  group  of  con- 
nectors capable  of  connecting  the 
proper  ringing  current  to  the  line. 

To  avoid  confusion,  which 
would  be  caused  by  a  subscriber 
on  a  party  line  attempting  to  make 
a  call  when  the  line  is  already  in 
use  by  some  other  subscriber,  the 
subscribers'  stations  are  so  ar- 
ranged that  when  the  line  is  in 
use  all  other  stations  on  the  line 

Pig.  399.    Wall  Set  for  Two- Wire  System 

are  locked  out. 

The  Two=Wire  Automatic  System.  The  two-wire  system  that  has 
recently  been  introduced  by  the  Automatic  Electric  Company  brings 
about  the  very  important  result  of  accomplishing  all  of  the  auto- 
matic switching  over  metallic  circuit  lines  without  the  use  of  ground 
or  common  returns.  The  system  is  thus  relieved  of  the  disturbing 
influences  to  which  the  three-wire  system  is  sometimes  subjected, 
due  to  differences  in  earth  potential  between  various  portions  of 
the  system,  which  may  add  to  or  subtract  from  the  battery  potential 
and  alter  the  net  potential  available  between  two  distant  points. 
The  introduction  of  this  system  has  also  made  possible  certain  other 
incidental  features  of  advantage,  one  of  which  is  a  great  simplifica- 
tion and  reduction  in  size  of  the  subscriber's  station  signal-trans- 
mitting apparatus. 

With  the  doing  away  of  the  ground  as  a  return  circuit,  it  becomes 
impossible  to  send  vertical  impulses  over  one  side  of  the  line  and  to 
follow  them  by  single  rotary  impulses  over  the  other  side  of  the  line. 


570 


TELEPHONY 


Yet  it  becomes  necessary  to  distinguish  between  the  pure  selective 
impulses  and  those  impulses  which  dictate  a  change  of  function  at 
the  central  office.  The  plan  has,  therefore,  been  adopted  of  accom- 
plishing the  selection  in  each  case  by  short  and  rapidly  recurring 
impulses  and  of  accomplishing  those  functions  formerly  brought 
about  by  the  single  impulse  over  the  rotary  side  of  the  line  by  a  pause 

between  the  respective  series  of 
selective  impulses.  This  is  ac- 
complished at  the  central  office 
by  replacing  the  vertical  and  the 
rotary  relays  of  the  three-wire 
system  by  a  quick-acting  and  a 
sluggish  relay,  respectively;  the 
quick-acting  relay  performing 
the  functions  previously  carried 
out  by  the  vertical  relay,  and 
the  sluggish  relay  acting  only 
during  the  pauses  between  the 
successive  series  of  quick  im- 
pulses to  do  the  things  formerly 
done  by  the  rotary  relay.  This 
has  resulted  in  a  delightful  sim- 
plification of  subscriber's  appa- 
ratus, since  it  is  now  necessary 
only  to  provide  a  device  which 
will  connect  the  two  sides  of  the 
line  together  the  required  number  of  times  in  quick  succession  and 
then  allow  a  pause  with  the  circuit  closed  while  the  subscriber  is 
getting  ready  to  transmit  another  set  of  impulses  corresponding  to 
another  digit.  The  calling  device  has  no  mechanical  function  co- 
acting  with  any  of  the  other  parts  of  the  telephone  and  may  be 
considered  as  a  separate  mechanical  device  electrically  connected 
with  the  line.  The  transmitting  device  is  not  much  larger  than 
a  large  watch  and  a  good  idea  of  it  may  be  had  from  Fig.  399, 
which  shows  the  latest  wall  set,  and  Fig.  400,  which  shows  the  latest 
desk  set  of  the  Automatic  Electric  Company.  We  regret  the  fact 
that  this  company  has  made  the  request  that  the  complete  details 
of  their  two-wire  system  be  not  published  at  this  time. 


Fig.  400.     Desk  Stand  for  Two- 
Wire  System 


CHAPTER  XXX 
THE  LORIMER  AUTOMATIC  SYSTEM 

The  Lorimer  automatic  telephone  system  has  not  been  com- 
mercially used  in  this  country  but  is  in  commercial  operation  in  a 
few  places  in  Canada.  It  is  interesting  from  several  points  of  view. 
It  was  invented,  built,  and  installed  by  the  Lorimer  Brothers — Hoyt, 
George  William,  and  Egbert — of  Brantford,  Ontario.  These  young 
men  without  previous  telephonic  training  and,  according  to  their 
statements,  without  ever  having  seen  the  inside  of  a  telephone  office, 
conceived  and  developed  this  system  and  put  it  in  practical  operation. 
With  the  struggles  and  efforts  of  these  young  men  in  accomplishing 
this  feat  we  have  some  familiarity,  and  it  impresses  us  as  one  of  the 
most  remarkable  inventive  achievements  that  has  come  to  our  attention, 
regardless  of  whatever  the  merits  or  demerits  of  the  system  may  be. 

The  Lorimer  system  is  interesting  also  from  the  fact  that,  in 
most  cases,  it  represents  the  mechanical  rather  than  the  electrical 
way  of  doing  things.  The  switches  are  power  driven  and  electrically 
controlled  rather  than  electrically  driven  and  electrically  controlled, 
as  in  the  system  of  the  Automatic  Electric  Company. 

The  subscriber's  station  apparatus  consists  of  the  usual  receiver, 
speech  transmitter,  call  bell,  and  hook  switch,  and  in  addition  a 
signal  transmitter  arranged  to  be  manipulated  by  the  subscriber 
so  as  to  control  the  operation  of  the  central-office  apparatus  in  con- 
necting with  any  desired  line  in  the  system. 

The  central-office  apparatus  is  designed  throughout  upon  the 
principle  of  switching  by  means  of  power-driven  switches  which  are 
under  the  control  of  the  signal  transmitters  of  the  calling  subscriber's 
station.  The  switches  employed  in  making  a  connection  are  all  so 
arranged  with  respect  to  constantly  rotating  shafts  that  the  movable 
member  of  such  switches  may  be  connected  to  the  shafts  by  means 
of  electromagnets  controlled  directly  or  indirectly  by  relays,  which, 
in  turn,  are  brought  under  the  control  of  the  signal  transmitters. 


572 


TELEPHONY 


The  circuits  are  so  designed  in  many  instances  that  the  changes 
necessary  for  the  different  steps  are  brought  about  by  the  movement 
of  the  switches  themselves,  thus  permitting  the  use  of  circuits  which 
are  rather  simple.  The  switches  employed  are  all  of  a  rotary  type; 
the  co-ordinate  selection,  which  is  accomplished  in  the  Automatic 
Electric  Company's  system  by  a  vertical  and  rotary  movement,  being 
brought  about  in  this  system  by  the  independent  rotation  of  two 
switches. 

Subscriber's  Station  Equipment.  A  subscriber's  desk-stand 
set,  except  the  call  bell,  is  shown  in  Fig.  401,  and  a  wall  set  complete 
in  Fig.  402.  In  both  of  these  illustrations  may  be  seen  the  familiar 
transmitter,  receiver,  and  hook  switch,  and 
in  the  wall  set,  the  call  bell.  The  portion  of 
these  telephone  sets  which  is  unfamiliar  at 
present  is  the  part  which  is  enclosed  in  the 
enlarged  base  of  the  desk  stand  and  the 
protruding  device  below  the  speech  trans- 
mitter in  the  wall  set — the  signal  transmitter 
referred  to  earlier  in  the  chapter.  The  small 
push  button  and  small  plate  through  which 
the  number  may  be  seen  directly  below  the 
transmitter  in  Fig.  402,  are  for  the  purpose 
of  registering  calls. 

The  signal  transmitter  is  a  device  whose 
function  is  to  record  mechanically  the  num- 
ber of  the  subscriber's  station  with  which 
connection  is  desired,  and  to  transmit  that 
record  to  the  central  office  by  a  system  of 
electrical  impulses  over  the  line  conductors.  Instead  of  operating 
by  its  own  initiative,  the  signal  transmitter  is  adapted  to  respond 
to  central-office  control  in  transmitting  electrically  the  number  which 
has  been  recorded  mechanically  upon  it. 

The  signal  transmitter  shown  removed  from  the  base  of  the  desk 
stand  at  the  left  in  Fig.  403  comprises  in  part  four  sets  of  contact 
pins  having  ten  pins  in  each  set,  one  set  for  each  of  the  digits  of  a  four- 
digit  number.  There  are  also  several  additional  contact  pins  for 
signaling  and  auxiliary  controlling  purposes.  All  of  these  contact 
pins  are  arranged  upon  the  circumference  of  a  circle  and  a  movable 


Pig.  401.     Lorimer  Auto- 
matic Desk  Stand 


LORIMER  AUTOMATIC  SYSTEM 


573 


brush  mounted  upon  a  shaft  at  the  center  of  the  circle  is  adapted  to  be 

rotated  by  a  clock  spring  and  to  make  contact  with  each  of  the  pins 

successively.    The  call  is  started,  after  the  number  desired  has  been 

set    on    the   dial,  by   giving 

the  crank  at  the  right  of  the 

signal  transmitter  a  complete 

turn   and   thus   winding  the 

spring.     The  shaft   carrying 

the  signal  transmitter  brush 

carries    also    an    escapement 

wyheel,  the  pallet  of  which  is 

directly     controlled     by    an 

electromagnet. 

The  four  dials  with  the 
numerals  printed  on  them  are 
attached  to  four  levers,  re- 
spectively, and  are  moved  by 
their  levers  opposite  windows, 
near  the  top  of  the  casing. 
Through  each  of  these  win- 
dows a  single  numeral  may 
be  seen  on  the  corresponding 
one  of  the  dials.  The  dials 

may  be  adjusted  so  that  the  four  numerals  seen  will  read  from  left 
to  right  to  correspond  to  the  number  of  the  line  with  which  connec- 
tion is  desired. 

The  setting  of  the  dials  so  that  the  number  desired  shows  at  the 
small  circular  opening  results  in  connecting  the  earth  or  a  common 
return  conductor  to  one  pin  of  each  set  of  ten  pins,  the  pin  grounded 
in  each  set  depending  upon  the  numerical  value  of  the  digit  for  which 
the  dial  is  set. 

The  circle  of  contact  pins  is  set  in  an  insulating  disk,  the  signal 
transmitting  brush  operates  upon  the  pins  on  one  side  of  the  disk, 
and  electrical  fingers  attached  to  the  dials  operate  upon  the 
pins  on  the  other  side  of  the  disk.  The  escapement  wheel  is  a 
single  toothed  disk  attached  directly  to  the  shaft  which  carries  the 
signal  brush  and  its  pallet  is  attached  rigidly  to  the  magnet  arma- 
ture. 


Fig.  402.     Lorimer  Automatic  Wall  Set 


574 


TELEPHONY 


Once  a  call  has  been  turned  in,  the  entire  subscriber's  station 
equipment  is  locked  beyond  power  of  the  subscriber  to  tamper 
with  it  in  any  way,  rendering  it  impossible  either  to  defeat  the  call 
which  has  been  started  or  to  prevent  the  subscriber's  station  as  a 
whole  from  returning  completely  to  normal  position  and  thus  re- 
storing itself  for  regular  service.  The  key  shown  just  below  the 
signal  transmitter  in  the  case  of  the  desk  stand,  and  at  the  right  in 
the  wall,  set,  is  for  the  purpose  of  operating  a  relay  at  the  central 


Fig.  403.     Desk  Stand  with  Signal  Transmitter  Removed 

~Jice  which,  in  turn,  connects  ringing  current  to  the  line  of  the  sub- 
scriber with  which  connection  has  been  made,  and  thus  actuates  the 
call  bell. 

As  the  number  set  up  at  the  signal  transmitter  remains  in  full 
view  until  reset  for  some  other  number,  it  is  easily  checked  by  in- 
spection and  also  lessens  the  labor  involved  in  making  a  second  call 
for  the  same  line,  which  is  frequently  necessary  when  the  line  is 
found  busy  the  first  time  called. 

Central=0ffice  Apparatus.  The  subscriber's  lines  are  di- 
vided into  groups  of  one  hundred  lines  each  at  the  central  office, 
each  group  being  served  by  a  single  unit  of  central-office  apparatus. 


LORIMER  AUTOMATIC  SYSTEM  575 

In  a  central-office  unit  there  is  "sectional  apparatus"  which  appears 
but  once  for  the  unit  of  one  hundred  lines;  "divisional  apparatus" 
which  appears  a  number  of  times  for  each  unit,  depending  upon 
the  traffic;  and  "line  apparatus"  which  appears  one  hundred  times 
for  each  unit  or  once  for  each  line. 

The  sectional  apparatus  comprises  devices  whose  duties  are, 
first,  to  detect  a  calling  line,  and  second,  to  assign  to  the  calling 
line  a  set  of  idle  divisional  apparatus  which  serves  to  perform  the 
necessary  switching  functions  and  complete  the  connection. 

The  sets  of  divisional  apparatus,  or,  as  called  in  this  system, 
"divisions,"  are  common  to  a  section  and  are  employed  in  a  man- 
ner similar  to  the  connecting  cords  of  a  manual  switchboard.  The 
number  of  these  divisions  provided  for  each  section  is,  therefore, 
determined  -by  the  number  of  simultaneous  connections  resulting 
from  calls  originating  in  the  section.  It  has  been  the  custom  in 
building  this  apparatus  to  provide  each  section  with  seven  divisions 
or  connective  elements. 

The  line  apparatus  comprises  one  relay,  having  a  single  wind- 
ing, and  two  pairs  of  contacts  operated  by  its  armature.  This  de- 
vice is  substantially  the  well  known  cut-off  relay  almost  universally 
employed  in  common-battery  systems.  The  fixed  multiple  contacts 
of  the  lines  in  the  switching  banks  of  the  connecting  apparatus  are 
considered  as  pertaining  to  the  various  pieces  of  apparatus  on  which 
they  are  found  rather  than  to  their  respective  lines.  A  good  idea  may 
be  obtained  of  the  arrangement  of  the  sectional  and  divisional  appara- 
tus by  referring  to  Fig.  404,  which  is  one  unit  of  a  thousand-line 
equipment.  The  apparatus  in  the  vertical  row  at  the  extreme  left 
of  the  illustration  is  the  sectional  apparatus,  while  the  remaining 
seven  vertical  rows  of  apparatus  are  the  divisions. 

The  Section.  The  sectional  appararus  for  each  unit  consists 
of  three  separate  devices  called  for  convenience  a  decimal  indicator,  a 
division  starter,  and  a  decimal-register  controller.  All  of  these  devices 
are  normally  motionless  when  idle.  The  energization  of  the  deci- 
mal indicator,  in  response  to  the  inauguration  of  a  call  at  a  subscrib- 
er's station,  results  immediately  in  an  action  of  the  division  starter 
which  starts  a  division  to  connect  with  the  line  calling.  It  results 
also  in  the  starting  of  the  decimal-register  controller,  the  remaining 
unit  of  sectional  apparatus. 


576  TELEPHONY 

It  is  thus  seen  that  upon  the  starting  of  a  call  by  a  subscriber, 
all  of  the  sectional  apparatus  belonging  to  his  one  hundred  lines 
immediately  becomes  active,  the  division  starter  acting  to  start  a 
division,  the  decimal  indicator  becoming  energized  to  indicate  the 
tens  group  in  which  the  call  has  appeared,  and  the  decimal-register 
controller  becoming  active  to  adjust  the  decimal  register  of  the 
division  assigned  by  the  division  starter.  The  division  starter  hav- 
ing assigned  a  division  for  the  exclusive  use  of  this  particular  call, 
passes  to  a  position  from  which  it  may  start  a  similar  idle  division 
when  the  next  call  is  received.  The  decimal  register  controller 
makes  its  half  revolution  for  the  call  and  comes  to  rest,  awaiting  a 
subsequent  call,  and  the  decimal  indicator  continues  energized  but 
only  momentarily,  since  it  is  released  by  the  action  of  the  cut-off  relay 
when  the  call  is  taken  in  charge  by  the  divisional  connective  devices. 

Calls  may  follow  each  other  rapidly,  the  connective  devices  being 
entirely  independent  of  each  other  after  having  been  assigned  to  the 
respective  calling  lines.  As  has  been  described,  the  decimal  indi- 
cator starts  the  division  starter  and  the  decimal-register  controller 
in  quick  succession.  The  division  starter,  shown  at  the  extreme 
bottom  of  the  left-hand  row  of  Fig.  404,  is  a  cylinder  switch  of  the 
same  general  type  as  used  throughout  this  system.  In  it  the  termi- 
nals of  a  switch  in  each  division  appear  as  fixed  contact  points  in  a 
circle  over  which  move  the  brushes  of  the  division  starter. 

The  decimal-register  controller  has  the  duties  of  transmitting 
to  the  divisional  apparatus  a  series  of  current  impulses  correspond- 
ing in  number  to  the  numerical  value  of  the  tens  digit  of  the  calling 
line.  This  is  effected  by  providing  before  a  movable  brush  ten  con- 
tacts from  which  the  brush  may  receive  current.  These  contacts 
are  normally  not  connected  to  battery,  so  that  the  brush  in  passing 
over  them  does  not  receive  current  from  them;  however,  when  the 
brush  has  reached  the  contact  corresponding  in  number  to  the  tens 
digit  of  the  calling  line,  a  relay  associated  with  the  decimal-register 
controller  charges  the  contacts  with  the  potential  of  the  main  bat- 
tery, and  each  of  the  remaining  contacts  passed  over  by  the  brush 
sends  a  current  impulse  to  a  device  designed  to  indicate  on  the 
division  selected  for  the  call  the  tens  digit  of  the  calling  line. 

The  Connective  Division.  The  connective  division,  seven  of 
which  are  shown  in  Fig.  404,  is  an  assemblage  of  switches  comprising, 


LORIMER  AUTOMATIC  SYSTEM 


577 


as  a  whole,  a  set  suitable  for  a  complete  connection  from  calling  to 
called  subscriber.  Each  connective  division  in  the  unit  illustrated 
is  completely  equipped  to  care  for  a  called  number  of  three  digits, 
i.  e.,  each  division  will  connect  its  calling  line  with  any  one  of  one 
thousand  lines  which  may  be  called.  By  a  system  of  interconnecting 
between  divisions,  each  division  may  be  equipped  with  interconnect- 
ing apparatus  so  as  to  make  it  possible  to  complete  a  call  with  any 
one  of  ten  thousand  lines.  Each  connecting  division  of  a  ten-thou- 
sand-line exchange  comprises  six  major  switches.  Of  the  six  major 


Fig.  404.     Unit  of  Switching  Apparatus 

switches,  one  is  termed  a  secondary  connector,  another  an  intercon- 
nector,  and  the  four  remaining  are  termed  the  primary  portion  of 
the  division. 

Before  taking  up  the  operation  of  the  switches,  the  mechanical 
nature  of  the  switches  themselves  will  be  described.  The  switches 
are  built  with  a  contact  bank  cylindrical  in  form  and  with  internal 
movable  brushes  traveling  in  a  rotary  manner  in  circular  paths  upon 
horizontal  rows  of  contacts  fixed  in  the  cylindrical  banks.  For  driv- 
ing these  brushes  a  constantly  rotating  main  power-driven  shaft  is 


578  TELEPHONY 

provided.  Between  each  shaft  and  the  rotating  brushes  of  each 
major  switch  is  an  electric  clutch,  which,  by  the  movement  of  an 
armature,  causes  the  brushes  of  the  switch  to  partake  of  the  motion 
of  the  shaft  and  by  the  return  of  the  armature  to  come  again  to  rest. 
The  motion  of  the  brushes  of  the  major  switches,  or  cylinder  switches, 
as  they  are  frequently  called  because  .of  their  form,  is  constantly 
in  the  same  direction.  They  have  a  normal  position  upon  a  set  of 
the  cylinder  contacts.  They  leave  their  normal  position  and  take 
any  predetermined  position  as  controlled  by  the  magnets  of  the  clutch, 
and,  having  served  the  transient  purpose,  they  return  to  their  nor- 
mal position  by  traversing  the  remainder  of  their  complete  revolu- 
tion and  stopping  in  their  position  of  rest  or  idleness. 

The  mechanical  construction  of  each  of  the  cylinder  switches 
is  such  that  it  may  disengage  its  clutch  and  bring  its  brushes  to  rest 
only  with  the  brushes  in  some  one  of  a  number  of  predetermined 
positions.  The  locations  of  the  brushes  in  these  positions  of  rest, 
or  "stop"  positions,  as  they  are  called,  may  differ  with  the  different 
cylinder  switches,  according  to  the  nature  of  the  duty  required  of 
the  switch,  and  the  total  number  of  stop  positions  also  may  vary. 
The  primary  and  secondary  connectors,  the  interconnector  selectors, 
and  the  interconnectors  each  have  eleven  stop  positions;  the  rotary 
switch  has  eight  stop  positions;  the  signal-transmitter  controller  has 
but  two. 

In  the  six  cylinder  switches  making  up  a  connective  division 
and  required  for  any  conversation,  in  a  ten-thousand-line  exchange 
some  of  the  switches  are  set  to  positions  which  are  determined  by  the 
control  of  the  calling  subscriber  and  represent  by  their  selective  po- 
sitions the  value  of  some  digit  of  the  calling  or  called  subscriber's 
number.  Others  are  switches  controlling  the  call  in  its  progress 
and  controlling  the  switches  responsive  to  the  call.  These  latter 
switches  take  positions  independent  of  the  numbers. 

In  addition  to  the  major  switches,  there  are  upon  each  division 
four  minor  switches  termed  registers.  Each  consists  of  an  arc  of 
fixed  contacts  accompanied  by  a  set  of  brushes  which  sweep  over  the 
contacts.  Instead  of  being  driven  by  an  electromagnet,  the  register 
brushes  are  placed  under  tension  of  a  spring  which  tends  at  all  times 
to  draw  them  forward.  They  are  then  restrained  by  an  escapement 
device  similar  to  a  pallet  escapement  in  a  clock,  the  pallet  being  con- 


579 

trolled  by  the  register's  magnets.  When  a  series  of  impulses  are 
received  by  the  register  magnets,  the  pallet  is  actuated  a  corresponding 
number  of  times  and  the  register  brushes  are  permitted  to  move  for- 
ward under  tension  of  their  powerful  propelling  spring.  Each  reg- 
ister is  associated  with  a  major  switch,  and  the  register  brushes  are 
engaged  by  a  cam  upon  the  associated  major  switch,  and  are  re- 
stored to  normal  position  against  the  tension  of  their  propelling 
spring,  the  force  of  restoration  being  obtained  from  the  main  shaft. 

The  electrical  clutches  which  connect  and  disconnect  the  mov- 
able brushes  of  the  major  switches  from  the  main  driving  shaft  are 
controlled  in  all  instances  by  circuits  local  to  the  central  office.  In 
some  instances  these  circuits  include  relay  contacts  and  are  controlled 
by  a  relay.  In  other  instances  they  are  formed  solely  through  switch 
contacts.  In  all  cases  the  control,  when  from  a  distance,  is  received 
upon  relays  suitable  for  being  controlled  by  the  small  currents  which 
are  adapted  to  flow  over  long  lines.  In  all  instances  the  power  for 
moving  a  brush  is  derived  from  the  main  shaft  and  only  the  control 
of  the  movement  is  derived  from  electromagnets,  relays,  or  other 
electric  sources.  In  many  instances  the  clutch  circuit  is  closed 
through  contacts  of  its  own  switch  and,  therefore,  may  be  closed  only 
when  its  switch  is  in  some  predetermined  position.  All  of  the  switches 
are  mechanically  powerful  and  designed  particularly  to  sustain  the 
wear  of  long-continued  and  oft-repeated  usage.  This  is  true  also 
of  the  moving  parts  which  carry  the  brushes  and  of  the  journals 
sustaining  those  parts. 

The  Switches  of  the  Connective  Division.  The  six  major  switches 
of  the  connecting  division  are  as  follows: 

The  Primary  Connector: — The  function  of  this  switch  is  to 
connect  the  conductors  of  the  calling  line  with  the  switching 
devices  of  the  connective  division.  Associated  with  this  switch  is 
a  register  termed  the  decimal  register.  The  one  hundred  lines  of 
the  section  are  terminated  in  fixed  multiole  contacts  in  the  cylinder 
switch  of  the  primary  connector.  The  calling  line  is  selected  arid 
connected  with  by  adjusting  the  decimal  register  to  a  position  cor- 
responding to  the  calling  line's  tens  digit  and  adjusting  the  brushes 
of  the  cylinder  switch  to  a  position  corresponding  to  the  calling  line's 
unit  digit. 

The  Rotary  Switch: — This  is  a  master  switch,  or  pilot  switch, 


580  TELEPHONY 

consisting  of  a  cylinder  switch  without  register.  Its  duty  is  the  con- 
trol of  other  switches  and  the  completion  of  circuits  formed  in  part 
through  other  switches.  It  is  the  pilot  switch  and  the  switch  of 
initiative  and  control  for  the  entire  connective  division. 

Signal-Transmitter  Controller: — The  primary  function  of 
this  switch  is  the  generation  of  signaling  impulses  of  two  classes. 
Impulses  of  the  first  class  pass  over  central-office  circuits  only  and 
are  effective  upon  magnets  of  the  divers  major  and  minor  switches; 
impulses  of  the  second  class  pass  over  a  line  conductor  of  the  calling 
line  and  are  effective  upon  the  signal  transmitter  at  the  subscriber's 
station.  The  impulses  sent  out  over  the  line  to  the  subscriber's 
station  cause  the  brush  to  pass  over  the  contacts  and  thereby  indicate 
the  numerical  values  of  the  various  digits  set  by  the  dials.  This 
switch  also  enters  in  an  important  manner  into  the  circuits  involved 
in  the  testing  of  the  called  line  for  the  busy  condition.  It  is  con- 
trolled by  the  rotary  switch. 

Interconnector  Selector: — In  an  exchange  using  four  digits  in 
the  numbers,  the  register  of  the  intercormector  selector  is  adjusted 
in  each  call  to  a  position  corresponding  to  the  numerical  value 
of  the  thousands  digit  of  the  called  number.  The  cylinder  switch 
then  acts  to  select  an  idle  trunk.  The  switch  is  controlled  by  the 
rotary  switch  in  connection  with  the  signal  transmitter  controller. 

Interconnector: — This  switch  is  similar  to  the  interconnector 
selector  in  design  and  in  function.  It  is  a  cylinder  switch  with 
register.  The  register  is  adjusted  in  each  call  to  a  position  cor- 
responding to  the  numerical  value  of  the  hundreds  digit  of  the 
number  called  and  the  cylinder  switch  then  operates  to  select  an 
idle  trunk.  The  switch  is  controlled  by  the  rotary  switch  in  con- 
nection with  the  signal  transmitter  controller. 

Secondary  Connector: — This  switch  contains  in  its  cylinder 
bank  of  contacts  the  multiple  points  of  one  hundred  subscribers' 
lines  and  its  function  is  to  connect  the  conductors  of  the  called 
line  to  the  conductors  of  the  connective  division.  This  is  accom- 
plished by  adjusting  the  register  to  correspond  to  the  value  of  the 
tens  digit  of  the  line  desired  and  by  adjusting  the  cylinder  brushes 
to  correspond  to  the  value  of  the  units  digit  of  the  line.  The  switch 
is  controlled  by  the  rotary  switch  in  connection  with  the  signal-trans- 
mitter controller. 


LORIMER  AUTOMATIC  SYSTEM  581 

Operation.  A  brief  description  of  the  progress  of  a  call  from 
its  institution  to  the  complete  connection  and  subsequent  discon- 
nection begins  with  the  adjustment  of  the  dial  indicators  of  the  tele- 
phone set  and  the  turning  of  the  crank  of  the  signal  transmitter  one 
revolution.  This  act,  performed  by  the  calling  subscriber,  connects 
one  of  the  line  conductors  to  earth.  Immediately  the  decimal  in- 
dicator associated  with  the  section  in  which  the  calling  line  termi- 
nates is  energized  and  starts  the  division  starter.  The  division  starter 
instantly  starts  the  rotary  switch  of  an  idle  division.  The  rotary 
switch  now  starts  the  decimal-register  controller  and  connects  to  it 
the  decimal  register  of  the  primary  connector  of  the  division  selected. 

All  of  the  above  acts  in  the  central  office  occur  practically  simul- 
taneously. The  impulses  generated  by  the  controller  are  effective 
upon  the  decimal  register  of  the  started  division  and,  therefore,  ad- 
just that  register  to  a  position  corresponding  to  the  tens  value  of  the 
calling  line. 

The  rotary  switch  now  disconnects  the  tens  register  and  starts 
the  cylinder  brushes  of  the  primary  connector  which  automatically 
stop  when  they  encounter  the  calling  line.  At  this  instant  the  cut- 
off relay  of  the  line  is  energized  and  the  decimal  indicator  is  released. 
The  call  now  is  clear  of  all  sectional  apparatus  and  another  call 
may  come  through  immediately,  being  assigned  in  charge  of  another 
idle  division. 

The  total  time  in  which  any  call  is  in  charge  of  the  sectional 
apparatus,  i.  e.,  the  total  time  from  the  grounding  of  the  line  con- 
ductor at  the  sub-station  until  the  line  has  been  connected  with  by 
the  primary  connector  of  some  division  of  that  section  and  the  sec- 
tional apparatus  has  been  released  by  the  operation  of  the  cut-off 
relay,  approximates  two-fifths  of  a  second. 

The  next  operation  initiated  by  the  rotary  switch  is  the  starting 
of  the  signal-transmitter  controller  of  the  connective  division,  which, 
in  turn,  adjusts  the  register  of  the  interconnector  selector  to  a  posi- 
tion corresponding  to  the  thousands  digit  of  the  number  of  the  called 
line  as  indicated  by  the  signal  transmitter  at  the  calling  station. 
This  selects  an  interconnector  serving  the  lines  of  the  selected  thou- 
sand. 

This  initial  selection  being  completed  the  rotary  switch  readjusts 
the  circuits  of  the  connective  division  in  such  manner  that  in  the 


582  TELEPHONY 

further  progress  of  the  signal-transmitter  controller,  its  impulses  will 
be  effective  upon  the  register  of  the  selected  interconnector.  In  this 
manner,  the  register  of  the  interconnector,  which  may  be  upon  the 
same  connective  division  as  the  rotary  switch  handling  the  call, 
or  which  may  be  the  interconnector  of  some  other  division,  as  deter- 
mined by  the  number  of  the  called  subscriber,  is  adjusted  to  a  po- 
sition corresponding  to  the  second  or  hundreds  digit  of  the  number 
called.  The  cylinder  'switch  of  the  interconnector  then  selects  and 
appropriates  an  idle  trunk  extending  to  a  secondary  connector  upon 
some  connective  division  serving  the  hundred  selected. 

The  rotary  switch  again'  shifts  the  circuits  of  the  connective 
division  in  such  manner  that  the  signal-transmitter  controller  is  effec- 
tive upon  the '  secondary  connector,  both  register  and  cylinder,  and 
adjusts  the  register  and  cylinder,  respectively,  with  their  brushes  in 
contact  with  the -tens  and.  units  digits^  respectively, 'of  the  number  of 
the  called  line. 

The  conductors' of  the  called  line  now  are  connected  through  the 
secondary  connector,  the  interconnector,  and  the  interconnector 
selector  to  the  rotary  switch;  the  conductors  of  the  calling  line  are 
connected  through  the  primary  connector  to  the  rotary  switch;  thus 
completely  connecting  the  lines  except  at  the  rotary  switch.  To  effect 
the  connecting  together  of  the  two  lines,  both  rotary  switch  and 
signal-transmitter  controller  must  pass  forward  into  their  next  po- 
sitions, the  connection  when  thus  effected  being  made  through  con- 
ductor's containing  a  repeating  coil  and  main  battery  connection  for 
supplying  talking  current  to  the  two  lines  and  containing  also  ringing 
and*  supervisory  relays. 

The  called  line  is  tested  to  determine  if  busy  during  the  short 
interval  in  which  the  rotary  switch  takes  a  short  step  to  connect  the 
calling  and  the  called  lines.  In  this  step  of  the  rotary  switch  the 
busy-test  relay  is  connected  to  the  guard  wire  or  busy-test  wire  of 
the  called  line,  and  if  that  line  be  busy,  the  relay  interferes  with  the 
control  exercised  by  the  rotary  switch  upon  the  signal-transmitter 
controller,  and  the  controller  is  prevented  from  taking  the  step  re- 
quired to  connect  the  line.  Thus,  when  a  busy  line  is  encountered, 
the  final  step  of  the  rotary  switch  is  taken  to  set  up  the  conversation 
conditions,  but  the  signal-transmitter  controller  does  not  take  its 
final  step;  by  this  failure  of  the  signal-transmitter  controller  due  to 


LORIMER  AUTOMATIC  SYSTEM  583 

the  action  of  the  busy-test  relay,  the  calling  line  is  not  connected  to 
the  called  line  but  is  connected  to  a  busy-back  tone  generator  in- 
stead. 

Whether  the  line  encountered  be  busy  or  idle,  the  connective 
division  remains  in  its  condition  as  then  adjusted  until  the  subscriber 
hangs  his  receiver  upon  the  hook  switch  to  obtain  disconnection. 
The  ringing  of  the  bell  of  the  called  station  is  done  directly  by  the 
calling  subscriber  in  pressing  the  ringing  key. 

The  disconnection  is  effected,  when  the  receiver  of  the  calling 
line  is  hung  up,  by  the  supervisory  relay  in  the  central  office,  whose 
winding  is  included  in  the  line  circuit,  and  whose  contacts  act  di- 
rectly to  start  the  rotary  switch.  In  disconnecting,  the  rotary  switch 
starts  the  primary  and  the  secondary  connectors  and  thus  instantly 
releases  both  the  calling  and  the  called  lines.  Thereafter  the  rotary 
switch  in  passing  from  position  to  position  restores  switch  after  switch 
of  the  connective  division  to  normal  and  finally  itself  returns  to  normal 
in  preparation  for  its  assignment  to  service  in  answering  a  subsequent 
call. 


CHAPTER  XXXI 
THE  AUTOMANUAL  SYSTEM 

Two  systems  of  telephony  are  now  in  common  use  in  this  country 
— the  manual  system  and  the  automatic.  With  the  growth  of  the 
automatic,  and  the  gradually  ripening  conviction,  which  is  now  fully 
matured  in  the  minds  of  most  telephone  engineers,  that  automatic 
switching  is  practical,  there  has  been  a  growing  tendency  toward 
doing  automatically  many  of  the  things  that  had  previously  been 
done  manually.  One  of  the  results  of  this  tendency  has  been  the 
production  of  the  automanual  system,  the  invention  of  Edward  E. 
Clement,  an  engineer  and  patent  attorney,  of  Washington,  D.  C. 
In  connection  with  Mr.  Clement's  name,  as  inventor,  must  be  men- 
tioned that  of  Charles  H  North,  whose  excellent  work  as  a  designer 
and  manufacturer  has  contributed  much  toward  the  present  excel- 
lence of  this  highly  interesting  system. 

Characteristics  of  System.  The  name  "automanual"  is  coined 
from  the  two  words,  automatic  and  manual,  and  is  intended  to  sug- 
gest the  idea  that  the  system  partakes  in  part  of  the  features  of  the 
automatic  system  and  in  part  of  those  of  the  manual  system. 

We  regret  that  neither  space  nor  the  professional  relation  which 
we  have  had  with  the  development  of  this  system  will  permit  us  to 
make  public  an  extended  and  detailed  description  of  its  apparatus 
and  circuits.  Only  the  general  features  of  the  system  may,  there- 
fore, be  dealt  with. 

The  underlying  idea  of  the  automanual  system  is  to  relieve  the 
subscriber  of  all  work  in  connection  with  the  building  up  of  his  con- 
nection, except  the  asking  for  it;  to  complicate  the  subscriber's 
station  equipment  in  no>  way,  it  being  left  the  same  as  in  the  common- 
battery  manual  system;  to  do  away  with  manual  apparatus,  such 
as  jacks,  cords  and  plugs,  at  the  central  office,  and  to  substitute  for 
it  automatic  switching  apparatus  which  will  be  guided  in  its  move- 


AUTOMANUAL  SYSTEM  585 

ments,  not  by  the  subscriber,  but  by  a  very  much  smaller  number 
of  operators  than  would  be  necessary  to  manipulate  a  manual  switch- 
board. 

General  Features  of  Operation.  A  broad  view  of  the  operation 
of  the  system  is  this.  The  subscriber  desiring  to  make  a  call  takes 
down  his  receiver,  and  this  causes  a  lamp  to  light  in  front  of  an 
operator.  The  operator  presses  a  button  and  is  in  telephonic  com- 
munication with  the  subscriber.  Receiving  the  number  desired, 
the  operator  sets  it  up  on  a  keyboard  in  just  about  the  same  way 
that  a  typist  will  set  up  the  letters  of  a  short  word  on  a  typewriting 
machine.  The  setting  up  of  the  number  on  the  keyboard  being  ac- 
complished, the  proper  condition  of  control  of  the  associated  auto- 
matic apparatus  at  the  central  office  is  established  and  the  operator 
has  no  further  connection  with  the  call.  The  automatic  switching 
apparatus  guided  by  the  conditions  set  up  on  the  operator's  key- 
board proceeds  to  make  the  proper  selection  of  trunks  and  to  es- 
tablish the  proper  connections  through  them  to  build  up  a  talking 
circuit  between  the  calling  subscriber  and  the  called  and  to  ring  the 
called  subscriber's  bell,  or,  if  his  line  is  found  busy,  the  apparatus 
refuses  to  connect  with  it  and  sends  a  busy  signal  back  to  the  calling 
subscriber.  The  operator  performs  no  work  in  disconnecting  the 
subscribers,  that  being  automatically  taken  care  of  when  they  hang 
up  their  receivers  at  the  clc^e  of  the  conversation. 

From  the  foregoing  it  will  be  seen  that  there  is  this  fundamental 
difference  between  the  automatic  and  the  automanual — the  auto- 
matic system  dispenses  entirely  with  the  central-office  operator  for 
all  ordinary  switching  functions;  the  automanual  employs  operators 
but  attempts  to  so  facilitate  their  work  that  they  may  handle  very 
many  more  calls  than  would  be  possible  in  a  manual  system,  and  at 
the  same  time  secures  the  advantages  of  secrecy  which  the  auto- 
matic system  secures  to  its  subscribers. 

Subscriber's  Apparatus.  One  of  the  main  points  in  the  contro- 
versy concerning  automatic  versus  manual  systems  is  whether  of 
not  it  is  desirable  to  have  the  subscriber  ask  for  his  connection  or 
to  have  him  make  certain  simple  movements  with  his  fingers  which 
will  lead  to  his  securing  it.  The  developers  of  the  automanual 
system  have  taken  the  position  that  the  most  desirable  way,  so  far 
as  the  subscriber  is  concerned,  is  to  let  him  ask  for  it.  It  is  probable 


586 


TELEPHONY 


that  this  point  will  not  be  a  deciding  one  in  the  choice  of  future  sys- 
tems, since  it  already  seems  to  be  proven  that  the  subscribers  in  auto- 
matic systems  are  willing  to  go  through  the  necessary  movements 
to  mechanically  set  up  the  call.  The  advantage  which  the  auto- 


Fig.  405.     Operators'  Key  Tables 

manual  system  shares  with  the  manual,  however,  in  the  greater 
simplicity  of  its  subscriber's  station  apparatus,  cannot  be  gainsaid. 
Operator's  Equipment.  The  general  form  of  the  operator's 
equipment  is  shown  in  Fig.  405.  A  closer  view  of  the  top  of  one  of 
the  key  tables  is  shown  in  Fig.  406.  As  will  be  seen,  the  equipment 


Fig.   40fi.     Top  View  of  Key  Table 


on  each  operator's  position  consists  of  three  separate  sets  of  push- 
button keys  closely  resembling  in  external  appearance  the  keys  of 
a  typewriter  or  adding  machine.  Immediately  above  each  set  of 
keys  are  the  signal  lamps  belonging  to  that  set. 


AUTOMANUAL  SYSTEM 


587 


The  operator's  keys  are  arranged  in  strips  of  ten,  placed  across 
rather  than  lengthwise  on  the  key  shelf.  One  of  these  strips  is  shown 
in  Fig.  407.  There  are  as  many  strips  of  keys  in  each  set  as  there 
are  digits  in  the  subscribers'  numbers,  i.  e.,  three  in  a  system  having 


Fig.  407.     Strip  of  Selecting  Keys 

a  capacity  of  less  than  one  thousand;  four  in  a  system  of  less  than 
ten  thousand;  and  so  on.  In  addition  to  the  number  keys  of  each 
set  is  a  partial  row  of  keys,  including  what  is  called  a  starting  key  and 
also  keys  for  making  the  party-line  selection. 

The  simplicity  of  the  operator's  key  equipment  is  one  of  its  at- 


Fig.  408.     Wiring  of  Key  Shelf 

tractive  features.  Fig.  408  shows  one  of  the  key  shelves  opened  so 
as  to  expose  to  view  all  of  the  apparatus  and  wiring  that  is  placed 
before  the  operator.  The  reason  for  providing  more  than  one  key 


588 


TELEPHONY 


set  on  each  operator's  position  is,  that  after  a  call  has  been  set  up  on 
one  key  set,  a  few  seconds  is  required  before  the  automatic  apparatus 
controlled  by  the  key  set  can  do  its  work  and  release  the  key  set  ready 
for  another  call.  The  provision  of  more  than  one  key  set  makes  it 


Fig.  409.     Switch  Room  of  Automanual  Central  Office 

possible  for  the  operator  to  start  setting  up  another  call  on  another 
key  set  without  waiting  for  the  first  to  be  released  by  the  automatic 
apparatus. 

Automatic  Switching  Equipment.  A  general  view  of  the  ar- 
rangement of  automatic  switches  in  an  exchange  established  by  the 
North  Electric  Company  at  Ashtabula,  Ohio,  is  shown  in  Fig.  409. 
The  desk  in  the  foreground  is  that  of  the  wire  chief.  This  automatic 
apparatus  consists  largely  of  relays  and  automatic  selecting  switches. 


AUTOMANUAL  SYSTEM 


589 


The  switches  are  of  the  step-by-step  type,  having  vertical  and  rotary 
movements,  and  an  idea  of  one  of  them,  minus  its  contact  banks,  is 
given  in  Fig.  410.  The  control  of  the  automatic  switches  by  the  op- 
erator's key  sets  is  through  the  medium  of  a  power-driven,  impulse- 
sending  machine.  From  this  machine  impulses  are  taken  corre- 
sponding to  the  numbers  of  the  keys  depressed. 

Automatic  Distribution  of  Calls.  A  feature  of  great  interest 
in  this  system  is  the  manner  in  which  the  incoming  calls  are  distrib- 
uted among  the  operators.  From  each  key  set  an  operator's  trunk 
is  extended  to  what  is  called  a  secondary  selector  switch,  through 
which  it  may  be  con- 
nected to  a  primary  se- 
lector trunk  and  calling 
line.  When  a  subscrib- 
er calls  by  taking  down 
his  receiver,  his  line  re- 
lay pulls  up  and  causes 
a  primary  selector  switch 
to  connect  his  line  with 
an  idle  local  trunk  or 
link  circuit,  at  the  same 
time  starting  up  a  sec- 
ondary selector  switch 
which  immediately  con- 
nects the  primary  trunk 
and  the  calling  line  to 
an  operator's  idle  key 
set.  If  an  operator  is 
at  the  time  engaged  in 
setting  up  a  call  on  a 
key  set,  or  if  that  key 
set  is  still  acting  to  con- 
trol the  sending  of  im- 
pulses to  the  automatic  switches,  it  may  be  said  to  be  busy,  and 
it  is  not  selected  by  this  preliminary  selecting  apparatus  in  response 
to  an  incoming  call.  As  soon,  however,  as  the  necessary  impulses 
have  been  taken  from  the  key  set  by  the  automatic  apparatus,  that 
key  set  is  released  and  is  again  ready  to  receive  a  call.  In  this  way 


Fig.  410.     Selecting  Switch 


590  TELEPHONY 

the  calls  come  before  each  operator  only  as  that  operator  is  able  and 
ready  to  receive  them. 

Setting  up  a  Connection.  As  soon  as  the  key-set  lamp  lights, 
in  response  to  such  an  incoming  call,  the  operator  presses  a  listening 
button,  receives  the  number  from  the  subscriber,  and  depresses  the 
corresponding  number  buttons  on  that  key  set,  thereby  determining 
the  numbers  in  each  of  the  series  of  impulses  to  be  sent  to  the  selector 
and  the  connector  switches  to  make  the  desired  connection.  The 
operator,  repeats  this  number  to  the  calling  subscriber  as  she  sets  it 
up,  and  then  presses  the  starting  button,  whereupon  her  work  is 
done  so  far  as  that  call  is  concerned.  If,  upon  repeating  the  call  to 
the  subscriber,  the  operator  finds  that  she  is  in  error,  she  may  change 
the  number  set  up  at  any  time  before  she  has  pressed  the  starting 
button. 

Building  up  a  Connection.  The  keys  so  set  up  determine 
the  number  of  impulses  that  will  be  transmitted  by  the  impulse- 
sending  machine  to  the  selector  and  the  connector  switches.  These 
switches,  impelled  by  these  impulses,  establish  the  connection  if 
the  line  called  for  is  not  already  connected  to.  If  a  party-line  sta- 
tion is  called  for,  the  proper  station  on  it  will  be  selectively  rung 
as  determined  by  the  party-line  key  depressed  by  the  operator.  If 
the  line  is  found  busy,  the  connector  switch  refuses  to  make  the  con- 
nection and  places  a  busy-back  signal  on  the  calling  line. 

Speed  in  Handling  Calls.  This  necessarily  brief  outline  gives 
an  idea  only  of  the  more  striking  features  of  the  automanual  system. 
A  study  of  the  rapidity  with  which  calls  may  be  handled  in  actual 
practice  shows  remarkable  results  as  compared  with  manual  meth- 
ods of  operating.  The  operators  set  up  the  number  keys  corre- 
sponding to  a  called  number  with  the  same  rapidity  that  the  keys  of  a 
typewriter  are  pressed  in  spelling  a  word.  In  fact,  even  greater 
speed  is  possible,  since  it  is  noticed  that  the  operators  frequently 
will  depress  all  of  the  keys  of  a  number  at  once,  as  by  a  single  striking 
movement  of  the  fingers.  The  rapidity  with  which  this  is  done 
defies  accurate  timing  by  a  stop  watch  in  the  hands  of  an  expert. 
It  is  practically  true,  therefore,  that  the  time  consumed  by  the  oper- 
ator in  handling  any  one  call  is  that  which  is  taken  in  getting  the 
number  from  the  subscriber  and  in  repeating  it  back  to  him. 

Owing   to   the    difficulty   of  securing  accurate    traffic  data  by 


AUTOMANUAL  SYSTEM 


591 


TABLE  XI 

Total  Time  Consumed  by  Operator  in  Handling  Calls  on 
Automanual  System 


First  100  Calls 


Longest  Individual  Period 12.40  seconds 

Average  five  longest  Individual  Periods 7.44  seconds 

Average  ten  longest  Individual  Periods 6.34  seconds 

Shortest  Individual  Period 1.60  seconds 

Average  five  shortest  Individual  Periods 1 .92  seconds 

Average  ten  shortest  Individual  Periods 1 .96  seconds 

Average  Entire  100  Calls 3.396  seconds 

Hourly  Rate  at  which  calls  were  being  handled 1060 

Second  100  Calls 

Longest  Individual  Period 7.60  seconds 

Average  five  longest  Individual  Periods 5.52  seconds 

Average  ten  longest  Individual  Periods 5.34  seconds 

Shortest  Individual  Period 2.00  seconds 

Average  five  shortest  Individual  Periods 2.04  seconds 

Average  ten  shortest  Individual  Periods. 2.18  seconds 

Average  Entire  100  Calls 3.374  seconds 

Hourly  Rate  at  which  calls  were  being  handled 1067 

Third  100  Calls 

Longest  Individual  Period : 5.40  seconds 

Average  five  longest  Individual  Periods 5.32  seconds 

Average  ten  longest  Individual  Periods 4.44  seconds 

Shortest  Individual  Period 1.60  seconds 

Average  five  shortest  Individual  Periods 1 .65  seconds 

Average  ten  shortest  Individual  Periods 1 .80  seconds 

Average  Entire  100  Calls 3.160  seconds 

Hourly  Rate  at  which  calls  were  being  handled 1139 


means  of  a  stop  watch,  an  automatic,  electrical  timing  device, 
capable  of  registering  seconds  and  hundredths  of  a  second,  has 
been  used  in  studying  the  performance  of  this  system  in  regular 
operation  at  Ashtabula  Harbor.  The  operators  were  not  informed 
that  the  records  were  being  taken,  and  the  data  tabulated  represents 
the  work  of  two  operators  in  handling  regular  subscribers'  calls. 
The  figures  in  Table  XI  are  given  by  C.  H.  North  as  representing  the 


592  TELEPHONY 

total  time  consumed  by  the  operator  from  the  time  her  line  lamp  was 
lighted  until  her  work  in  connection  with  the  call  was  finished,  and  it 
included,  therefore,  the  pressing  of  the  listening  button,  the  receiv- 
ing of  the  number  from  the  subscriber,  repeating  it  back  to  him,  set- 
ting up  the  connection  on  the  keys,  and  pressing  the  starting  key. 

It  will  be  seen  that  the  average  time  for  each  100  calls  is  quite 
uniform  and  is  slightly  over  three  seconds.  The  considerable  varia- 
tion in  the  individual  calls,  ranging  from  a  maximum  of  12.40  seconds 
down  to  a  minimum  of  1.60  seconds,  is  due  almost  entirely  .to  the 
difference  between  the  subscribers  in  the  speed  with  which  they 
can  give  their  numbers.  These  figures  indicate  that,  in  each  of  the 
tests,  calls  were  being  handled  at  the  rate  of  more  than  one  thou- 
sand per  hour  by  each  operator. 

The  test  of  the  subscriber's  waiting  time,  i.  e.,  the  time  that  he 
waited  for  the  operator  to  answer,  for  one  hundred  calls  made  with- 
out the  knowledge  of  the  operator,  showed  the  results  as  given  in 
Table  XII,  in  which  a  split  second  stop  watch  was  used  in  making 
the  observations. 

TABLE  XII 
Subscribers'  Waiting  Time 


Number  of  Calls  Tested 100 

Longest  Individual  Period 5.20  seconds 

Average  5  Longest  Individual  Periods 4.64  seconds 

Average  10  Longest  Individual  Periods 3.80  seconds 

Shortest  Individual  Period 1 .00  seconds 

Average  5  Shortest  Individual  Periods 1.28  seconds 

Average  10  Shortest  Individual  Periods 1.34  seconds 

Average  Entire  100  Calls 2.07  seconds 


The  length  of  time  which  the  subscriber  has  to  wait  before  re- 
ceiving an  answer  from  the  operator  is,  of  course,  one  of  the  factors 
that  enters  into  the  giving  of  good  telephone  service,  and  the  times 
shown  by  this  test  are  considerably  shorter  than  ordinarily  main- 
tained in  manual  practice.  The  waiting  time  of  the  subscriber  is 
not,  of  course,  a  part  of  the  time  that  is  consumed  by  the  operator, 
and  the  real  economy  so  far  as  the  operator's  time  is  concerned  is 
shown  in  the  tests  recorded  in  Table  XL 


CHAPTER   XXXII 

POWER  PLANTS 

The  power  plant  is  an  organization  of  devices  to  furnish  to  a  tel- 
ephone system  the  several  kinds  of  current,  at  proper  pressures,  for  the 
performance  of  the  several  general  electrical  tasks  within  the  exchange. 

Kinds  of  Currents  Employed.  Sources  of  both  direct  and  alter- 
nating current  are  required  and  a  single  exchange  may  employ  these 
for  one  or  more  of  the  following  purposes: 

Direct  Current.  Current  which  flows  always  in  one  direction 
whether  steady  or  varying,  is  referred  to  as  direct  current,  and  may 
be  required  for  transmitters,  for  relays,  for  line,  supervisory,  and 
auxiliary  signals,  for  busy  tests,  for  automatic  switches,  for  call  reg- 
isters, for  telegraphy,  and  in  the  form  of  pulsating  current  for  the 
ringing  of  biased  bells. 

Alternating  Current.  Sources  of  alternating  current  are  re- 
quired for  the  ringing  of  bells,  for  busy-back  and  other  automatic 
signals  to  subscribers,  for  howler  signals  to  attract  the  attention  of 
subscribers  who  have  left  their  receivers  off  their  hooks,  and  for  sis:- 

7  O 

naling  over  composite  lines. 

Types  of  Power  Plants.  Clearly  the  requirements  for  cur- 
rent supply  differ  greatly  for  magneto  and  common-battery  systems. 
There  is,  however,  no  great  difference  between  the  power  plants 
required  for  the  automatic  and  the  manual  common-battery  systems. 

In  the  simplest  form  of  telephone  system — two  magneto  tele- 
phones on  a  private  line — the  power  plant  at  each  station  consists  of 
two  elements:  one,  the  magneto  generator,  which  is  a  translating 
device  for  turning  hand  power  into  alternating  current  for  ringing 
the  bell  of  the  distant  station ;  and  the  other,  a  primary  battery  which 
furnishes  current  to  energize  the  transmitter.  In  such  a  system, 
therefore,  each  telephone  has  its  own  power  plant.  The  term  power 
plant,  however,  as  commonly  employed  in  telephone  work,  refers  more 
particularly  to  the  organization  of  devices  at  the  central  office  for 
furnishing  the  required  kinds  of  current,  and  it  is  to  power  plants 
in  this  sense  that  this  chapter  is  devoted. 


594  TELEPHONY 

Magneto  Systems.  If  magneto  lines  be  connected  to  a  switch- 
board, the  current  for  throwing  the  drop  at  the  switchboard  is  fur- 
nished by  the  subscriber's  generator,  and  the  current  for  energizing 
the  subscriber's  transmitter  is  furnished  by  the  local  battery  at  his 
station;  but  sources  of  current  must  be  provided  for  enabling  the 
central-office  operator  to  signal  or  talk  to  the  subscribers.  These 
are  about  the  only  needs  for  which  current  must  be  furnished  in  an 
ordinary  magneto  central  office.  If  a  multiple  board  is  employed, 
direct  current  is  also  needed  for  the  purpose  of  the  busy  test  and 
also  for  operating  the  drop  restoring  circuits,  if  the  electrical  method 
of  restoring  the  drops  is  employed. 

Common-Battery  Systems.  In  common-battery  systems  the 
requirements  are  very  much  more  extensive.  The  subscribers'  tele- 
phones have  no  power  plants  of  their  own,  but  are  provided  with  a 
common  source  of  direct  current  located  at  the  central  office  for  sup- 
plying the  talking  current,  and  for  operating  the  central-office  sig- 
nals, and  the  operators  -are  provided  with  one  or  more  common 
sources  of  alternating  or  pulsating  current  for  ringing  the  subscrib- 
ers' bells.  Common-battery  equipment  requires  the  use  of  currents 
of  different  kinds  for  a  greater  number  of  auxiliary  purposes  than 
does  magneto  equipment.  These  facts  make  the  power  plant  in  a 
common-battery  office  much  more  important  than  in  a  magneto 
office. 

Operators'  Transmitter  Supply.  In  a  small  magneto  exchange, 
the  transmitter  current  may  be  had  from  primary  batteries,  a  separate 
battery  being  employed  for  each  operator's  set.  When  there  are  more 
than  three  or  four  operators,  however,  it  is  usual,  even  in  magneto 
offices,  to  obtain  the  transmitter  current  from  a  common  storage 
battery.  A  storage  battery  has  the  fortunate  quality  of  very  low 
internal  resistance,  therefore  a  number  of  operators'  transmitters  may 
be  actuated  by  one  source  without  introducing  cross-talk.  In  other 
words,  a  storage  battery  is  a  current-furnishing  device  of  good  regu- 
lation, the  variation  of  consumption  in  one  circuit  leading  from  it 
causing  slight  variation  in  the  currents  of  other  circuits  leading  from 
it.  If  this  were  not  so,  cross-talk  would  exist  between  the  telephones 
of  the  operators'  positions  connected  to  the  same  battery.  This 
regulating  quality  enables  the  multiple  feeding  of  telephone  circuits 
to  be  carried  further  than  the  mere  supplying  of  operators'  sets  and 


POWER  PLANTS  595 

is  the  quality  which  makes  possible  the  successful  use  of  a  storage 
battery  as  the  single  source  of  transmitter  current  for  common- 
battery  central-office  equipment. 

In  furnishing  a  plurality  of  operators'  transmitters  from  a  com- 
mon battery,  the  importance  of  low  "resistance  and  inductance  in  the 
portion  of  the  path  that  is  common  to  all  of  the  circuits  must  not  be 
overlooked.  Not  only  is  a  battery 'of  extremely  low  resistance  re- 
quired, but  also  conductors  leading  from  it  that  are  common  to  two 
or  more  of  the  circuits  should  be  of  very  low  resistance  and  conse- 
quently large  in  cross-section  arid  as  short  as  possible.  In  com- 
mon-battery offices  there  is  obviously  no  need  of  employing  a  sep- 
arate battery  for  the  operators'  transmitters,  since  they  may  readily 
be  supplied  from  the  common  storage  battery  which  supplies  direct 
current  to  the  subscribers'  lines. 

Ringing=Current  Supply.  Magneto  Generators.  As  a  central- 
office  equipment  is  required  to  ring  many  subscribers'  bells,  only  the 
small  ones  find  it  convenient  to  ring  them  by  means  of  hand-operated 
magneto  generators.  Small  magneto  switchboards  are  usually 
equipped  so  that  each  operator  is  provided  with  a  hand-generator, 
but  even  where  such  is  the  case  some  source  of  ringing  current  not 
manually  operated  is  desirable.  In  larger  switchboards  the  hand 
generators  are  entirely  dispensed  with. 

The  magneto  generator  may  be  driven  by  a  belt  from  any  con- 
venient constantly  moving  pulley,  and  the  early  telephone  exchanges 
were  often  equipped  with  such  generators  having  better  bearings 
and  more  current  capacity  than  those  in  magneto  telephones.  These 
were  adapted  to  be  run  constantly  from,  some  source  of  power,  de- 
livering ringing  current  to  the  operators'  keyboards  at  from  16  to  20 
cycles  per  second. 

Pole  Changers.  Vibrating  pole  changers  were  also  used  in  the 
early  exchanges,  but  passed  out  of  use,  partly  because  of  poor  design, 
but  more  because  of  the  absence  of  good  forrns  of  .primary  batteries 
for  vibrating  them  and  for  furnishing  the  /direct  currents  to  be  trans- 
formed into  alternating  line  current  for  ringing  the  bells.  The  pole 
changer  was  redesigned  after  the  beginning  of  the  great  spread  of 
telephony  in  the  United  States  in  1893.  Today  it  is  firmly  established 
as  an  element  of  good  telephone  practice.  Fig.  411  illustrates  the 
principle  upon  which  one  of  the  well-known  pole  changers— the. 


596 


TELEPHONY 


Warner — operates.  In  this  1  is  an  electromagnet  supplied  by  a 
constant-current  battery  2  to  keep  the  vibratory  system  continually 
in  motion.  This  motor  magnet  and  its  battery  work  in  a  local  cir- 
cuit and  cause  vibration  in  exactly  the  same  manner  as  the  armature 
of  an  ordinary  electric  door  bell  is  caused  to  vibrate.  The  battery 

from  which  the  ringing  current 
is  derived  is  indicated  at  3,  and 
the  poles  of  this  are  connected, 
respectively,  to  the  vibrating  con- 
tacts 4  and  5.  These  contacts 
are  merely  the  moving  members 
of  a  pole  changing  switch,  and  a 
study  of  the  action  will  readily 
show  that  when  these  moving 
parts  engage  the  right-hand  con- 
tacts, current  will  flow  to  the  line 
supposed  to  be  connected  to  the 
terminals  6  and  7  in  one  direc- 
tion, while,  when  these  parts  en- 
gage the  left-hand  contacts,  cur- 
rent will  flow  to  the  line  in  the 
reverse  direction.  The  circuit  of 


1 

( 

>- 

/ 

II*    , 

Pig.  411.     Warner  Pole  Changer 


the  condenser  shown  is  controlled 
by  the  armature  of  the  relay  8. 

The  winding  of  this  relay  is  put  directly  in  the  circuit  of  the 
main  battery  3,  so  that  whenever  current  is  drawn  from  this  battery 
to  ring  a  distant  bell,  this  relay  will  be  operated  and  will  bridge 
the  condenser  across  the  circuit  of  the  line.  The  purpose  of  the  con- 
denser is  to  make  the  impulses  flowing  from  the  pole  changer  less 
abrupt,  and  the  reason  for  having  its  bridged  circuit  normally  broken 
is  to  prevent  a  waste  of  current  from  the  battery  3,  due  to  the  energy 
which  would  otherwise  be  consumed  by  the  condenser  if  it  were  left 
permanently  across  the  line. 

Pole  changers  for  ringing  bells  of  harmonic  party  lines  are  required 
to  produce  alternating  currents  of  practically  constant  frequencies. 
The  ideal  arrangement  is  to  cause  the  direct  currents  from  a  storage 
battery  to  be  alternated  by  means  of  the  pole  changers,  and  then 
transformed  into  higher  voltages  required  for  ringing  purposes,  the 


POWER  PLANTS 


597 


transformer  also  serving  to  smooth  the  current  wave,  making  it  more 
suitable  for  ringing  purposes.  In  Fig.  412  such  an  arrangement, 
adapted  to  develop  currents  for  harmonic  ringing  on  party  lines, 
is  shown.  The  regular  common  battery  of  the  central  office  is  in- 
dicated at  1,  2  being  an  auxiliary  battery  of  dry  cells,  the  purpose  of 
which  will  be  presently  referred  to.  At  the  right  of  the  battery  1 
there  is  shown  the  calling  plug  with  its  associated  party-line  ringing 
keys  adapted  to  impress  the  several  frequencies  on  the  subscribers' 
lines.  The  method  by  which  the  current  from  the  main  storage  bat- 
tery passes  through  the  motor  magnets  of  the  several  vibrators, 


Fig.  412.     Pole  Changers  for  Harmonic  Ringing 

and  by  which  the  primary  currents  through  the  transformers  are 
made  to  alternate  at  the  respective  frequencies  of  these  vibrators, 
will  be  obvious  from  the  drawing.  It  is  also  clear  that  the  secondary 
currents  developed  in  these  transformers  are  led  to  the  several  ring- 
ing keys  so  as  to  be  available  Tui  connection  with  the  subscribers' 
lines  at  the  will  of  the  operator.  The  condensers  are  bridged  across 
the  primary  windings  of  the  transformers  for  the  purpose  of  aiding  in 
smoothing  out  the  current  waves.  The  use  of  the  auxiliary  battery 
2  and  the  retardation  coil  3  in  the  main  supply  lead  is  for  the  pur- 


598  TELEPHONY 

pose  preventing  the  pulsating  currents  drawn  from  the  main 
battery  1  from  making  the  battery  "noisy."  These  two  batter- 
ies have  like  poles  connected  to  the  supply  lead,  and  the  auxiliary 
battery  furnishes  no  current  to  the  system  except  when  the  electro- 
motive force  of  the  impulse  flowing  from  the  main  battery  is  choked 
down  by  the  impedance  coil  and  the  deficiency  is  then  momentarily 
supplied  for  each  wave  by  the  auxiliary  battery.  This  is  the  method 
developed  by  the  Dean  Electric  Company  for  preventing  the  pole- 
changer  system  from  causing  disturbances  on  lines  supplied  from 
the  same  main  battery. 

Ringing  Dynamos.  Alternating  and  pulsating  currents  for 
ringing  purposes  are  also  largely  furnished  from  alternating-current 
dynamos  similar  to  those  used  in  commercial  power  and  lighting 
work,  but  specially  designed  to  produce  ringing  currents  of  proper 
frequency  and  voltage.  These  are  usually  driven  by  electric  motors 
deriving  their  current  either  from  the  commercial  supply  mains  or 
from  the  central-office  battery.  In  large  exchanges  harmonic  ring- 
ers are  usually  operated  by  alternating-current  generators  driven  by 
motors,  a  separate  dynamo  being  provided  to  furnish  the  current  of 
each  frequency.  Fig.  413  shows  a  set  of  four  such  generators  directly 
connected  to  a  common  motor.  As  no  source  of  commercial  power 


Fig.  413.    Multi-Cyclic  Generator  Set 

for  driving  such  generators  is  absolutely  uniform,  and  since  the  fre- 
quency of  the  ringing  current  must  remain  very  close  to  a  constant 
predetermined  rate,  some  means  must  be  employed  for  holding 
the  generators  at  a  constant  speed  of  revolution,  and  this  is  done 
by  means  of  a  governor  shown  at  the  right-hand  end  of  the  shaft  in 


POWER  PLANTS 


599 


Fig.  413.  The  principle  of  this  governor  is  shown  in  Fig.  414.  A 
weighted  spring  acts,  by  centrifugal  force,  to  make  a  contact  against 
an  adjustable  screw,  when  the  speed  of  the  shaft  rises  a  predeter- 
mined amount.  This  spring  and  its  contact  are  connected  to  two 
collector  rings  1  and  2  on  the  motor  shaft,  and  connection  is  made 
with  these  by  the  brushes  3  and  4-  The  closing  of  the  governor 


SHAFT 


Fig.  414.     Governor  for  Harmonic  Ringing  Generators 

contact  serves,  therefore,  merely  to  short-circuit  the  resistance  5, 
which  is  normally  included  in  the  shunt  field  of  the  motor.  This 
governor  is  based  on  the  principle  that  weakening  the  field  increases 
the  speed.  It  acts  to  insert  the  resistance  in  series  with  the  field 
winding  when  the  speed  falls,  and  this,  in  turn,  results  in  restoring 
the  speed  to  normal. 

Auxiliary  Signaling  Currents.  Alternating  currents,  such 
as  those  employed  for  busy  signals  to  subscribers  in  automatic  systems, 
those  for  causing  loud  tones  in  receivers  which  have  been  left  off 
the  hook  switch,  and  those  for  producing  loud  tones  in  calling  receivers 
connected  to  composite  lines,  all  need  to  be  of  much  higher  frequency 
than  alternating  current  for  ringing  bells.  The  simplest  way  of 
producing  such  tones  is  by  means  of  an  interrupter  like  that  of  a 
vibrating  bell ;  but  this  is  not  the  most  reliable  way  and  it  is  usual 
to  produce  busy  or  "busy-back"currents  by  rotating  commutators 
to  interrupt  a  steady  current  at  the  required  rate.  As  the  usual 
busy-back  signal  is  a  series  of  recurrent  tones  about  one-half  second 
long,  interspersed  with  periods  of  silence,  the  rapidly  commuted 
direct  current  is  required  to  be  further  commuted  at  a  slow  rate,  and 
this  is  conveniently  done  by  associating  a  high-speed  commutator  with 
a  low-speed  one  Such  an  arrangement  may  be  seen  at  the  left-hand 


600  TELEPHONY 

end  of  the  multicyclic  alternating  machine  shown  in  Fig.  413.  This 
commuting  device  is  usually  associated  with  the  ringing  machine 
because  that  is  the  one  thing  about  a  central  office  that  is  available  for 
imparting  continuous  rotary  motion. 

Primary  Sources.  Most  telephone  power  plants  consume  com- 
mercial electric  power  and  deliver  special  electric  current.  Usually 
some  translating  device,  such  as  a  motor-generator  or  a  mercury-arc 
rectifier,  is  employed  to  transform  the  commercial  current  into  the 
specialized  current  required  for  the  immediate  uses  of  the  exchange. 

Charging  from  Direct-Current  Mains.  In  some  cases  com- 
mercial direct  current  is  used  to  charge  the  storage  batteries  without 
the  intervention  of  the  translating  devices,  resistances  being  used  in 
series  with  the  battery  to  regulate  the  amount  of  current.  Commer- 
cial direct  current  usually  is  available  at  pressures  from  110  volts 
and  upward,  while  telephone  power  plants  contain  storage  batteries 
rarely  of  pressures  higher  than  50  volts.  To  charge  a  50-volt  stor- 
age battery  direct  from  110-volt  mains  results  in  the  loss  of  about  half 
the  energy  purchased,  this  lost  energy  being  set  free  in  the  form  of 
heat  generated  in  the  resistance  devices.  Notwithstanding  this,  it 
is  sometimes  economical  to  charge  directly  from  the  commercial 
direct-current  power  mains,  but  only  in  small  offices  where  the  total 
amount  of  current  consumed  is  not  large  and  where  the  greatest 
simplicity  in  equipment  is  desirable.  It  is  better,  however,  in  nearly 
all  cases,  to  convert  the  purchased  power  from  the  received  voltage 
to  the  required  voltage  by  some  form  of  translating  device,  such 
as  a  rotary  converter  or  a  mercury-arc  rectifier. 

Rotary  Converters.  Broadly  speaking,  a  rotary  converter 
consists  of  a  motor  adapted  to  the  voltage  and  kind  of  current  re- 
ceived, mechanically  coupled  to  a  generator  adapted  to  produce 
current  of  the  required  kind  and  voltage.  The  harmonic  ringing 
machine  shown  in  Fig.  413  is  an  example  of  this,  this  particular  one 
being  adapted  to  receive  direct  current  at  ordinary  commercial  pres- 
sure and  to  deliver  four  different  alternating  currents  of  suitable 
pressures  and  frequencies.  It  is  to  be  understood,  however,  that  the 
conversion  may  be  from  direct  current  to  direct  current,  from  alter- 
nating to  direct,  or  from  direct  to  alternating.  Such  a  device  where 
the  motor  is  a  separate  and  distinct  machine  from  the  generator  or 
generators  is  called  a  motor-generator.  It  is  usual  to  connect  the 


POWER  PLANTS  601 

motors  and  the  generators  together  directly  by  a  coupling  having  some 
flexibility,  as  shown  in  Fig.  413,  so  as  to  prevent  undue  friction  in 
the  bearings. 

As  an  alternative  to  the  converting  device  made  up  of  a  motor 
coupled  to  a  generator,  both  motor  and  generator  windings  may 
be  combined  on  the  same  core  and  rotate  within  the  same  field. 
Such  a  rotary  converter  has  been  called  a  dynamotor.  As  a  rule  the 
dynamotor  is  only  suitable  for  small  power-plant  work.  It  has  the 
following  objectionable  features:  (a)  It  is  difficult  to  regulate 
its  output,  since  the  same  field  serves  for  both  the  motor  and  the  dy- 
namo windings.  For  this  reason  its  main  use  is  as  a  ringing  ma- 
chine where  the  regulation  of  the  output  is  not  an  important  factor. 
(b)  Furthermore,  the  fact  that  the  motor  and  dynamo  armature 
windings  are  on  the  same  core  makes  it  difficult  to  guard  against 
breakdowns  of  the  insulation  between  the  two  windings,  especially 
when  the  driving  current  is  of  high  voltage. 

Charging  Dynamos.  The  dynamo  for  charging  the  stoiage 
battery  is,  of  course,  a  direct-current  machine  and  may  be  a  part  of 
a  motor  generator  or  it  may  derive  its  power  from  some  other  than 
an  electric  motor,  such  as  a  gas  or  steam  engine.  It  should  be  able 
to  develop  a  voltage  slightly  above  that  of  the  voltage  of  the  storage 
battery  when  at  its  maximum  charge,  so  as  always  to  be  able  to  de- 
liver current  to  the  charging  battery  regardless  of  the  state  of  charge. 
A  30-volt  generator,  for  example,  can  charge  eleven  cells  in  series 
economically;  a  60- volt  generator  can  charge  twenty-five  cells  in 
series  economically. 

Battery-charging  generators  are  controlled  as  to  their  output 
by  varying  a  resistance  in  series  with  their  fields.  Such  machines 
are  usually  shunt-wound.  Sometimes  they  are  compound-wound, 
but  compounding  is  less  important  in  telephone  generators  than  in 
some  other  uses.  A  feature  of  great  importance  in  the  design  of 
charging  generators  is  smoothness  of  current.  If  it  were  possible 
to  design  generators  to  produce  absolutely  even  or  smooth  current, 
the  storage  battery  would  not  be  such  an  essential  feature  to  com- 
mon-battery exchanges,  because  then  the  generator  might  deliver 
its  current  directly  to  the  bus  bars  of  the  office  without  any  storage- 
battery  connection  and  without  causing  noise  on  the  lines.  Such 
generators  have  been  built  in  small  units.  Even  if  these  smooth  cur- 


602  TELEPHONY 

rent  generators  were  commercially  developed  to  a  degree  to  produce 
absolutely  no  noise  on  the  lines,  the  storage  battery  would  still  be  used, 
since  its  action  as  a  reservoir  for  electrical  energy  is  important.  It 
not  only  dispenses  with  the  necessity  of  running  the  generators  con- 
tinuously, but  it  also  affords  a  safeguard  against  breakdowns  which 
is  one  of  its  important  uses. 

The  ability  to  carry  the  load  of  a  central  office  directly  on  the 
charging  generator  without  the  use  of  a  storage  battery  is  of  no  im- 
portance except  in  an  emergency  which  takes  the  storage  battery 
wholly  out  of  service.  Since  the  beginning  of  common-battery 
working  such  emergencies  have  happened  a  negligible  number  of 
times.  Far  more  communities  have  lacked  telephone  service  because 
of  accidents  beyond  human  control  than  because  of  storage-battery 
failures. 

In  power  plants  serving  large  offices,  the  demand  upon  the 
storage  battery  is  great  enough  to  require  large  plate  areas  in  each 
cell.  The  internal  resistance,  therefore,  is  small  and  considerable 
fluctuations  may  exist  in  the  charging  current  without  their  being 
heard  in  the  talking  circuits.  The  amount  of  noise  to  be  heard  de- 
pends also  on  the  type  of  charging  generator.  Increasing  the  num- 
ber of  armature  coils  and  commutator  segments  increases  the  smooth- 
ness of  the  charging  current.  The  shape  of  the  generator  pole 
pieces  is  also  a  factor  in  securing  such  smoothness. 

If,  with  a  given  machine  and  storage  battery,  the  talking  circuits 
are  disturbed  by  the  charging  current,  relief  may  be  obtained  by 
inserting  a  large  impedance  in  the  charging  circuit.  This  impe- 
dance requires  to  be  of  low  resistance,  because  whatever  heat  is  de- 
veloped in  it  is  lost  energy.  This  means  that  the  best  conditions 
exist  when  the  resistance  is  low  and  the  inductance  large.  These 
conditions  are  satisfied  by  using  in  the  impedance  coil  many  turns 
of  large  wire  and  an  ample  iron  core. 

Dynamotors  are  not  generally  suitable  for  charging  purposes. 
Not  only  is  the  difficulty  in  regulating  their  output  a  disadvantage, 
but  the  fact  that  the  primary  and  secondary  windings  are  so  closely 
associated  on  the  armature  core  makes  them  carry  into  the  charg- 
ing current,  not  only  the  commutator  noises  of  the  generator  end,  but 
of  the  motor  end  as  well. 

Mercury-Arc    Rectifiers.      In  common-battery  offices  serving  a 


POWER  PLANTS 


603 


few  hundred  lines,  and  where  the  commercial  supply  is  alternating 
current,  it  is  good  practice  to  transform  it  into  direct-battery  charg- 
ing current  by  means  of  a  mercury-arc  rectifier.  It  is  a  device 
broadly  similar  to  the  mercury-arc  lamp  produced  by  Peter  Cooper 
Hewitt.  It  contains  no 
moving  parts  and  operates 
at  high  efficiency  without 
introducing  noises  into 
the  telephone  lines.  It 
requires  little  care  and 
has  good  length  of  life. 

The  circuit  of  a  mer- 
cury-arc rectifier  charging 
outfit  is  shown  in  Fig.  415. 
The  mercury-arc  rectifier 
proper  consists  of  a  glass 
bulb  containing  vacuum 
and  a  small  amount  of 
mercury.  When  its  termi- 
nals are  connected,  as  in- 
dicated— the  two  anodes 
across  an  alternating-current  source  and  the  cathode  with  a  circuit  that 
is  to  be  supplied  with  direct  current — this  device  has  the  peculiarity 
of  action  that  current  will  flow  alternately  from  the  two  anodes  always 
to  the  cathode  and  never  from  it.  The  cathode,  therefore,  becomes 
a  source  of  positive  potential  and,  as  such,  is  used  in  charging  the 
storage  battery  through  the  series  reactance  coil  and  the  compen- 
sating reactances,  as  indicated.  The  line  transformer  shown  at  the 
upper  portion  of  Fig.  415,  is  the  one  for  converting  the  high-poten- 
tial alternating  current  to  the  comparatively  low-potential  current 
required  for  the  action  of  the  rectifier.  The  transformer  below  this 
has  a  one-to-one  ratio,  and  is  called  the  insulating  transformer.  Its 
purpose  is  to  safeguard  the  telephone  apparatus  and  circuits  against 
abnormal  potentials  from  the  line,  and  also  to  prevent  the  ground, 
which  is  commonly  placed  on  the  neutral  wire  of  transformers  on 
commercial  lighting  circuits,  from  interfering  with  the  ground  that 
is  commonly  placed  on  the  positive  pole  of  the  central-office  battery. 

Provision    Against    Breakdown.     In  order  to    provide  against 


Fig.  415.     Mercury-Arc  Rectifier  Circuits 


604  TELEPHONY 

breakdown  of  service,  a  well-designed  telephone  power  plant  should 
have  available  more  than  one  primary  source  of  power  and  more 
than  one  charging  unit  and  ringing  unit. 

Duplicate  Primary  Sources.  In  large  cities  where  the  com- 
mercial power  service  is  highly  developed  and  a  breakdown  of  the 
generating  station  is  practically  impossible,  it  is  customary  to  de- 
pend on  that  service  alone.  In  order  to  insure  against  loss  of  power 
due  to  an  accident  to  portions  of  the  distributing  system,  it  is  the  com- 
mon custom  to  run  two  entirely  separate  power  leads  into  the  office, 
coming,  if  possible,  from  different  parts  of  the  system  so  that  a  break- 
down on  one  section  will  not  deprive  the  telephone  exchange  of 
primary  power.  In  smaller  places  where  the  commercial  service 
is  not  so  reliable,  it  is  usual  to  provide,  in  addition  to  the  commer- 
cial electric-power  service,  an  independent  source  of  power  in  the 
form  of  a  gas  or  steam  engine.  This  may  be  run  as  a  regular  source, 
the  commercial  service  being  employed  as  an  emergency  or  vice 
versa,  as  economy  may  dictate.  In  providing  a  gas  engine  for  driv- 
ing charging  dynamos,  it  is  important  to  obtain  one  having  as  good 
regulation  as  possible,  in  order  to  obtain  a  charging  current  of  prac- 
tically constant  voltage. 

Duplicate  Charging  Machines.  The  storage  batteries  of 
telephone  exchanges  are  usually  provided  of  sufficient  capacity  to 
supply  the  direct-current  needs  of  the  office  for  twenty-four  hours 
after  a  full  charge  has  been  given  them.  This  in  itself  is  a  strong 
safeguard  against  breakdown.  In  addition  to  this  the  charging  ma- 
chines should  be  in  duplicate,  so  that  a  burnt-out  armature  or  other 
damage  to  one  of  the  charging  units  will  not  disable  the  plant. 

Duplicate  Ringing  Machines.  It  is  equally  important  that 
the  ringing  machines,  whether  of  the  rotary  or  vibrating  type,  be  in 
duplicate.  For  large  exchanges  the  ringing  machines  are  usually 
dynamos,  and  it  is  not  unusual  to  have  one  of  these  driven  from  the 
commercial  power  mains  and  the  other  from  the  storage  battery. 
With  this  arrangement  complete  failure  of  all  sources  of  primary 
power  would  still  leave  the  exchange  operative  as  long  as  sufficient 
charge  remains  in  the  storage  battery. 

Capacity  of  Power  Units.  In  designing  telephone  switch- 
boards it  is  the  common  practice  to  so  design  the  frameworks  that 
the  space  for  multiple  jacks  is  in  excess  of  that  required  for  the  orig- 


POWER  PLANTS  605 

inal  installation.  In  a  like  manner,  the  power  plant  is  also  designed 
with  a  view  of  being  readily  increased  in  capacity  to  an  amount 
sufficient  to  provide  current  for  the  ultimate  number  of  subscribers' 
lines  for  which  the  switchboard  is  designed.  The  motor  genera- 
tors, or  whatever  means  are  provided  for  charging  the  storage  bat- 
teries, are  usually  installed  of  sufficient  size  to  care  for  the  ultimate 
requirements  of  the  office.  The  ringing  machines  are  also  provided 
for  the  ultimate  equipment.  However,  in  the  case  of  the  storage 
battery,  it  is  common  practice  to  provide  the  battery  tanks  of  suffi- 
cient size  to  care  for  the  ultimate  capacity,  while  the  plates  are  in- 
stalled for  a  capacity  only  slightly  in  excess  of  that  required  for  the 
original  installation.  As  the  equipment  of  subscribers'  lines  is  in- 
creased, additional  plates  may,  therefore,  be  added  to  the  cells  with- 
out replacing  the  storage  battery  as  a  whole,  and  without  making 
extraordinary  provisions  to  prevent  the  interruption  of  service.  It 
is  also  customary  to  provide  charging  and  supply  leads  from  the 
storage  battery  of  carrying  capacity  sufficient  for  the  ultimate  re- 
quirements of  the  office. 

Storage  Battery.  The  storage  battery  is  the  power  plant 
element  which  has  made  common-battery  systems  possible.  The 
common-battery  system  is  the  element  which  has  made  the  present 
wide  development  of  telephony  possible. 

A  storage-battery  cell  is  an  electro-chemical  device  in  which 
a  chemical  state  is  changed  by  the  passage  of  current  through  the 
cell,  this  state  tending  to  revert  when  a  current  is  allowed  to  flow 
in  the  opposite  direction.  A  storage  cell  consists  of  two  con- 
ductors in  a  solution,  the  nature  and  the  relation  of  these  three  ele- 
ments being  such  that  when  a  direct  current  is  made  to  pass  from 
one  conductor  to  the  other  through  the  solution,  the  compelled 
chemical  change  is  proportional  to  the  product  of  the  current  and 
its  duration.  When  the  two  conductors  are  joined  by  a  path  over 
which  current  may  flow,  a  current  does  flow  in  the  opposite  direc- 
tion to  that  which  charged  the  cell. 

All  storage  batteries  so  far  in  extensive  use  in  telephone  systems 
are  composed  of  lead  plates  in  a  solution  of  sulphuric  acid  in  water 
called  the  electrolyte.  In  charging,  the  current  tends  to  oxidize 
the  lead  of  one  plate  and  de-oxidize  the  other.  In  discharging,  the 
tendency  is  toward  equilibrium. 


606 


TELEPHONY 


The  containers,  employed  in  telephone  work,  for  the  plates 
and  electrolyte  are  either  of  glass  or  wood  with  a  lead  lining,  the 
glass  jars  being  used  for  the  smaller  sized  plates  of  small  capacity 
cells,  while  the  lead-lined  wooden  tanks  are  employed  with  the 
larger  capacity  cells.  The  potential  of  a  cell  is  slightly  over  two 
volts  and  is  independent  of  the  shape  or  size  of  the  plates  for  a  given 
type  of  battery.  The  storage  capacity  of  a  cell  is  determined  by  the 

size  and  the  number  of  plates. 
Therefore,  by  increasing  the  num- 
ber of  plates  and  the  areas  of  their 
surfaces,  the  ampere-hour  capacity 
of  the  cell  is  correspondingly  in- 
creased. The  desired  potential  of 
the  battery  is  obtained  by  connect- 
ing the  proper  number  of  cells  in 
series.  Storage-battery  cells  used 
in  telephone  work  vary  from  2 
plates  having  an  area  of  12  square 
inches  each,  to  cells  having  over 
50  plates,  each  plate  having  an 
area  of  240  square  inches.  The 
ampere-hour  capacity  of  these  bat- 
teries varies  from  6  ampere  hours 
to  4,000  ampere  hours,  respectively, 
when  used  at  an  average  8-hour 
discharge  rate.  In  Fig.  416  is  illus- 
trated a  storage  cell  employing  a  glass1  container  and  having  fifteen 
plates.  Each  plate  is  11  inches  high  and  10?  inches  wide,  with  an 
area,  therefore,  of  115.5  square  inches.  Such  a  cell  has  a  normal 
capacity  of  560  ampere  hours.  The  type  illustrated  is  one  made 
by  the  Electric  Storage  Battery  Company  of  Philadelphia,  Pa.* 

Installation.  In  installing  the  glass  jars  it  is  customary  to 
place  them  in  trays  partially  filled  with  sand.  They  are,  however, 
at  times  installed  on  insulators  so  designed  as  to  prevent  moisture 
f^om  causing  leakage  between  the  cells.  The  cells  using  wooden 
tanks  are  placed  on  glass  or  porcelain  insulators,  and  the  tanks  are 


Fig.   416.     Storage  Cell 


*The  instructions  given  later  in  this  chapter  are  for  batteries  of  this  make, 
although  they  are  applicable  in  many  respects  to  all  types  commonly  used  in  telephone 
work. 


POWER  PLANTS  607 

placed  with  enough  clearance  between  them  to  prevent  the  lead 
lining  of  adjacent  tanks  from  being  in  contact  and  thereby  short- 
circuiting  the  cells.  After  the  positive  and  the  negative  plates  have 
been  installed  in  the  tanks,  their  respective  terminals  are  connected 
to  bus  bars,  these  bus  bars  being,  for  the  small  types  of  battery,  lead- 
covered  clamping  bolts,  while  in  the  larger  types  reinforced  lead 
bus  bars  are  employed,  to  which  the  plates  are  securely  joined  by  a 
process  called  lead  burning.  This  process  consists  in  melting  a  por- 
tion of  the  bus  bar  and  the  terminal  lug  of  the  plate  by  a  flame  of 
very  high  temperature,  thus  fusing  each  individual  plate  to  the  proper 
bus  bar.  The  plates  of  adjacent  cells  are  connected  to  the  same  bus 
bar,  thus  eliminating  the  necessity  of  any  other  connection  between 
the  cells. 

Initial  Charge.  As  soon  as  the  plates  have  been  installed  in 
the  tanks  and  welded  to  the  bus  bars,  the  cell  should  be  filled  with 
electrolyte  having  a  specific  gravity  of  1.180  to  1.190  to  one-half 
inch  above  the  tops  of  the  plates  and  then  the  charge  should  be  im- 
mediately started  at  about  the  normal  rate.  In  the  case  of  a  battery 
consisting  of  cells  of  large  capacity,  it  is  customary  to  place  the  elec- 
trolyte in  the  cells  as  nearly  simultaneously  as  possible  rather  than 
to  completely  fill  the  cells  in  consecutive  order.  When  the  electro- 
lyte is  placed  in  the  cells  simultaneously,  the  charge  is  started  at  a 
very  much  reduced  rate  before  the  cells  are  completely  filled,  the  rate 
being  increased  as  the  cells  are  filled,  the  normal  rate  of  charge  being 
reached  when  the  cells  are  completely  filled.  Readings  should  be 
taken  hourly  of  the  specific  gravity  and  temperature  of  the  elec- 
trolyte, voltage  of  the  cells,  and  amperage  of  charging  current.  A 
record  or  log  should  be  kept  of  the  specific  gravity  and  voltage  of 
each  of  the  cells  of  the  battery  regularly  during  the  life  of  the  bat- 
tery and  it  is  well  to  commence  this  record  with  the  initial  charge. 

The  initial  charge  should  be  maintained  for  at  least  ten  hours 
after  the  time  when  the  voltage  and  specific  gravity  have  reached  a 
maximum.  If  for  any  reason  it  is  impractical  to  continue  the  initial 
charge  uninterrupted,  the  first  period  of  charging  should  be  at  least 
from  twelve  to  fifteen  hours.  However,  every  effort  should  be  made 
to  have  the  initial  charge  continuous,  as  an  interruption  tends  to 
increase  the  time  necessary  for  the  initial  charge,  and  if  the  time  be 
too  long  between  the  periods  of  the  initial  charge,  the  efficiency  and 


608  TELEPHONY 

capacity  of  the  cells  are  liable  to  be  affected.  In  case  of  a  large 
battery,  precaution  should  be  taken  to  insure  that  vthe  ventilation  is 
exceptionally  good,  because  if  it  is  not  good  the  temperature  is  lia- 
ble to  increase  considerably  and  thereby  cause  an  undue  amount 
of  evaporation  from  the  cells. 

The  object  of  the  temperature  readings  taken  during  the 
charge  is  to  enable  corrections  to  be  made  to  the  specific  gravity 
readings  as  obtained  by  the  hydrometer,  in  order  that  the  correct 
specific  gravity  may  be  ascertained.  This  correction  is  made  by 
adding  .001  specific  gravity  for  each  three  degrees  in  temperature 
above  70°  Fahrenheit,  or  subtracting  the  same  amount  for  each  three 
degrees  below  70°  Fahrenheit.  At  the  time  the  cells  begin  to  gas 
they  should  be  gone  over  carefully  to  see  that  they  gas  evenly,  and 
also  to  detect  and  remedy  early  in  the  charging  period  any  defects 
which  may  exist.  If  there  is  any  doubt  in  regard  to  the  time  at  which 
the  cells  reach  a  maximum  voltage  and  specific  gravity,  the  charge 
should  be  continued  sufficiently  long  before  the  last  ten  hours  of  the 
charge  are  commenced  to  eliminate  any  such  doubt,  as  in  many  cases 
poor  efficiency  and  low  capacity  of  a  cell  later  in  its  life  may  be 
traced  to  an  insufficient  initial  charge. 

Operation.  After  the  battery  has  been  put  in  commission  the 
periodic  charges  should  be  carefully  watched,  as  excessive  charg- 
ing causes  disintegration  and  decreases  the  life  and  capacity  of  the 
battery;  while,  on  the  other  hand,  undercharging  will  result  in  sul- 
phating  of  the  plates  and  decrease  of  capacity,  and,  if  the  under- 
charge be  great,  will  result  in  a  disintegration  of  the  plates.  It  is, 
therefore,  essential  that  the  battery  be  charged  regularly  and  at  the 
rate  specified  for  the  particular  battery  in  question.  In  order  to 
minimize  the  chance  of  either  continuously  overcharging  or  under- 
charging the  battery,  the  charges  are  divided  into  two  classes,  name- 
ly, regular  charges  and  overcharges.  The  regular  charges  are  the 
periodic  charges  for  the  purpose  of  restoring  the  capacity  of  the 
battery  after  discharge.  The  overcharges,  which  should  occur 
once  a  week  or  once  in  every  two  weeks,  according  to  the  use  of  the 
battery,  are  for  the  purpose  of  insuring  that  all  cells  have  received 
their  proper  charge,  for  reducing  such  sulphating  as  may  have  oc- 
curred on  cells  undercharged,  and  for  keeping  the  plates,  in  general, 
in  a  healthy  condition.  The  specific  gravity  of  the  electrolyte,  the 


POWER  PLANTS  609 

voltage  of  the  battery,  and  the  amount  of  gasing  observed  are  all 
indications  of  the  amount  of  charge  which  the  battery  has  received 
and  should  all  be  considered  when  practicable.  Either  the  specific 
gravity  or  voltage  may  be  used  as  the  routine  method  of  determining 
the  proper  charge,  but,  however,  if  the  proper  charge  is  determined 
by  the  voltage  readings,  this  should  be  frequently  checked  by  the 
specific  gravity,  and  vice  versd. 

During  the  charging  and  discharging  of  a  battery  the  level  of 
the  electrolyte  in  the  cells  will  fall.  As  the  portion  of  the  electro- 
lyte which  is  evaporated  is  mainly  water,  the  electrolyte  may  be 
readily  restored  to  its  normal  level  by  adding  distilled  water  or  care- 
fully collected  rain  water. 

Pilot  Cell.  As  the  specific  gravity  of  all  the  cells  of  a  battery, 
after  having  once  been  properly  adjusted,  will  vary  the  same  in  all 
the  cells  during  use,  it  has  been  found  satisfactory  to  use  one  cell, 
commonly  termed  the  pilot  cell,  for  taking  the  regular  specific  gravity 
readings  and  only  reading  the  specific  gravity  of  all  the  cells  occa- 
sionally or  on  the  overcharge.  This  cell  must  be  representative  of 
all  the  cells  of  the  battery,  and  if  the  battery  is  so  subdivided  in  use 
that  several  sets  of  cells  are  liable  to  receive  different  usage,  a  pilot 
cell  should  be  selected  for  each  group. 

Overcharge.  If  the  battery  is  charged  daily,  it  should  receive 
an  overcharge  once  a  week,  or  if  charged  less  frequently,  an  over- 
charge should  be  given  at  least  once  every  two  weeks.  In  making 
an  overcharge  this  should  be  done  at  a  constant  rate  and  at  a  rate 
specified  for  the  battery.  During  the  overcharge  the  voltage  of  the 
battery  and  the  specific  gravity  of  the  pilot  cell  should  be  taken  every 
fifteen  minutes  from  the  time  the  gasing  begins.  The  charge  should 
be  continued  until  five  consecutive,  specific-gravity  readings  are 
practically  the  same.  The  voltage  of  the  battery  should  not  increase 
during  the  last  hour  of  the  charge. 

As  the  principal  object  of  the  overcharge  is  to  insure  that  all  of 
the  cells  have  received  the  proper  charge,  it  must,  therefore,  be  con- 
tinued long  enough  to  not  only  properly  charge  the  most  efficient 
cells,  but  also  to  properly  charge  those  which  are  lower  in  efficiency. 
The  longer  the  interval  between  overcharges,  the  greater  will  be  the 
variation  between  the  cells  and,  therefore,  it  is  necessary  to  continue 
the  overcharge  longer  when  the  interval  between  overcharges  is  as 


.610  TELEPHONY 

great  as  two  weeks.  Before  the  overcharge  is  made  the  cells  should 
be  carefully  inspected  for  short  circuits  and  other  abnormal  condi- 
tions. These  inspections  may  best  be  made  by  submerging  an 
electric  lamp  in  the  cell,  if  the  cell  be  of  wood,  or  of  allowing  it  to 
shine  through  from  the  outside,  if  it  be  of  glass.  By  this  means  any 
foreign  material  may  be  readily  detected  and  removed  before  serious 
damage  is  caused.  In  making  these  inspections  it  must  be  borne 
in  mind  that  whatever  tools  or  implements  are  used  must  be  non- 
metallic  and  of  some  insulating  material. 

Regular  Charge.  Regular  charges  are  the  periodic  charges 
for  restoring  the  capacity  of  the  battery,  and  should  be  made  as  fre- 
quently as  the  use  of  the  battery  demands.  The  voltage  of  the  cells 
is  a  good  guide  for  determining  when  the  battery  should  be  recharged. 
The  voltage  of  a  cell  should  never  be  allowed  to  drop  below  1.8 
volts,  and  it  is  usually  considered  better  practice  to  recharge  when 
the  battery  has  reached  1.9  volts.  If  a  battery  is  to  remain  idle  for 
even  a  short  time,  it  should  be  left  in  a  completely  charged  condition. 

The  regular  charges  for  cells  completely  equipped  with  plates 
should  be  continued  until  the  specific  gravity  of  the  pilot  cell  has 
risen  to  five  points  below  the  maximum  attained  on  the  preceding 
overcharge,  or,  if  only  partially  equipped  with  plates,  until  it  has  risen 
to  three  points  below  the  previous  maximum.  The  voltage  per  cell 
at  this  time  should  be  from  .05  volts  to  .1  volts  below  that  obtained 
on  the  previous  overcharge.  At  this  time  all  the  cells  should  be 
gasing,  but  not  as  freely  as  on  an  overcharge. 

Low  Cells.  An  unhealthy  condition  in  a  cell  usually  manifests 
itself  in  one  of  the  following  ways:  Falling  off  in  specific  gravity 
or  voltage  relative  to  the  rest  of  the  cells,  lack  of  gasing  when  charged, 
and  color  of  the  plates,  either  noticeably  lighter  or  darker  than  those 
of  other  cells  of  the  battery.  When  any  of  the  above  conditions  are 
found  in  a  cell,  the  cell  should  receive  immediate  attention,  as  a 
delay  may  mean  serious  trouble.  The  cell  should  be  thoroughly 
inspected  to  determine  if  a  short-circuit  exists,  either  caused  by  some 
foreign  substance,  by  an  excess  of  sediment  in  the  bottom  of  the  tank, 
or  by  portions  of  the  plates  themselves.  If  such  a  condition  is  found, 
the  cause  should  be  immediately  removed  and,  if  the  defect  has  been 
of  short  duration,  the  next  overcharge  will  probably  restore  it  to  nor- 
mal condition.  If  the  defect  has  existed  for  some  time,  it  is  often 


POWER  PLANTS  611 

necessary  to  give  the  cell  a  separate  charge.  This  may  be  done  by 
connecting  it  directly  to  the  charging  generator  with  temporary 
leads  and  thus  bring  it  back  to  its  normal  condition.  It  is  some- 
times found  necessary  to  replace  the  cell  in  order  to  restore  the  bat- 
tery to  its  normal  condition. 

Sediment.  The  cells  of  the  battery  should  be  carefully  watched 
to  prevent  the  sediment  which  collects  in  the  bottom  of  the  jar  or  tank 
during  use  from  reaching  the  bottom  of  the  plates,  thereby  causing 
short  circuits  between  them.  When  the  sediment  in  the  cell  has 
reached  within  one-half  inch  of  the  bottom  of  the  plates,  it  should 
be  removed  at  once.  With  small  cells  using  glass  jars  this  can  most 
easily  be  done  directly  after  an  overcharge  by  carefully  drawing  off 
the  electrolyte  without  disturbing  the  sediment  and  then  removing 
it  from  the  jar.  The  plates  and  electrolyte  should  be  replaced  in 
the  jar  as  soon  as  convenient  to  prevent  the  plates  from  becoming 
dry.  If  the  plates  are  large  and  in  wooden  tanks,  the  sediment  can 
most  easily  be  removed  by  mean's  of  a  scoop  made  especially  for  the 
purpose.  The  preferable  time  to  clean  the  tanks  is  just  before  an 
overcharge. 

Replacing  Batteries.  There  comes  a  time  in  the  life  of  nearly 
every  central-office  equipment  when  the  storage  battery  must  be 
completely  renewed.  This  is  due  to  the  fact  that  the  life  of  even 
the  best  of  storage  batteries  is  not  as  great  as  the  life  of  the  average 
switchboard  equipment.  It  may  also  be  due  to  the  necessity  for 
greater  capacity  than  can  be  secured  with  the  existing  battery  tanks, 
usually  caused  by  underestimating  the  traffic  the  office  will  be  re- 
quired to  handle.  Again,  it  is  sometimes  necessary  to  make  exten- 
sive alterations  in  an  existing  battery,  perhaps  due  to  the  necessity 
for  changing  its  location.  To  change  a  battery  one  cell  at  a  time, 
keeping  the  others  in  commission  meanwhile,  has  often  been  done, 
but  it  is  always  expensive  and  unsatisfactory  and  is  likely  to  shorten 
the  life  of  the  battery,  due  to  improper  and  irregular  forming  of  the 
plates  during  the  initial  charge.  The  advent  of  the  electric  auto- 
mobile industry  has  brought  with  it  a  convenient  means  for  over- 
coming this  difficulty.  Portable  storage  cells  for  automobile  use 
are  available  in  almost  every  locality  and  may  often  be  rented  at 
small  cost.  A  sufficient  number  of  such  cells  may  be  temporarily 
installed,  enough  of  them  being  placed  in  multiple  to  give  the  neces- 


612  TELEPHONY 

sary  output.  By  floating  a  temporary  battery  so  formed  across  the 
charging  mains  and  running  the  generators  continuously,  a  tem- 
porary source  of  current  supply  may  be  had  at  small  expense  for  run- 
ning the  exchange  during  the  period  required  for  alterations.  Us- 
ually a  time  of  low  traffic  is  chosen  for  making  the  changes,  such  as 
from  Saturday  evening  to  Monday  morning.  Very  large  central- 
office  batteries,  serving  as  many  as  6,000  lines,  have  thus  been  taken 
out  of  service  and  replaced  without  interfering  with  the  traffic  and 
with  the  use  of  but  a  comparatively  few  portable  cells.  One  pre- 
caution has  to  be  observed  in  such  work,  and  that  is  not  to  subject 
the  portable  cells  to  too  great  an  overcharge,  due  to  the  great  ex- 
cess of  generator  over  battery  capacity.  This  is  easily  avoided  by 
watching  the  ammeters  to  see  that  the  input  is  not  in  too  great  excess 
of  the  output,  and  if  necessary,  by  frequently  stopping  the  machines 
to  avoid  this. 

Power  Switchboard.  The  clearing-house  of  the  telephone 
power  plant  is  the  power  board.  In  most  cases,  it  carries  switches, 
meters,  and  protective  devices. 

Switches.  The  switches  most  essential  are  those  for  opening 
and  closing  the  motor  and  the  generator  circuits  of  the  charging 
sets  and  with  these  usually  are  associated  the  starting  rheostats  of 
the  motors  and  the  field  rheostats  of  the  generators.  The  starting 
rheostats  are  adapted  to  allow  resistance  to  be  removed  from  the 
motor  armature  circuit,  allowing  the  armature  to  gain  speed  and  in- 
crease its  counter-electromotive  force  without  overheating.  The 
accepted  type  has  means  for  opening  the  driving  circuit  automatically 
in  case  its  voltage  should  fall,  thus  preventing  a  temporary  interrup- 
tion of  driving  current  from  damaging  the  motor  armature  on  its 
return  to  normal  voltage. 

Meters.  The  meters  usually  are  voltmeters  and  ammeters, 
the  former  being  adapted  to  read  the  several  voltages  of  direct  cur- 
rents in  the  power  plant.  An  important  one  to  be  known  is  the 
voltage  of  the  generator  before  beginning  a  battery  charge,  so  that 
the  generator  may  not  be  thrown  on  the  storage  battery  while  gen- 
erating a  voltage  less  than  that  of  the  battery.  If  this  were  done,  the 
battery  would  discharge  through  the  generator  armature.  The  volt- 
meter enables  the  voltage  of  the  charging  generator  to  be  kept  above 
that  of  the  battery,  as  the  latter  rises  during  charge.  It  enables  the 


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614  TELEPHONY 

performance  of  several  cells  of  the  battery  to  be  observed.  A 
convenient  way  is  to  connect  the  terminals  of  the  several  cells  to  jacks 
on  the  power  board  and  to  terminate  the  voltmeter  in  a  plug. 

The  ammeter,  with  suitable  connections,  enables  the  battery- 
charge  rate  to  be  kept  normal  and  the  battery  discharge  to  be  ob- 
served. In  order  to  economize  power,  it  is  best  to  charge  the  battery 
during  the  hours  of  heavy  load.  The  generator  output  then  divides, 
the  switchboard  taking  what  the  load  requires,  the  battery  receiving 
the  remainder. 

In  systems  requiring  the  terminal  voltage  of  the  equipment  to 
be  kept  constant  within  close  limits,  either  it  is  necessary  to  use  two 
batteries — never  drawing  current  from  a  battery  during  charge— 
or  to  provide  means  of  compensating  for  the  rise  of  voltage  while 
the  battery  is  under  charge.  The  latter  is  the  more  modern  method 
and  is  done  either  by  using  fewer  cells  when  the  voltage  per  cell  is 
higher  or  by  inserting  counter-electromotive  force  cells  in  the  dis- 
charge leads,  opposing  the  discharge  by  more  or  fewer  cells  as  the 
voltage  of  the  battery  is  higher  or  lower.  In  either  method,  switches 
on  the  power  board  enable  the  insertion  and  removal  of  the  neces- 
sary end  cells  or  counter-electromotive  force  cells. 

Protective  Devices.  The  protective  devices  required  on  a 
power  board  are  principally  circuit-breakers  and  fuses.  Circuit- 
breakers  are  adapted  to  open  motor  and  generator  circuits  when 
their  currents  are  too  great,  too  small,  or  in  the  wrong  direction. 
Fuses  are  adapted  to  open  circuits  when  the  currents  in  them  are 
too  great.  The  best  type  is  that  in  which  the  operation  of  the  fuses 
sounds  or  shows  an  alarm,  or  both. 

Power=Plant  Circuits.  The  circuit  arrangement  of  central- 
office  power  plants  is  subject  to  wide  variation  according  to  condi- 
tions. The  type  of  telephone  switchboard  equipment,  whether 
magneto  or  common-battery,  automatic  or  manual,  will,  of  coilrse, 
largely  affect  the  circuit  arrangement  of  the  power  plant.  Fig.  417 
shows  a  typical  example  of  good  practice  in  this  respect  for  use  with 
a  common-battery  manual  switchboard  equipment.  Besides  show- 
ing the  switches  for  handling  the  various  machines  and  the  charge- 
and-discharge  leads  from  the  storage  battery,  this  diagram  shows 
how  current  from  the  storage  battery  is  delivered  to  various  parts 
of  the  central-office  equipment. 


CHAPTER    XXXIII 
HOUSING  CENTRAL=OFFICE  EQUIPMENT 

The  Central=0ffice  Building.  Proper  arrangement  of  the 
central-office  equipment  depends  largely  upon  the  design  of  the 
central-office  building.  The  problem  involved  should  not  be  solved 
by  the  architect  alone.  The  most  careful  co-operation  between  the 
engineer  and  the  architect  is  necessary  in  order  that  the  various  parts 
of  the  telephonic  equipment  may  be  properly  related,  and  that  the 
wires  connecting  them  with  each  other  and  with  the  outside  lines  be 
disposed  of  with  due  regard  to  safety,  economy,  and  convenience. 
So  many  factors  enter  into  the  design  of  a  central-office  building 
that  it  is  impossible  to  lay  down  more  than  the  most  general 
rules.  The  attainment  of  an  ideal  is  often  impossible,  because  of 
the  fact  that  the  building  is  usually  in  congested  districts,  and  its 
very  shape  and  size  must  be  governed  by  the  lot  on  which  it  is  built, 
and  by  the  immediate  surroundings.  Frequently,  also,  the  build- 
ing must  be  used  for  other  purposes  than  those  of  a  telephone  office, 
so  that  the  several  purposes  must  be  considered  in  its  design.  Again, 
old  buildings,  designed  for  other  purposes,  must  sometimes  be  al- 
tered to  meet  the  requirements  of  a  telephone  office,  and  this  is  per- 
haps the  most  difficult  problem  of  all. 

The  exterior  of  the  building  is  a  matter  that  may  be  largely 
decided  by  the  architect  and  owner  after  the  general  character  of 
the  building  has  been  determined.  One  important  feature,  however, 
and  one  that  has  been  overlooked  in  many  cases  that  we  know  of, 
is  to  so  arrange  the  building  that  switchboard  sections  and  other 
bulky  portions  of  the  apparatus,  which  are  necessarily  assembled  at 
the  factory  rather  than  on  the  site,  may  be  brought  into  the  building 
without  tearing  down  the  walls. 

Fire  Hazard.  The  apparatus  to  be  housed  in  a  central-office 
building  often  represents  a  cost  running  into  the  hundreds  of  thou- 
sands of  dollars;  but  whether  of  large  or  small  first  cost,  it  is 


616  TELEPHONY 

evident  that  its  destruction  might  incur  a  very  much  greater  loss 
than  that  represented  by  its  replacement  value.  In  guarding 
the  central-office  equipment  against  destruction  by  fire  or  other 
causes,  the  telephone  company  is  concerned  to  a  very  much  greater 
extent  than  the  mere  cost  of  the  physical  property;  since  it  is  guard- 
ing the  thing  which  makes  it  possible  to  do  business.  While  the 
cost  of  the  central  office  and  its  contents  may  be  small  in  compar- 
ison with  the  total  investment  in  outside  plant  and  other  portions 
of  the  equipment,  it  is  yet  true  that  these  larger  portions  of  the  in- 
vestment become  useless  with  the  loss  of  the  central  office. 

There  is  another  consideration,  and  that  is  the  moral  obliga- 
tion of  the  operating  company  to  the  public.  A  complete  break- 
down of  telephone  service  for  any  considerable  period  of  time  in  a 
large  city  is  in  the  nature  of  a  public  calamity. 

For  these  reasons  the  safeguarding  of  the  central  office  against 
damage  by  fire  and  water  should  be  in  all  cases  a  feature  of  funda- 
mental importance,  and  should  influence  not  only  the  character  of 
the  building  itself,  but  in  many  cases  the  choice  of  its  location. 

Size  of  Building.  It  goes  without  saying  that  the  building 
must  be  large  enough  to  accommodate  the  switchboards  and  other 
apparatus  that  is  required  to  be  installed.  The  requirement  does 
not  end  here,  however.  Telephone  exchange  systems  have,  with 
few  exceptions,  grown  yery  much  faster  than  was  expected  when 
they  were  originally  installed.  Many  buildings  have  had  to  be 
abandoned  because  outgrown.  In  planning  the  building,  therefore, 
the  engineer  should  always  have  in  mind  its  ultimate  requirements. 
It  is  not  always  necessary  that  the  building  shall  be  made  large  enough 
at  the  outset  to  take  care  of  the  ultimate  requirements,  but  where 
this  is  not  done,  the  way  should  be  left  clear  for  adding  to  it  when 
necessity  demands. 

Strength  of  Building.  The  major  portion  of  telephone  central- 
office  apparatus,  whether  automatic  or  manual,  is  not  of  such  weight 
as  to  demand  excessive  strength  in  the  floors  and  walls  of  buildings. 
Exceptions  to  this  may  be  found  in  the  storage  battery,  in  the  power 
machinery,  especially  where  subject  to  vibration,  and  in  certain 
cases  in  the  cable  runs.  After  the  ultimate  size  of  the  equipment  has 
been  determined,  the  engineer  and  the  architect  should  confer  on 
this  point,  particularly  with  reference  to  the  heavier  portions  of  the 


HOUSING  CENTRAL-OFFICE  EQUIPMENT  617 

apparatus,  to  make  sure  that  adequate  strength  is  provided.  The 
approximate  weights  of  all  parts  of  central-office  equipments  may 
readily  be  ascertained  from  the  manufacturers. 

Provision  for  Employes.  In  manual  offices  particularly  it  has 
been  found  to  be  not  only  humane,  but  economical  to  provide  ade- 
quate quarters  for  the  employes,  both  in  the  operating  rooms  and 
places  where  they  actually  perform  their  work,  and  in  the  places 
where  they  may  assemble  for  recreation  and  rest.  The  work  of 
the  telephone  operator,  particularly  in  large  cities,  is  of  such  a  na- 
ture as  often  to  demand  frequent  periods  of  rest.  This  is  true  not 
only  on  account  of  the  nervous  strain  on  the  operator,  but  also  on 
account  of  the  necessity,  brought  about  by  the  demands  of  economy, 
for  varying  the  number  of  operators  in  accordance  with  the  traffic 
load.  These  features  accentuate  the  demand  for  proper  rooms 
where  recreation,  rest,  and  nourishment  may  be  had. 

Provision  for  Cable  Runways.  In  very  small  offices  no  special 
structural  provision  need  be  made  in  the  design  of  the  building  it- 
self for  the  entrance  of  the  outside  cables,  and  for  the  disposal  of  the 
cables  and  wires  leading  between  various  portions  of  the  apparatus. 
For  large  offices,  however,  this  must  necessarily  enter  as  an  important 
feature  in  the  structure  of  the  building  itself.  It  is  important  that 
the  cables  be  arranged  systematically  and  in  such  a  way  that  they 
will  be  protected  against  injury  and  at  the  same  time  be  accessible 
either  for  repairs  or  replacement,  or  for  the  addition  of  new  cables  to 
provide  for  growth.  Disorderly  arrangement  of  the  wires  or  cables 
results  in  disorder  indeed,  with  increased  maintenance  cost,  uneco- 
nomical use  of  space,  inaccessibility,  liability  to  injury,  and  general 
unsightliness. 

The  carrying  of  cables  from  the  basement  to  the  upper  floors 
or  between  floors  elsewhere  must  be  provided  for  in  a  way  that  will 
not  be  wasteful  of  space,  and  arrangements  must  be  made  for  sup- 
porting the  cables  in  their  vertical  runs.  In  the  aggregate  their 
weight  may  be  great,  and  furthermore  each  individual  cable  must 
be  so  supported  that  its  sheath  will  not  be  subject  to  undue  strain. 
Another  factor  which  must  be  considered  in  vertical  cable  runs  is 
the  guarding  against  such  runs  forming  natural  flues  through  which 
flames  or  heated  gases  would  pass,  in  the  event  of  even  an  un- 
important fire  at  their  lower  ends. 


618 


TELEPHONY 


Arrangement  of  Apparatus  in  Small  Manual  Offices.  Where 
a  common-battery  multiple  switchboard  equipment  is  used,  at  least 
three  principal  rooms  should  be  provided— one  for  the  multiple 
switchboard  proper;  one  for  the  terminal  and  power  apparatus, 
including  the  distributing  frames,  racks,  and  power  machinery;  and 
the  third  for  the  storage  battery.  These  should  adjoin  each  other  for 
purposes  of  convenience  and  of  economy  in  wiring. 

Floor  Plans  for  Small  Manual  Offices.  As  was  pointed  out, 
there  are  several  plans  of  disposing  of  the  main  and  intermediate 
distributing  frames  and  the  line  and  cut-off  relay  racks.  The  one 
most  practiced  is  to  mount  the  relay  rack  alongside  the  main  and 
intermediate  distributing  frame  in  the  terminal  room.  A  typical 


Fig.  418.     Typical  Small  Office  Floor  Plan 


floor  plan  of  such  an  arrangement  for  a  small  office,  employing  as  a 
maximum  five  sections  of  multiple  switchboards,  is  shown  in  Fig. 
418.  This  is  an  ideal  arrangement  well  adapted  for  a  rectangular 
floor  space  and  on  that  account  may  often  be  put  into  effect.  It 
should  be  noted  that  the  switchboard  grows  from  left  to  right,  and 
that  alternative  arrangements  are  shown  for  disposing  of  those  sec- 
tions beyond  the  second.  The  cable  turning  section  through  which 
the  multiple  and  answering  jacks  are  led  to  the  terminal  frames  is 
placed  as  close  as  possible  to  the  terminal  frames.  This  results  in  a 
considerable  saving  in  cable.  An  interesting  feature  of  this  floor 
plan  is  the  arrangement  of  unitary  sections  of  main  and  intermedi- 
ate frames  and  relay  racks,  representing  recent  practice  of  the  West- 


HOUSING  CENTRAL-OFFICE  EQUIPMENT 


619 


ern  Electric  Company.  The  iron  work  of  the  three  racks  is  built 
in  sections  and  these  are  structurally  connected  across  so  that  the 
first  section  of  the  main  frame,  the  intermediate  frame,  and  the  relay 
rack  form  one  unit,  the  structural  iron  work  which  ties  them  to- 
gether forming  the  runway  for  the  cables  between  them.  But  two 
of  these  units,  including  two  sections  of  each  frame,  are  shown  in- 
stalled, the  provision  for  growth  being  indicated  by  dotted  lines. 

The  battery  room  in  this  case  provides  for  the  disposal  of  the 
battery  cells  in  two  tiers.  This  room  is  merely  partitioned  off  from 
the  distributing  or  terminal  room.  Where 
this  is  done  the  partition  walls  should  be 
plastered  on  both  sides  so  as  to  prevent, 
as  far  as  possible,  the  entrance  of  any  bat- 
tery fumes  into  the  apparatus  rooms. 

The  wire  chief's  desk,  as  will  be  noted, 
is  located  in  such  a  position  as  to  give  easy 
access  from  it  not  only  to  the  distributing 
frames  and  relay  rack,  but  to  the  power 
apparatus  as  well. 

Combined  Main  and  Intermediate 
Frames.  For  use  in  small  exchanges,  the 
Western  Electric  Company  has  recently 
put  on  the  market  a  combined  main  and 
intermediate  distributing  frame.  This  is 
constructed  about  the  same  as  an  ordinary 
main  frame,  the  protectors  being  on  one 
side  and  the  line  and  intermediate  frame 
terminals  on  the  other.  The  lower  half  of 
the  terminals  on  each  vertical  bay  is  devoted 
to  the  outside  line  terminals  and  the  upper  half  is  devoted  to  inter- 
mediate frame  terminals.  This  arrangement  is  indicated  in  the 
elevation  in  Fig.  419.  With  the  use  of  this  combined  main  and 
intermediate  frame,  the  floor  plan  of  Fig.  418  may  be  modified, 
as  shown  in  Fig.  420. 

In  Fig.  421  is  given  an  excellent  idea  of  terminal-room  appara- 
tus carried  out  in  accordance  with  the  more  usual  plan  of  employing 
separate  main  and  intermediate  distributing  frames.  At  the  extreme 
right  of  this  figure  the  protector  side  of  the  main  frame  is  shown, 


Fig.  419.     Combined  Main 
and  Intermediate  Frames 


620 


TELEPHONY 


It  will  be  understood  that  the  line  cables  terminate  on  the  horizontal 
terminal  strips  on  the  other  side  of  this  frame  and  are  connected 


Fig.   420.     Small  Office  Floor  Plan 


through  the  horizontal  and  vertical  runways  of  the  frame  to  the 
protector  terminals.    The  intermediate  frame  is  shown  in  the  cen- 


Fig.  421.     Terminal  Apparatus — Small  Office 


tral  portion  of  the  figure,  the  side  toward  the  left  containing  the  an- 
swering-jack  terminals,  and  the  side  toward  the  right  the  multiple 


HOUSING  CENTRAL-OFFICE  EQUIPMENT  621 

jack  terminals,  these  latter  being  arranged  horizontally.  This 
horizontal  and  vertical  arrangement  of  the  terminals  on  the  main 
and  intermediate  distributing  frames  has  been  the  distinguishing 
feature  between  the  Bell  and-  Independent  practice,  the  Bell  Com- 
panies adhering  to  the  horizontal  and  vertical  arrangement,  while 
the  Independent  Companies  have  employed  the  vertical  arrangement 
on  both  sides.  We  are  informed  that  in  the  future  the  new  smaller 
installations  of  the  Bell  Companies  will  be  made  largely  with  the 
vertical  arrangement  on  both  sides.  At  the  left  of  Fig.  421  is  shown 
the  relay  rack  in  two  sections  of  two  bays  each.  This  illustration 
also  gives  a  good  idea  of  the  common  practice  in  disposing  of  the 
cables  between  the  frames  in  iron  runways  just  below  the  ceiling  of 
the  terminal  room. 

Types  of  Line  Circuits.  The  design  of  the  terminal-room 
floor  plan  will  depend  largely  on  the  arrangement  of  apparatus  in 
the  subscribers'  line  circuits  with  respect  to  the  distributing  frames 
and  relay  racks.  The  Bell  practice  in  this  respect  has  already  been 
referred  to  and  is  illustrated  in  Fig.  348.  In  this  the  line  and  cut- 
off relays  are  permanently  associated  with  the  answering  jacks  and 
lamps,  resulting  in  the  answering-jack  equipment  being  subject  to 
change  with  respect  to  the  multiple  and  the  line  through  the  jump- 
ers of  the  intermediate  frame.  The  practice  of  the  Kellogg  Com- 
pany, on  the  other  hand,  has  been  illustrated  in  Fig.  353,  and  in  this 
the  line  and  cut-off  relays  are  permanently  associated  with  the  multi- 
ple and  with  the  line,  only  the  answering  jacks  and  lamps  being 
subject  to  change  through  the  jumper  wires  on  the  intermediate 
frame.  This  latter  arrangement  has  led  to  a  very  desirable  paral- 
lel arrangement  of  the  two  distributing  frames  and  the  relay  rack. 
These  are  made  of  equal  length  so  as  to  correspond  bay  for  bay, 
and  are  placed  side  by  side  with  only  enough  space  between  them 
for  the  passage  of  workmen — the  relay  rack  lying  between  the  main 
and  intermediate  frames.  In  this  scheme  all  the  multiple  and  an- 
swering-jack cables  run  from  the  intermediate  distributing  frame, 
and  the  cabling  between  the  intermediate  frame  and  the  relay  rack 
and  between  the  relay  rack  and  the  main  frame  is  run  straight  across 
from  one  rack  to  the  other.  This  results  in  a  great  saving  of  cable 
within  the  terminal  room,  over  that  arrangement  wherein  the  cabling 
from  one  frame  to  another  is  necessarily  led  along  the  length  of  the 


622 


TELEPHONY 


frame  to  its  end  and  then  passes  through  a  single  runway  to  the  end 
of  the  other  frame. 

Large  Manual  Offices.  For  purposes  of  illustrating  the  practice 
in  housing  the  apparatus  in  very  large  offices  equipped  with  manual 
switchboards,  we  have  chosen  the  Chelsea  office  of  the  New  York 
Telephone  Company  as  an  excellent  example  of  modern  practice. 


Fig.  422.     Floor  Plan,  Operating  Room,  Chelsea  Office,  New  York  City 

The  ground  plan  of  the  building  is  U-shaped,  in  order  to  provide 
the  necessary  light  over  the  rather  large  floor  areas.  The  plan  of 
the  operating  floor — the  sixth  floor  of  the  building — is  shown  in  Fig. 
422.  As  will  be  seen,  this  constitutes  a  single  operating  room,  the 
.4-board  being  located  in  the  right  whig  and  the  .B-board  in  the  left. 
The  point  from  which  both  boards  grow  is  near  the  center  of  the 
front  of  the  building,  the  boards  coming  together  at  this  point  in  a 
common  cable  turning  section,  The  disposal  of  the  various  desks 


HOUSING  CENTRAL-OFFICE  EQUIPMENT 


623 


for  the  manager,  chief  operator,  and  monitors  is  indicated.  Those 
switchboard  sections  which  are  shown  in  full  lines  are  the  ones  at 
present  installed,  the  provision  for  growth  being  indicated  in  dotted 
lines. 

The  fifth  floor  is  devoted  to  the  terminal  room  and  operators' 
quarters,  the  terminal  room  occupying  the  left-hand  wing  and  the 


Fig.  423      Terminal  Room  and  Operators'  Quarters,  Chelsea  Office,  New  York  City 

major  portion  of  the  front  of  the  building,  and  the  operators'  quar- 
ters the  right-hand  wing.  The  line  and  the  trunk  cables  come  up 
from  the  basement  of  the  building  at  the  extreme  left,  being  sup- 
ported directly  on  the  outside  wall  of  the  building.  Arriving  at  the 
fifth  floor,  they  turn  horizontally  and  are  led  under  a  false  flooring 
provided  with  trap  doors,  to  the  protector  side  of  the  main  frame. 
The  disposal  of  the  cables  between  the  various  frames  will  be  more 
readily  understood  by  reference  to  the  following  photographs. 


624  TELEPHONY 

A  general  view  of  a  portion  of  the  A  -board  of  the  Chelsea  office 
is  shown  in  Fig.  424,  this  view  being  taken  from  a  point  in  the  left- 
hand  wing  looking  toward  the  front.  In  Fig.  425  is  shown  a  closer 
view  of  a  smaller  portion  of  the  board.  Fig.  426  gives  an  excellent 
idea  of  the  rear  of  this  switchboard  and  of  the  disposal  of  the  cables 
and  wires.  The  main  mass  of  cables  at  the  top  are  those  of  the 
multiple.  Immediately  below  these  may  be  seen  the  outgoing 
trunk  cables.  The  forms  of  the  answering-jack  cables  lie  below 
these  and  are  not  so  readily  seen,  but  the  cables  leading  from  these 
forms  are  led  down  to  the  runway  at  the  bottom  of  the  sections,  and 
thence  along  the  length  of  the  board  to  the  intermediate  distributing 
frame  on  the  floor  below.  The  layer  of  cables,  supported  on  the  iron 
rack  immediately  above  the  answering-jack  cable  runway,  shown 
at  the  extreme  bottom  of  the  view,  are  those  containing  the  wires 
leading  from  the  repeating  coils  to  the  cord  circuits. 

An  interesting  feature  of  this  board  is  the  provisions  for  protec- 
tion against  injury  by  fire  and  water.  On  top  of  the  boards  through- 
out their  entire  length  there  is  laid  a  heavy  tarpaulin  curtain  with 
straps  terminating  in  handles  hanging  down  from  its  edges.  These 
may  be  seen  in  Fig.  426  and  also  in  Fig.  425.  The  idea  of  this  is 
that  if  the  board  is  exposed  to  a  water  hazard,  as  in  the  case  of  fire, 
the  board  may  be  completely  covered,  front  and  rear,  with  this  tar- 
paulin curtain,  by  merely  pulling  the  straps.  The  entire  force — 
both  operators  and  repairmen — is  drilled  to  assure  the  carrying  out 
of  this  plan. 

The  rear  of  the  boards  is  adapted  to  be  enclosed  by  wooden 
curtains,  similar  to  those  employed  in  roll-top  desks.  These  are 
all  raised  in  the  rear  view  of  Fig.  426,  the  housing  for  the  rolled-up 
curtain  being  shown  at  the  extreme  top  of  the  sections.  In  order 
to  guard  the  multiple  cables  and  the  multiple  jacks  against  fire 
which  might  originate  in  the  cord-circuit  wiring,  a  heavy  asbestos 
partition  is  placed  immediately  above  the  cord  racks  and  is  clearly 
shown  in  Fig.  426. 

A  view  of  the  terminal  and  power  room  is  shown  in  Fig.  427. 
In  the  upper  left-hand  corner  the  cables  may  be  seen  in  their  passage 
downward  from  the  cable  turning  section  between  the  A-  and  B- 
boards.  The  large  group  of  cables  shown  at  the  extreme  left  is  the 
A  board  multiple.  This  passes  down  and  then  along  the  horizontal 


HOUSING  CENTRAL-OFFICE  EQUIPMENT  629 

shelves  of  the  intermediate  frame,  which  is  the  frame  in  the  extreme 
left  of  this  view.  The  5-board  multiple  comes  down  through  an- 
other opening  in  the  floor,  and  as  is  shown,  after  passing  under  the 
A-board  multiple  joins  it  in  the  same  vertical  run  from  which  it  passes 
to  the  intermediate  frame.  The  cord-circuit  cables  lead  down  through 
the  same  opening  as  that  occupied  by  the  ^4-board  multiple  and  pass 
off  to  the  right-hand  one  of  the  racks  shown,  which  contains  the 
repeating  coils.  The  cables  leading  from  the  opening  in  the  ceiling 
to  the  right-hand  side  of  the  intermediate  distributing  frame  are  the 
answering-jack  cables,  and  from  the  terminals  on  this  side  of  this 
frame  other  cables  pass  in  smaller  groups  to  the  relay  terminals  on 
the  relay  racks  which  lie  between  the  intermediate  frame  and  the 
coil  rack. 

The  power  board  is  shown  at  the  extreme  right.  The  fuse 
panel  at  the  left  of  the  power  board  contains  in  its  lower  portion  fuses 
for  the  battery  supply  leads  to  the  operator's  position  and  to  private- 
branch  exchanges,  and  in  its  upper  portion  lamps  and  fuses  for 
the  ringing  generator  circuits  for  the  various  operators'  positions 
and  also  for  private-branch  exchanges. 

At  the  lower  left-hand  portion  of  this  view  is  shown  the  battery 
cabinet.  It  is  the  practice  of  the  New  York  Telephone  Company 
not  to  employ  separate  battery  rooms,  but  to  locate  its  storage  bat- 
teries directly  in  the  terminal  room  and  to  enclose  them,  as  shown, 
in  a  wooden  cabinet  with  glass  panels,  which  is  ventilated  by  means 
of  a  lead  pipe  extending  to  a  flue  in  the  wall. 

One  unit  of  charging  machines,  consisting  of  motor  and  gener- 
ator, is  shown  in  the  immediate  foreground.  A  duplicate  of  this 
unit  is  employed  but  is  not  shown  in  this  view.  The  various  ringing 
and  message  register  machines  are  shown  beyond  the  charging  ma- 
chines. Three  of  these  smaller  machines  are  for  supplying  ringing 
current  and  the  remainder  are  for  supplying  30-volt  direct  cur- 
rent for  operating  the  message  registers.  One  of  the  machines  of 
each  set  is  wound  to  run  from  the  main  storage  battery  in  case  of  a 
failure  of  the  general  lighting  service  from  which  the  current  for 
operating  is  normally  drawn. 

Another  view  of  the  terminal-room  apparatus  is  given  in  Fig. 
428.  This  is  taken  from  the  point  marked  B  on  the  floor  plan  of 
Fig.  423.  At  the  right  may  be  seen  the  message  registers  on  which 


630 


TELEPHONY 


the  calls  of  the  subscribers  in  this  office  are  counted  as  a  basis  for 
the  bills  for  their  service.     At  the  extreme  left  is  shown  the  private- 


Fig.  428.    Terminal  Apparatus.    Chelsea  Office 

line  test  board.     Through  this  board  run  all  of  the  lines  leased  for 
private  use,  and  also  all  of  the  order  wire  or  call  lines  passing  through 


Fig.  429.     Floor  Plan,  Automatic  Office,  Lansing,  Michigan 

this  office.     The  purpose  of  such  an  arrangement  is  to  facilitate  the 
testing  of  such  line  wires.     At  the  right   of   this  private-line  test 


HOUSING  CENTRAL-OFFICE  EQUIPMENT 


631 


board   is  shown  a  four-position  wire  chief's  desk,  upon  which  are 
provided  facilities  for  making  all  of  the  tests  inside  and  outside. 


430.     Line-Switch  Units 


The  main  frame  is  shown  at  the  right  of  Fig.  428,  just  to  the  right 
of  a  gallery  from  which  a  step-ladder  leads.     The  left-hand  side  of 


Fig.  431.     Automatic  Apparatus  at  Lansing  Office 

this  frame  is  the  line  or  protector  side,  but  the  portion  toward  the 
observer  in  this  picture  is  unequipped.     These  equipped  protector 


632 


TELEPHONY 


Fig.  432.     Main  Distributing  Frame,  Lansing  Office 


Fig.  433.    Line  Switches 


HOUSING  CENTRAL-OFFICE  EQUIPMENT 


633 


strips  carry  400  pairs  of  terminals  each,  and  the  consequent  length 
of  these  strips  makes  necessary  the  gallery  shown,  in  order  that  all 
of  them  may  be  readily  accessible. 

Automatic  Offices.  There  is  no  great  difference  in  the  amount 
of  floor  space  required  in  central  offices  employing  automatic  and 
manual  equipment.  Whatever  difference  there  is,  is  likely  to  be  in 
favor  of  the  automatic.  The  fact  that  no  such  rigid  requirement 


Fig.  434.     Secondary  Line  Switches  and  First  Selectors 

exists  in  the  arrangement  of  automatic  apparatus,  as  that  which 
makes  it  necessary  to  place  the  sections  of  a  multiple  board  all  in 
one  row,  makes  it  possible  to  utilize  the  available  space  more  eco- 
nomically with  automatic  than  with  manual  equipment. 


634 


TELEPHONY 


Fig.  435.     Second  Selectors 


Fig.  436.     Toll  Distributing  Frame  and  Harmonic  Converters 


HOUSING  CENTRAL-OFFICE  EQUIPMENT          635 

In  manual  practice  it  is  necessary  to  place  the  distributing  frames 
and  power  apparatus  in  a  separate  room  from  that  containing  the 
switchboard,  but  in  an  automatic  exchange  no  such  necessity  exists ; 
in  fact,  so  far  as  the  distributing-frame  equipment  is  concerned,  it 
is  considered  desirable  to  have  it  located  in  the  same  room  as  the 
automatic  switches. 

The  battery  room  in  an  automatic  exchange  should  be  entirely 
separate  from  the  operating  room,  since  the  fumes  from  the  battery 
would  be  fatal  to  the  proper  working  of  the  automatic  switches. 

Typical  Automatic  Office.  The  floor-plan  and  views  of  a  medium- 
sized  automatic  office  at  Lansing,  Michigan,  have  been  chosen  as 
representing  typical  practice.  The  floor  plan  is  shown  in  Fig.  429. 
The  apparatus  indicated  in  full  lines  represents  the  present  equip- 
ment, and  that  in  dotted  lines  the  space  that  will  be  required  by  the 
expected  future  equipment. 

In  Fig.  430  is  shown  a  group  of  five  line-switch  units,  represent- 
ing a  total  of  five  hundred  lines.  The  length  of  such  a  unit  is  prac- 
tically fourteen  feet  and  the  breadth  over  all  about  twenty-two  inches. 

Fig.  431  shows  a  general  view  of  this  Lansing  office,  taken 
from  a  point  of  view  indicated  at  A  on  the  floor  plan  of  Fig.  429. 
Fig.  432  shows  the  main  distributing  frame,  which  is  of  ordinary 
type;  Fig.  433  shows  a  closer  view  of  some  of  the  primary  line  switches; 
Fig.  434  is  a  view  of  the  secondary  line  switches  and  first  selectors, 
the  latter  being  on  the  right;  Fig.  435  is  a  view  of  the  frequency  se- 
lectors and  second  selectors,  the  former  being  used  in  connection 
with  party-line  work;  and  Fig.  436  is  a  view  of  the  toll  distributing 
frame  and  harmonic  converters  for  party-line  ringing. 

A  general  view  of  the  main  switching  room  in  the  Grant  Avenue 
office  of  the  Home  Telephone  Company  of  San  Francisco  is  given  in 
Fig.  437,  this  being  taken  before  the  work  of  installation  had  been 
fully  completed.  The  present  capacity  of  the  equipment  is  6,000  and 
the  ultimate  12,000  lines.  This  office  is  one  of  a  number  of  similar 
ones  recently  installed  for  the  Home  Telephone  Company  in  San 
Francisco,  the  combination  of  which  forms  by  far  the  largest  auto- 
matic exchange  yet  installed.  The  scope  of  the  plans  is  such  as  to  en- 
able 125,000  subscribers  to  be  served  without  any  change  in  the  fun- 
damental design,  and  by  means  merely  of  addition  in  equipment  and 
lines  as  demanded  by  the  future  subscriptions  for  telephone  service. 


CHAPTER  XXXIV 
PRIVATE  BRANCH  EXCHANGES 

Definitions.  A  telephone  exchange  devoted  to  the  purejy  local 
uses  of  a  private  establishment  such  as  a  store,  factory,  or  business 
office,  is  a  private  exchange.  If,  in  addition  to  being  used  for  such 
local  communication,  it  serves  also  for  communication  with  the 
subscribers  of  a  city  exchange,  it  becomes  in  effect  a  branch  of  the 
city  exchange  and,  therefore,  a  private  branch  exchange.  The  term 
"P.  B.  X."  has  become  a  part  of  the  telephone  man's  vocabulary 
as  an  abbreviation  for  private  branch  exchange. 

Private  exchanges  for  purely  local  use  require  no  separate  treat- 
ment as  any  of  the  types  of  switching  equipments  for  interconnecting 
the  lines  for  communication,  that  have  been  or  that  will  be  described 
herein,  may  be  used.  The  problem  becomes  a  special  one,  however, 
when  communication  must  also  be  had  with  the  subscribers  of  a 
public  exchange,  since  then  trunking  is  involved  in  which  the  condi- 
tions differ  materially  from  those  encountered  in  trunking  between 
the  several  offices  in  a  multi-office  exchange. 

For  such  communication  one  or  more  trunk  lines  are  led  from 
the  private  branch  office  usually  to  the  nearest  central  office  of  the 
public  exchange  and  such  trunks  are  called  private  branch-exchange 
trunks.  They  are  the  paths  for  communication  between  the  private 
exchange  and  the  public  exchange.  For  establishing  the  connections 
either  between  the  local  lines  themselves  or  between  the  local  lines  and 
the  trunks,  and  for  performing  other  duties  that  will  be  referred  to, 
one  or  more  private  branch-exchange  operators  are  employed  at  the 
switchboard  of  the  private  establishment. 

The  private  branch  exchange  may  operate  in  conjunction  with 
a  manual  or  an  automatic  public  exchange,  but  whether  manual 
or  automatic,  the  private  exchange  is  usually  manually  operated, 
although  it  is  quite  possible  to  make  a  private  branch  exchange  that 
is  wholly  automatic  and  will,  therefore,  involve  no  operator  at  all. 


638  TELEPHONY 

Functions   of   the    Private    Branch=Exchange   Operator.     It  is 

possible,  as  just  stated,  entirely  to  dispense  with  the  private  branch- 
exchange  operator  so  far  as  the  mere  connection  and  disconnection 
of  the  lines  is  concerned.  But  the  real  function  of  the  private 
branch-exchange  operator  is  a  broader  one  than  this,  and  it  is  for 
this  reason  that  even  in  connection  with  automatic  public  exchanges, 
operators  are  desirable. at  the  private  branches.  The  private  branch- 
exchange  operator  is,  as  it  were,  the  doorkeeper  of  the  telephone 
entrance  to  the  private  establishment.  She  is  the  person  first  met 
by  the  public  in  entering  this  telephone  door.  There  is  the  same 
reason,  therefore,  why  she  should  be  intelligent,  courteous,  and 
obliging  as  that  the  ordinary  doorkeeper  should  possess  these 
characteristics. 

As  to  incoming  traffic  to  a  private  branch  exchange,  an  intelli- 
gent operator  may  do  much  toward  directing  the  calls  to  the  proper 
department  or  person,  even  though  the  person  calling  may  have  little 
idea  as  to  whom  he  desires  to  reach.  This  saves  the  time  of  the  per- 
son who  makes  the  call  as  well  as  that  of  the  people  at  the  private 
branch  stations,  since  it  prevents  their  being  unnecessarily  called. 

The  functions  of  the  private  branch-exchange  operator  are  no 
less  important  with  respect  to  outgoing  calls.  It  is  the  duty  of  the 
operator  to  obtain  connections  through  the  city  exchange  for  the 
private  branch  subscriber,  who  merely  asks  for  a  certain  connection 
and  hangs  up  his  receiver  to  await  her  call  when  she  shall  have 
obtained  it.  This  saving  of  time  of  busy  people  by  having  the  branch- 
exchange  operator  make  their  calls  for  them  has  one  attending  dis- 
advantage, which  is  that  the  person  in  the  city  exchange  who  is  called 
does  not,  when  he  answers  his  telephone,  find  the  real  party  with 
whom  he  is  to  converse,  but  has  to  wait  until  that  party  responds  to 
the  private  branch  operator's  call.  This  is  akin  to  asking  a  person 
to  call  at  one's  office  and  then  being  out  when  he  gets  there.  This 
drawback  is  greatly  accentuated  where  both  the  parties  that  are  to 
be  involved  in  the  connection  are  people  high  in  authority  in  certain 
establishments  at  private  branch  exchanges.  Some  business  houses 
have  made  the  rule  that  the  private  branch  operator  shall  not  con- 
nect with  their  lines  until  she  has  actually  heard  the  voice  of  the 
proper  party  at  the  other  end.  When  two  subscribers  in  two  different 
private  branch  exchanges  where  this  rule  is  enforced,  attempt  to  get 


PRIVATE  BRANCH  EXCHANGES  639 

into  communication  with  each  other,  the  possibilities  of  trouble  are 
obvious. 

All  that  may  be  said  oh  this  matter  is  that  the  person  who  calls 
another  by  telephone  should  extend  that  person  the  same  courtesies 
that  he  would  had  he  called  him  in  person  to  his  office;  and  that  a 
person  who  is  called  by  telephone  by  another  should  meet  him  with 
the  same  consideration  as  if  he  had  received  a  personal  call  at  his 
office  or  home.  The  arbitrary  ruling  made  by  some  corporations 
and  persons,  which  results  always  in  the  "other  fellow's"  doing  the 
waiting,  is  not  ethically  correct  nor  is  it  good  policy. 

Private  Branch  Switchboards.  Private  branch  switchboards  may 
be  of  common-battery  or  magneto  types  regardless  of  whether 
they  work  in  conjunction  with  main  office  equipments  having  com- 
mon-battery or  magneto  equipments.  Usually  a  magneto  private 
branch  exchange  works  in  conjunction  with  a  magneto  main  office, 
but  this  is  not  always  true.  There  are  cases  where  the  private 
branch  equipment  of  modern  common-battery  type  works  in  conjunc- 
tion with  main  office  equipment  of  the  magneto  type;  and  in  some 
of  these  cases  the  private  branch  exchange  has  a  much  larger  number 
of  subscribers  than  the  main  office.  This  is  likely  to  be  true  in  large 
summer  resort  hotels  located  in  small  and  otherwise  unimportant 
rural  districts.  In  one  such  case  within  our  knowledge  the  private 
branch  exchange  has  a  larger  number  of  stations  than  the  total  cen- 
sus population  of  the  town,  resulting  in  an  apparent  telephone 
development  considerably  greater  than  one  hundred  per  cent. 

Magneto  Type.  Where  both  the  private  branch  and  the  main 
office  equipments  are  of  the  magneto  type,  the  private  branch  re- 
quirements are  met  by  a  simple  magneto  switchboard  of  the  requisite 
size,  and  the  trunking  conditions  are  met  by  ring-down  trunks  ex- 
tending to  the  main  office.  In  this  case  the  supervision  is  that  of 
the  ordinary  clearing-out  drop  type,  the  operators  working  together 
as  best  they  may. 

Common- Battery  Type.  The  cases  where  the  private  branch 
board  is  of  common-battery  type  and  the  main  office  of  magneto 
type  are  comparatively  so  few  that  they  need  not  be  treated  here. 
Where  they  do  occur  they  demand  special  treatment  because  the 
main  portion  of  the  traffic  over  the  trunk  lines  to  the  city  or  town 
central  office  is  likely  to  be  toll  traffic  through  that  office  over 


640 


TELEPHONY 


long-distance  lines.  The  principal  reason  why  the  equipment  of 
the  town  offices  under  such  conditions  is  magneto  rather  than  com- 
mon battery  is  that  the  traffic  conditions  are  those  of  short  season  and 
heavy  toll,  and  common-battery  switching  equipment  at  the  main 
office  has  no  especial  advantages  for  toll  work. 

For  small   private  branch   exchanges  the  desk  type  of  switch 
board,  shown  in  Fig.  438,  is  largely  used.     The  operator  frequently 


Fig.  438.     Desk  Type,  Private  Branch  Board 

has  other  work  to  do  and  the  desk  is,  therefore,  a  convenience.  In 
larger  private  exchanges,  such  as  those  requiring  more  than  one 
operator,  some  form  of  upright  cabinet  is  employed,  and  if,  as  some- 
times occurs,  the  branch  exchange  is  of  such  size  as  to  demand 
a  multiple  board,  then  the  general  form  of  the  board  does  not  differ 


PRIVATE  BRANCH  EXCHANGES 


641 


materially  from  the  standard  types  of  multiple  board  employed  in 
regular  central  office  work.  The  most  common  private  branch-ex- 
change condition  is  that  of  a  common-battery  branch  working  into 
a  common-battery  main  office.  In  such  the  main  point  to  be  con- 
sidered is  that  of  supervision  of  trunk-line  connections. 

Cord  Type.  For  the  larger  sizes  of  branch  exchange  switch- 
boards, the  switching  apparatus  is  practically  the  same  as  that  of  or- 
dinary manual  switchboards  wherein  the  connections  are  made  be- 
tween the  various  lines  by  means  of  pairs  of  cords  and  plugs.  The 
private  branch-exchange  trunk  lines  usually  terminate  on  the  private 
branch  board  in  jacks  but  in  some  cases  plug-ended  trunks  are  used. 

The  line  signals  may  consist  in  mechanical  visual  signals  or  in 
lamps,  the  choice  between  these  depending  largely  on  the  source 


Fig.  439.     Key  Type,  Private  Branch  Board 

of  battery  supply  at  the  branch  exchange,  a  matter  which  will  be 
considered  later.  The  trunk-line  signals  at  the  private  branch  board 
are  usually  ordinary  drops  which  are  thrown  when  the  main-exchange 
operator  rings  on  the  line  as  she  would  on  an  ordinary  subscriber's 
line.  Frequently,  however,  lamp  signals  are  used  for  this  purpose, 
being  operated  by  locking  relays  energized  when  the  main-office 
operator  rings  or,  in  some  cases,  operated  at  the  time  when  the 
main-office  operator  plugs  into  the  trunk-line  jack. 

Key  Type.  For  small  private  branch-exchange  switchboards, 
a  type  employing  no  cords  and  plugs  has  come  into  great  favor  dur- 
ing recent  years.  Instead  of  connecting  the  lines  by  jacks  and  plugs, 


642 


TELEPHONY 


they  are  connected  by  means  of  keys  closely  resembling  ordinary 
ringing  and  listening  keys.  Such  a  switchboard  is  shown  in  Fig. 
439,  this  having  a  capacity  of  three  trunks,  seven  local  lines,  and  the 
equivalent  of  five  cord  circuits.  The  drops  associated  with  the  three 
trunks  may  be  seen  in  the  upper  left-hand  side  of  the  face  of  the 
switchboard.  Immediately  below  these  in  three  vertical  rows  are  the 
keys  which  are  used  in  connecting  the  trunks  with  the  "cord  cir- 
cuits" or  connecting  bus  wires.  At  the  right  of  the  drop  associated 


ii;  ii;  i 
fofWirrofTOiTOfroti 

c         I  C    I   >  4         >  4    I     J  c         >  c         i 


Fig.  440.     Circuits,  Key-Type  Board 

with  the  trunks  are  seven  visual  signals,  these  being  the  calling  sig- 
nals of  the  local  lines.  The  seven  vertical  rows  of  keys,  immediately 
to  the  right  of  the  three  trunk-line  rows,  are  the  line  keys.  The 
throwing  of  any  one  of  these  keys  and  of  a  trunk-line  key  in  the  same 
horizontal  row  in  the  same  direction  will  connect  a  line  with  a  trunk 
through  the  corresponding  bus  wires,  leaving  one  of  the  super- 
visory visual  signals,  shown  at  the  extreme  top  of  the  board,  con- 
nected with -the  circuit.  The  keys  in  a  single  row  at  the  right  are  those 
by  means  of  which  the  operator  may  bridge  her  talking  set  across  any 
of  the  "cord  circuits."  The  circuits  of  this  particular  board  are 


PRIVATE  BRANCH  EXCHANGES  643 

shown  in  Fig.  440.  This  is  equipped  for  common-battery  working, 
the  battery  feed  wires  being  shown  at  the  left. 

Supervision  of  Private  Branch  Connections.  At  the  main  office 
where  common-battery  equipment  is  used,  the  private  branch  trunks 
terminate  before  the  .4-operators  exactly  in  the  same  way  as  ordinary 
subscribers'  lines,  i.  e.,  each  in  an  answering  jack  and  lamp  at  one 
position  and  in  a  multiple  jack  on  each  section.  It  goes  without  say- 
ing, therefore,  that  the  handling  of  a  private  branch  call,  either  in- 
coming or  outgoing,  should  be  done  by  the  A  -operator  in  the  same  man- 
ner as  a  call  on  an  ordinary  subscriber's  line,  and  that  the  supervision 
of  the  connection  should  impose  no  special  duties  on  the  yl-operator. 

There  has  been  much  discussion,  and  no  final  agreement,  as  to 
the  proper  method  of  controlling  the  supervisory  lamp  at  the  main 
office  of  a  cord  that  is,  at  the  time,  connected  to  a  private  branch 
trunk.  Three  general  methods  have  been  practiced : 

The  first  method  is  to  have  the  private  branch  subscriber  directly 
control  the  supervisory  lamp  at  the  main  office  without  producing 
any  effect  upon  the  private  branch  supervisory  signal;  this  latter 
signal  being  displayed  only  after  the  connection  has  been  taken  down 
at  the  main  office  and  in  response  to  the  withdrawal  of  the  main  office 
plug  from  the  private  branch  jack.  This  is  good  practice  so  far  as 
the  main-office  discipline  is  concerned  but  it  results  in  a  considerable 
disadvantage  to  both  the  city  and  private  branch  subscribers  in  that 
it  is  impossible  for  the  private  branch  subscriber,  when  connected 
to  the  other,  to  re-signal  the  private  branch  operator  without  the 
connection  being  first  taken  down. 

The  second  method  is  to  have  the  private  branch  subscriber 
control  both  the  supervisory  signal  at  the  private  branch  board  and 
at  the  main  board.  This  has  the  disadvantage  of  bringing  both 
operators  in  on  the  circuit  when  the  private  branch  subscriber  signals. 

The  third  method,  and  one  that  seems  best,  is  to  place  the  super- 
visory lamp  of  the  private  branch  board  alone  under  the  control  of  the 
private  branch  subscriber,  so  that  he  may  attract  the  attention  of  the 
private  branch  operator  without  disturbing  the  supervisory  signal 
at  the  main  office.  The  supervisory  signal  at  the  main  office  in 
this  case  is  displayed  only  when  the  private  branch  operator  takes 
down  the  connection.  This  practice  results  in  a  method  of  operation 
at  the  main  office  that  involves  no  special  action  on  the  part  of  the 


644  TELEPHONY 

yl-operator.  She  takes  down  the  connection  only  when  the  main- 
office  subscriber  has  hung  up  his  telephone  and  the  private  branch 
subscriber  has  disconnected  from  the  trunk. 

Whatever  method  is  employed,  private  branch  disconnection  is 
usually  slow,  and  for  this  reason  many  operating  companies  instruct 
the  A  -opera tors  to  disconnect  on  the  lighting  of  the  supervisory  lamp 
of  the  city  subscriber. 

With  Automatic  Offices.  Private  branch  exchanges  most 
used  in  connection  with  automatic  offices  employ  manual  switch- 
boards, with  the  cord  circuits  of  which  is  associated  a  signal  transmit- 
ting device  by  which  the  operator  instead  of  the  subscriber  may 
manipulate  the  automatic  apparatus  of  the  public  exchange  by 
impulses  sent  over  the  private  branch-exchange  trunk  lines.  The 
subscriber's  equipment  at  the  private  branch  stations  may  be  either 
automatic  or  manual.  Frequently  the  same  private  branch  exchange 
will  contain  both  kinds.  With  the  manual  sub-station  equipment 
the  operation  is  exactly  the  same  as  in  a  private  branch  of  a  manual 
exchange,  except  that  the  private  branch  operator  by  means  of  her 
dial  makes  the  central-office  connection  instead  of  telling  the  main- 
office  operator  to  do  so  for  her.  With  automatic  sub-station  equip- 
ment at  the  private  branch  the  subscribers,  by  removing  their 
receivers  from  their  hooks,  call  the  attention  of  the  private  branch 
operator,  who  may  receive  their  orders  and  make  the  desired  central- 
office  connection  for  them,  or  who  may  plug  their  lines  through  to  the 
central  office  and  allow  the  subscribers  to  make  the  connection  them- 
selves with  their  own  dials. 

In  automatic  equipment  of  the  common-battery  type,  some 
change  always  takes  place  in  the  calling  line  at  the  time  the  called 
subscriber  answers.  In  the  three-wire  system  during  the  time  of 
calling,  both  wires  are  of  the  same  polarity  with  respect  to  earth. 
At  the  time  of  the  answering  of  the  called  subscriber,  the  two  wires 
assume  different  polarities,  one  being  positive  to  the  other.  Such  a 
change  is  sufficient  for  the  actuation  of  devices  local  to  the  private 
exchange  switchboard  and  may  be  interpreted  through  the  calling 
supervisory  signal  in  such  a  way  as  to  allow  it  to  glow  during  calling 
and  not  to  glow  after  the  called  subscriber  has  answered.  In  the 
two-wire  automatic  system  a  similar  change  can  be  arranged  for, 
with  similar  advantageous  results. 


PRIVATE  BRANCH  EXCHANGES  645 

Secrecy.  In  private  exchanges  operating  in  connection  with 
automatic  central  offices,  the  secret  feature  of  individual  lines  may 
or  may  not  be  carried  into  the  private  exchange  equipment.  Some 
patrons  of  automatic  exchanges  set  a  high  value  on  the  absence  of 
any  operator  in  a  connection  and  transact  business  over  such  lines 
which  they  would  not  transact  at  all  over  manual  lines  or  would  not 
transact  in  the  same  way  over  manual  lines.  To  some  such  patrons, 
the  presence  of  a  private  exchange  operator,  even  though  employed 
and  supervised  by  themselves,  seems  to  be  a  disadvantage.  To 
meet  such  a  feeling,  it  is  not  difficult  to  arrange  the  circuits  of  a 
private  exchange  switchboard  so  that  the  operator  may  listen  in  upon 
a  cord  circuit  at  any  time  and  overhear  what  is  being  said  upon  it 
so  long  as  two  subscribers  are  not  in  communication  on  that  cord  circuit. 
That  is,  she  may  answer  a  call  and  may  speak  to  the  calling  person  at 
any  time  she  wishes  until  the  called  person  answers.  When  he  does 
answer  and  conversation  can  take  place,  some  device  operates  to 
disconnect  her  listening  circuit  from  the  cord  circuit,  not  to  be  con- 
nected again  until  at  least  one  of  the  subscribers  has  hung  up  his 
receiver.  With  private  exchange  apparatus  so  arranged,  the  secrecy 
of  the  system  is  complete. 

Battery  Supply.  There  are  three  available  methods  of  sup- 
plying direct  current  for  talking  and  signaling  purposes  to  private 
branch  exchanges,  each  of  which  represents  good  practice  under 
certain  conditions.  First,  by  means  of  pairs  of  wires  extended  from 
the  central-office  battery;  second,  by  means  of  a  local  storage  battery 
at  the  private  branch  exchange  charged  over  wires  from  the  central 
office;  and  third,  by  means  of  a  local  storage  battery  at  the  private 
exchange  charged  from  a  local  source. 

The  choice  of  these  three  methods  depends  always  on  the  local 
conditions  and  it  is  a  desirable  feature,  to  be  employed  by  large 
operating  companies,  to  have  all  private  branch-exchange  switch- 
boards provided  with  simple  convertible  features  contained  within 
the  switchboard  for  adapting  it  to  any  one  of  these  methods  of  sup- 
plying current. 

If  a  direct-current  power  circuit  is  available  at  the  private 
branch  exchange,  it  may  be  used  for  charging  the  local  storage  bat- 
tery by  inserting  mere  resistance  devices  in  the  charging  leads.  If 
the  local  power  circuit  carries  alternating  current,  a  converting  de- 


646  TELEPHONY 

vice  of  some  sort  must  be  used  and  for  this  purpose,  if  the  exchange 
is  large  enough  to  warrant  it,  a  mercury  rectifier  is  an  economical  and 
simple  device. 

The  supply  of  current  to  private  branch  exchanges  over  wires 
leading  to  the  central-office  battery  has  the  disadvantage  of  requiring 
one  or  several  pairs  of  wires  in  the  cables  carrying  the  trunk  wires. 
No  special  wires  are  run,  regular  pairs  in  the  paper  insulated  line  or 
trunk  cables  being  admirably  suited  for  the  purpose.  Sufficient  con- 
ductivity may  be  provided  by  placing  several  such  pairs  in  multiple. 

If  the  amount  of  current  required  by  the  private  exchange 
warrants  it,  pairs  of  charging  wires  from  the  central  office  may  be 
fewer  if  a  battery  is  charged  over  them  than  if  they  are  used  direct 
to  the  bus  bars  of  the  private  exchange  switchboard.  If  they  are 
used  in  the  latter  way,  and  this  is  simpler  for  reasons  of  maintenance, 
some  means  must  be  provided  to  prevent  the  considerable  resistance 
of  the  supply  wires  from  introducing  cross-talk  into  the  circuit  of 
the  private  exchange.  This  is  accomplished  by  bridging  a  con- 
siderable capacity  across  the  supply  pairs  at  the  private  exchange — 
ten  to  twelve  microfarads  usually  suffice.  This  point  has  already 
been  referred  to  and  illustrated  in  connection  with  Fig.  141. 

The  number  of  pairs  of  wires,  or,  in  other  words,  the  amount  of 
copper  in  the  battery  lead  between  the  central  office  and  the  private 
branch-exchange  switchboard  needs  to  be  properly  determined  not 
only  to  eliminate  cross-talk  when  the  proper  condensers  are  used  with 
them,  but  to  furnish  the  proper  difference  of  potential  at  the  private 
exchange  bus  bars,  so  that  the  line  and  supervisory  signals  will  receive 
the  proper  current.  It  is  a  convenience  in  installing  and  maintaining 
private  exchange  switchboards  of  this  kind  to  prepare  tables  show- 
ing the  number  of  pairs  of  No.  19  gauge  and  No.  22  gauge  wires 
required  for  a  private  exchange  at  a  given  distance  from  its  central 
office  and  of  a  probable  amount  of  traffic.  The  traffic  may  be  ex- 
pressed in  the  maximum  number  of  pairs  of  cords  which  will  be  in 
use  at  one  time.  With  this  fact  and  the  distance,  the  number  of  pairs 
of  wires  required  may  be  determined. 

Ringing  Current.  The  ringing  current  may  be  provided  in 
two  ways:  over  pairs  of  wires  from  the  city-office  ringing  machines 
or  by  means  of  a  local  hand  generator,  or  both.  A  key  should  enable 
either  of  these  sources  of  ringing  current  to  be  chosen  at  will. 


PRIVATE  BRANCH  EXCHANGES  647 

Marking  of  Apparatus.  All  apparatus  should  be  marked  with 
permanent  and  clear  labels.  That  private  exchange  switchboard  is 
best  at  which  an  almost  uninformed  operator  could  sit  and  operate 
it  at  once.  It  is  not  difficult  to  lay  out  a  scheme  of  labels  which  will 
enable  such  a  board  to  be  operated  without  any  detailed  instructions 
being  given. 

Desirable  Features.  The  board  should  contain  means  of  con- 
necting certain  of  the  local  private  exchange  lines  to  the  central- 
office  trunks  when  the  board  is  unattended.  Also,  it  is  desirable 
that  it  should  contain  means  whereby  any  local  private  exchange 
line  may  be  connected  to  the  trunk  so  that  its  station  will  act  as  an 
ordinary  subscriber's  station.  Whether  the  trunks  of  the  private 
exchange  lead  to  a  manual  or  an  automatic  equipment,  it  often  is 
desired  to  connect  a  local  line  through  in  that  way,  either  so  that  the 
calling  person  may  make  his  calls  without  the  knowledge  of  the  pri- 
vate exchange  operator,  because  he  wishes  to  make  a  large  number 
of  calls  in  succession,  or  because  for  some  other  reason  he  prefers  to 
transact  his  business  directly  with  or  through  the  exchange  than  to 
entrust  it  to  his  operator, 


CHAPTER  XXXV 
INTERCOMMUNICATING  SYSTEMS 

Definition.  The  term  "intercommunicating"  has  been  given 
to  a  specialized  type  of  telephone  system  wherein  the  line  belonging 
to  each  station  is  extended  to  each  of  the  other  stations,  resulting  in 
all  lines  extending  to  all  stations.  Each  station  is  provided  with 
apparatus  by  means  of  which  the  telephone  user  there  may  connect 
his  own  telephone  with  the  line  of  the  station  with  which  he  wishes 
to  communicate,  enabling  him  to  signal  and  talk  with  the  person  at 
that  station. 

Limitations.  The  idea  is  simple.  Each  person  does  his  own 
switching  directly,  and  no  operator  is  required.  It  is  easy  to  see. 
however,  that  the  system  has  limitations.  The  amount  of  line  wire 
necessary  in  order  to  run  each  line  to  each  station  is  relatively  great, 
and  becomes  prohibitive  except  in  exchanges  involving  a  very  small 
number  of  subscribers,  none  of  which  is  remote  from  the  others. 
Again,  the  amount  of  switching  apparatus  required  becomes  pro- 
hibitive for  any  but  a  small  number  of  stations.  As  a  result,  twenty- 
five  or  thirty  stations  are  considered  the  usual  practical  limit  for 
intercommunicating  systems. 

Types.  An  intercommunicating  system  may  be  either  mag- 
neto or  common-battery,  according  to  whether  it  uses  magneto  or 
common-battery  telephones.  The  former  is  the  simpler;  the  latter 
is  the  more  generally  used. 

Simple  Magneto  System.  The  schematic  circuit  arrangement 
of  an  excellent  form  of  magneto  intercommunicating  system  is  given 
in  Fig.  441.  In  this,  five  metallic  circuit  lines  are  led  to  as  many 
stations,  an  ordinary  two-contact  open  jack  being  tapped  off  of  each 
line  at  each  station.  A  magneto  l>ell  of  the  bridging  type  is  per- 
manently bridged  across  each  line  at  the  station  to  which  that  line 
belongs.  The  telephone  at  each  station  is  an  ordinary  bridging 
magneto  set  except  that  its  bell  is,  in  each  case,  connected  to  the 


INTERCOMMUNICATING  SYSTEMS 


649 


line  as  just  stated.  Each  telephone  is  connected  through  a  flexible 
cord  to  a  two-contact  plug  adapted  to  fit  into  any  of  the  jacks  at  the 
same  station. 

The  operation  is  almost  obvious.  If  a  person  at  Station  A  desires 
to  call  Station  E,  he  inserts  his  plug  into  the  jack  of  line  E  at  his 
station  and  turns  his  generator  crank.  The  bell  of  Station  E  rings 
regardless  of  where  the  plug  of  that  station  may  be.  The  per- 
son at  Station  E  responds  by  inserting  his  own  plug  in  the  jack 
of  line  E,  after  which  the  two  parties  are  enabled  to  converse  over 
a  metallic  circuit.  It  makes  no  difference  whether  the  persons, 


^STATION    a 


3  TAT/OH     C 


I .J 

I — '  f- 


1 — ?*ff 


UmLJ 


LMJ 


Fig.  441.     Magneto  Intercommunicating  System 


after  talking,  leave  these  plugs  in  the  jacks  or  take  them  out,  since 
the  position  of  the  plug  does  not  alter  the  relation  of  the  bell  with 
the  line. 

This  system  has  the  advantage  of  great  simplicity  and  of  being 
about  as  "fool  proof"  as  possible.  It  is,  however,  not  quite  as  con- 
venient to  use  as  the  later  common-battery  systems  which  require 
no  turning  of  a  generator  crank. 

Common=Battery  Systems.  In  the  more  popular  common- 
battery  systems  two  general  plans  of  operation  are  in  vogue,  one 
employing  a  plug  and  jacks  at  each  station  for  switching  the  "home" 
instrument  into  circuit  with  any  line,  and  the  other  employing  merely 


650 


TELEPHONY 


push  buttons  for  doing  the  same  thing.     These  may  be  referred  to  as 
the  plug  type  and  the  push-button  type,  respectively. 

Kellogg  Plug  Type.     The  circuits  of  a  plug  type  of  intercom- 


Fig.  442.     Plug  Type  of  Common-Battery  Intercommunicating  System 

municating  system,  as  manufactured  by  the  Kellogg  Company,  are 
shown  in  Fig.  442.     While  only  three  stations  are  shown,  the  method 

of  connecting  more  will  be  obvious. 
This  system  requires  as  many 
pairs  of  wires  running  to  all  sta- 
tions as  there  are  stations,  and  in 
addition,  two  common  wires  for 
ringing  purposes.  The  talking  bat- 
tery feed  is  through  retardation 
coils  to  each  line.  When  all  the 
hooks  are  down,  each  call  bell  is 
connected  between  the  lower  com- 
mon wire  and  the  tip  side  of  the 
talking  circuit  individual  to  the 
corresponding  station.  The  ringing 
buttons  at  each  station  are  con- 
nected between  the  tip  of  the  plug 
at  that  station  and  the  upper  com- 
mon wire.  As  a  result,  when  a  per- 
son at  one  station  desires  to  call  an- 
Fig.  443.  Push-Button  Wall  Set  other,  it  is  only  necessary  for  him  to 


INTERCOMMUNICATING  SYSTEMS 


651 


insert  his  plug  in  the  jack  of  the  desired  station  and  press  his  ringing 
button;  the  circuit  being  traced  from  one  pole  of  the  ringing  battery 
through  the  upper  common  ringing  wire,  ringing  key  of  the  station 
making  the  call,  tip  of  plug,  tip  conductor  of  called  station's  line,  bell 
of  called  station,  and  back  to  the  ringing  battery  through  the  lower 
common  ringing  wire. 

Kellogg  Push-Button  Type.  Fig.  443  shows  a  Kellogg  wal)- 
type  intercommunicating  set  employing  the  push-button  method  of 
selecting,  and  Fig.  444  shows  the  internal  arrangement  of  this  set. 


Fig.  444.     Push-Button  Wall  Set 


Western  Electric  System.  The  method  of  operation  of  the  push- 
button key  employed  in  the  intercommunicating  system  of  the  West- 
ern Electric  Company  is  well  shown  in  Fig.  445.  When  the  button 
is  depressed  all  the  way  down,  as  shown  in  the  center  cut  of  Fig.  445, 
which  represents  the  ringing  position  of  the  key,  contact  is  made  with 
the  line  wires  of  the  station  called,  and  ringing  current  is  placed  on 
the  line.  When  the  pressure  is  released,  the  button  assumes  an  in- 
termediate position,  as  shown  in  the  right-hand  cut,  which  represents 
the  talking  position  of  the  key  and  in  which  the  ringing  contacts  1  at. 


652 


TELEPHONY 


2  are  open,  but  contact  with  the  line  for  talking  purposes  is  maintained. 
The  key  is  automatically  held  in  this  intermediate  position  by  locking 
plate  3  until  this  plate  is  actuated  by  the  operation  of  another  but- 
ton which  releases  the  key  so  that  it  assumes  its  normal  position  as 
shown  in  the  left-hand  cut.  When  a  button  is  depressed  to  call  a 


Fig.  445.     Push-Button  Action,  Western  Electric  System 

station,  it  first  connects  the  called  station's  line  to  the  calling  station 
through  the  two  pairs  of  contacts  4  and  5  and  then  connects  the  ring- 
ing battery  to  that  line  by  causing  the  spring  /  to  engage  the  contact 
2.  The  ringing  current  then  passes  through  the  bell  at  the  called 
station,  through  the  back  contacts  of  the  switch  hook  at  that  station, 
over  one  side  of  the  line,  and  through  the  "way-down"  contact  1  of 


Fig.  446.     Push-Button  Wall  Set 

the  button  at  the  calling  station,  thence  over  the  other  side  of  the 
battery  line  back  to  the  ringing  battery,  operating  the  bell  at  the 
called  station. 

The  circuits  of  the  Western  Electric  system  are  similar  to  those 
of  Fig.  442,  but  adapted,  of  course,  to  the  push-button  arrangement 
of  switches.  Two  batteries  are  employed,  one  for  ringing  and  the 


INTERCOMMUNICATING  SYSTEMS 


653 


other  for  talking,  talking  current  being  fed  to  the  lines  through  re- 
tardation coils  to  prevent  interference  or  cross-talk  from  other  sta- 
tions which  might  be  connected  together  at  the  same  time. 

Monarch  System.  As  the  making  of  connections  in  an  inter- 
communicating system  is  entirely  in  the  hands  of  the  user,  it  is  de- 
sirable that  the  operation  be  simple  and  that  carelessness  on  the 
part  of  the  user  result  in  as  few  evil  effects  as  possible.  For  instance, 
the  leaving  of  the  receiver  off  its  hook  will,  in  many  systems,  result 
in  such  a  drain  on  the  battery  as  to  greatly  shorten  its  life. 

The  system  of  the  Monarch  Company  has  certain  distinctive 
features  in  this  respect.     It  is  of  the  push-button  type  and  as  in  the 
system  just  discussed,  one  pressure  of  the  finger  on  one  button  clears 
the  station  of  previous  connections,   rings   the  station  called,  and 
establishes  a  talking  connection 
between    the    caller's    telephone 
and    the  line  desired.     In  addi- 
tion to  this,  the  system  is  designed 
to  eliminate  battery  waste  by  so 
arranging   the    circuits    that   the 
battery    current    does    not    flow 
through  either  called  or  calling 
instrument  until  a  complete  con- 
nection is  made — the  calling  but- 
ton down  at  one  station,  the  home  button  down  at  the  called  sta- 
tion, and  both  receivers  off  the  hook.      It  does  not  hurt  the  bat- 
teries, therefore,  if  one  neglects  to  hang  up  his  receiver. 

Three  views  of  the  wall  set  of  this  system  are  shown  in  Fig. 
446,  which  illustrates  how  both  the  door  and  the  containing  box 
are  separately  hinged  for  easy  access  to  the  apparatus  and  connecting 
rack.  As  in  the  Western  Electric  and  Kellogg  push-button  systems, 
each  push-button  key  has  three  positions,  as  shown  in  Fig.  447.  The 
first  button  shows  all  the  springs  open,  the  normal  position  of  the 
key.  The  second  button  is  in  the  half-way  or  talking  position  with 
all  the  springs,  except  the  ringing  spring,  in  contact.  The  third 
button  shows  the  springs  all  in  contact,  the  condition  which  exists 
when  ringing  a  station. 

The  mechanical  construction  of  the  key  is  shown  in  Fig.  448. 
Each  button  has  a  separate  frame  upon  which  the  springs  are  mounted. 


TALKING  fOSH 


Fig.  447.     Push-Button  Action, 
Monarch  System 


654 


TELEPHONY 


Any  one  of  the  frames  with  its  group  of  contact  springs  may  be  re- 
moved without  interfering  with  either  the  electrical  or  the  mechan- 
ical operation  of  the  others.  This  is  a  convenient  feature,  making 
possible  the  installation  of  as  few  stations  as  are  needed  at  first,  and 
the  subsequent  addition  of  buttons  as  other  stations  are  added. 

The  restoring  feature  is  a  horizontal  metal  carriage,  in  construc- 
tion very  much  like  a  ladder — one  round  pressing  against  each  key 
frame,  due  to  the  tension  on  the  carriage  exerted  by  a  single  flat 
spring.  The  plunger  of  each  button  is  equipped  with  a  shoulder, 


Fig.  448.     Push-Button  Keys 

which  normally  is  above  the  round  of  the  ladder.  When  the  button 
is  operated,  this  shoulder  presses  against  a  round  of  the  carriage 
forcing  it  over  far  enough  so  that  the  shoulder  can  slip  by.  The 
upper  surface  of  the  shoulder  is  flat,  and  on  passing  below  the  pin, 
allows  the  carriage  to  slip  back  into  its  normal  position  and  the  pin 
rests  on  the  top  of  the  shoulder  holding  the  plunger  down.  This 
position  places  the  talking  springs  in  contact.  The  ringing  springs 
are  open  until  the  plunger  is  pressed  all  the  way  down,  then  the  ring- 
ing contact  is  made.  When  the  pressure  is  released,  the  plunger 
comes  back  to  the  half-way  or  talking  position,  leaving  the  ringing 
contacts  open  again. 


INTERCOMMUNICATING  SYSTEMS 


655 


When  another  button  is  pressed,  the  same  operation  takes  place 
and,  by  virtue  of  the  carriage  being  temporarily  displaced,  the  orig- 
inal key  is  left  free  to  spring  back  to  its  normal  position. 

Each  station  is  provided  with  a  button  for  each  other  station  and 
a  "home"  button.  The  salient  feature  of  the  system  is  that  before 
a  connection  may  be  established,  the  button  at  the  calling  station 
corresponding  to  the  station  called  and  also  the  home  button  of  the 
station  called  must  be  depressed,  if  it  is  not  already  down.  The 
home  key  at  any  station,  when  depressed,  transposes  the  sides  of  the 
line  with  respect  to  the  talking  apparatus.  The  home  key  also  has 
a  spring  which  changes  the  normal  connection  of  the  line  at  that 
station  from  the  negative  to  the  positive  side  of  the  talking  battery. 


-Ofr - 

Fig.  449.     Monarch  Intercommunicating  System 

Unless,  therefore,  a  connection  between  two  stations  is  made  through 
the  calling  key  at  one  station  and  the  home  key  at  the  other,  no  cur- 
rent can  flow  even  though  both  receivers  are  off  their  hooks,  because 
in  that  case  no  connection  will  exist  with  the  positive  side  of  the  bat- 
tery. This  relation  is  shown  in  Fig.  449,  which  gives  a  simplified 
circuit  arrangement  for  two  connected  stations. 

Referring  to  Fig.  449,  when  the  station  called  depresses  the  home 
button  the  talking  circuit  is  then  completed  after  the  hook  switch  is 
raised.  This  is  because  the  talking  battery  is  controlled  by  the 
home  key.  Conductors  from  both  the  negative  and  the  positive 
sides  of  the  battery  enter  this  key.  In  the  normal  position  of  the 
springs,  the  negative  side  of  the  battery  is  in  contact  with  the  master 
spring  in  the  home  key  and  through  these  springs  the  negative  bat- 
tery is  applied  to  all  the  calling  keys,  and  from  there  on  to  the  hook 
switch.  When,  however,  the  home  button  is  operated,  the  spring 


656  TELEPHONY 

which  carries  the  negative  battery  to  the  home  key  is  opened,  and 
the  spring  which  carries  the  positive  battery  is  closed.  This  puts 
the  positive  battery  on  at  the  hook  switch  instead  of  the  negative 
battery,  as  in  its  normal  condition. 

In  this  system"  it  is  seen  that  a  separate  pair  of  line  wires  is  used 
for  each  station,  and  in  addition  to  these,  two  common  pairs  are  run 
to  all  stations,  one  for  ringing  and  one  for  talking  battery  connections. 

For  Private  Branch  Exchanges.  So  far  the  intercommunicating 
system  has  been  discussed  only  with  respect  to  its  use  in  small  iso- 
lated plants.  It  has  a  field  of  usefulness  in  connection  with  city  ex- 
change work,  as  it  may  be  made  to  serve  admirably  as  a  private 
branch  exchange.  Where  this  is  done,  one  or  more  trunk  lines  lead- 
ing to  an  office  of  the  city  exchange  are  run  through  the  intercom- 
municating system  exactly  as  a  local  line  in  that  system,  being 
tapped  to  a  jack  or  push  button  at  every  station.  A  person  at  any 
one  of  the  stations  may  originate  a  call  to  the  main  office  by  inserting 
his  plug  in  the  trunk  jack,  or  pushing  his  trunk  push  button.  Also 
any  station,  within  hearing  or  sight  of  the  trunk-line  signal  from 
the  main  office,  may  answer  a  main-office  call  in  .he  same  way.  In 
order  that  the  convenience  of  a  private  branch  exchange  may  be 
fully  realized,  however,  it  is  customary  to  provide  an  attendant's 
station  at  which  is  placed  the  drop  or  bell  on  which  the  incoming 
trunk  signal  is  received.  The  duty  of  this  attendant  during  busi- 
ness hours  is  to  answer  trunk  calls  from  the  main  office  and  finding 
out  what  party  is  desired,  call  up  the  proper  station  on  the  intercom- 
municating system.  The  party  at  that  station  may  then  connect 
himself  with  the  trunk. 

The  practice  of  the  Dean  Company,  for  instance,  is  as  follows 
in  regard  to  trunking  between  intercommunicating  systems  and 
main  offices  with  common-battery  equipment.  The  attendant's 
station  telephone  cabinet  contains,  besides  the  push-button  keys 
for  local  and  trunk  connections,  a  drop  signal  and  release  key,  to- 
gether with  relays  in  each  trunk  circuit.  The  latter  are  used  to  hold 
the  trunks  until  the  desired  party  responds. 

The  main-exchange  trunk  lines,  besides  terminating  at  the  at- 
tendant's station,  are  wired  through  the  complete  intercommunicating 
system  so  that  any  intercommunicating  telephone  can  be  connected 
direct  to  the  central  office  by  depressing  the  trunk  key,  which  is  pro- 


INTERCOMMUNICATING  SYSTEMS 


657 


vided  with  a  button  of  distinctive  color.  The  pressing  of  the  trunk 
key  allows  the  telephone  to  take  its  current  from  the  main-office 
storage  battery  and  to  operate 
the  main-office  line  and  super- 
visory signals  direct,  without 
making  it  necessary  to  call  on 
the  attendant  to  set  up  the 
connection. 

Incoming  calls  from  the 
common-battery  main  office  to 
the  intercommunicating  system 
are  all  handled  by  the  attend- 
ant. The  main-office  operator  signals  the  intercommunicating  sys- 
tem by  ringing,  the  same  as  for  a  regular  subscriber's  line.  This 
will  operate  a  drop  in  the  attendant's  station  cabinet,  and  through 


Pi?.  450.     Junction  Box 


JUNCTION  BOX 

//PAIR  SA  TUffA  TED     


JUHCT/ONBOX 


[UPAJR LEAD  COVERED  CABL  E 

TOWEATHEJf 


WALL  SET         WALL  SET 
Wm/OKEYS     W/TH/OKEY5 


//PA/ff  SATURATED  CABLE  FO/f 

MTEff/o/r  ttsetttormocA  T/ONJ 


Tfr(/f//f RELEASE      ATTENDANT'S 
HEY  STAT/OH  CAB/NET 

Tff(/m  LtNE 


<?caw.co>FD--~-<j    / 

KAY  BE  LOCATED  OH   / 

~s«X 

//NTE/TC 


TffUVX L/NE 5 
TOMAffEXCttWGE 


FX/ff  W/ffES 
Fig.  451.     Typical  Arrangement  of  Intercommunicating  System 

an  armature  contact,  give  a  signal  on  a  low-pitched  buzzer.  This 
alarm  buzzer  operates  only  when  the  main  exchange  is  ringing  and, 
therefore,  does  not  require  that  the  drop  shutter  be  restored  imme- 
diately. An  extra  key  may  be  provided  for  an  extension  night-alarm 
bell,  for  use  where  the  attendant  also  does  work  in  a  room  separate 
from  that  containing  the  attendant's  station  telephone  equipment. 


658  TELEPHONY 

The  attendant  operator  answers  the  main-line  signal  by  press- 
ing the  proper  trunk  button,  as  designated  by  the  operated  drop 
on  the  attendant's  cabinet.  The  answering  of  the  trunk  connects 
a  locking  relay  across  the  circuit  so  that  the  attendant  may  call 
the  desired  party  on  the  intercommunicating  system  without  having 
to  hold  the  trunk  manually.  The  party  desired  is  then  notified  which 
trunk  to  use  and  the  attendant  operator  hangs  up  her  receiver,  no 
further  attention  being  necessary  on  her  part. 

The  trunk-holding  relay  is  automatically  released  when  the 
desired  party  (with  the  telephone  receiver  off  the  hook)  depresses  the 
proper  trunk  button,  thus  clearing  the  trunk  line  of  all  bridged 
apparatus  and  making  the  talking  circuit  the  same  as  in  the  regular 
type  of  private  branch-exchange  switchboard. 

The  most  convenient  way  of  installing  the  wires  of  an  inter- 
communicating system  is  to  run  a  cable  containing  the  proper  number 
of  pairs  to  provide  for  the  ultimate  number  of  stations  to  all  the  sta- 
tions, tapping  off  from  the  conductors  in  the  cable  to  the  jacks  or 
push  buttons  at  each  station.  These  tap  connections  are  best  made 
by  means  of  junction  boxes  which  contain  terminals  for  all  the  con- 
ductors. 

Such  a  junction  box,  with  the  through  cable  and  the  tap  cable 
in  place,  is  illustrated  in  Fig.  450.  A  schematic  lay-out  of  the  vari- 
ous parts  of  a  Dean  intercommunicating  system,  provided  with  an 
attendant's  station  and  with  trunks  to  a  city  office,  is  given  in  Fig.  451. 


CHAPTER  XXXVI 
LONQ=DISTANCE  SWITCHING 

Definitions.  Telephone  messages  between  communities  are 
called  long-distance  messages.  They  are  also  called  toll  messages. 
Almost  all  long-distance  traffic  is  handled  by  message-rate  (meas- 
ured-service) methods  of  charge.  All  measured-service  messages  are 
toll  messages,  whether  they  are  completed  within  a  given  community 
or  between  communities.  The  term  "long-distance,"  therefore,  is 
more  descriptive  than  the  term  "toll."  The  subject  of  local  and 
long-distance  measured  service  is  treated  exhaustively  in  a  chapter  of 
its  own. 

Some  telephone-exchange  operating  companies  call  their  own 
inter-city  business  "toll,"  and  use  the  term  "long-distance"  for  busi- 
ness carried  between  exchanges  for  them  by  another  company.  The 
distinction  seems  to  be  unwarranted. 

Use  of  Repeating  Coil.  Most  long-distance  lines  are  mag- 
neto circuits.  If  they  are  switched  to  grounded  circuits,  repeating 
coils  need  to  be  inserted.  Toll  switching  equipments  contain  means 
of  inserting  repeating  coils  in  the  connecting  cords  when  required. 
Their  use  reduces  the  volume  of  transmitted  speech,  but  often  is 
essential  even  in  connecting  metallic  circuit  lines,  as  a  quiet  local 
metallic  circuit  may  have  a  ground  upon  it  which  will  cause  excessive 
noises  when  a  quiet  long-distance  line  is  connected  to  it. 

Switching  through  Local  Board.  In  the  simplest  form  of 
long-distance  switching,  the  lines  terminate  in  switchboards  with 
local  lines  and  may  be  connected  with  each  other  and  with  the  local 
lines  through  the  regular  cord  circuits,  if  the  equipment  be  of  the 
magneto  type.  The  waystations  on  such  a  line  are  equipped  with 
magneto  generators.  These  waystations  may  signal  each  other  by 
bell  ringing;  the  central  office  may  call  any  waystation  by  ringing  the 
proper  signal  and  may  supervise  in  a  way  all  traffic  on  such  lines 
by  noting  the  calls  for  other  stations  than  the  supervising  exchange. 


060  TELEPHONY 

Operators'  Orders.  By  Call  Circuits.  Where  the  long-dis- 
tance traffic  between  two  communities  is  large,  economy  requires 
that  the  sending  of  signals  by  ringing  over  the  line,  waiting  for  an 
answer,  and  then  reciting  the  details  of  the  call,  be  improved  upon. 
If  the  traffic  is  large  and  the  distance  between  communities  small, 
call  circuits  are  established  in  the  same  way  as  between  the  switch- 
boards in  several  manual  central  offices  of  an  exchange.  The  long- 
distance operator  handling  the  originating  call  passes  the  necessary 
details  to  the  distant  operator  by  telephone  over  the  call  circuit. 
Such  circuits  also  are  known  as  order  circuits.  They  are  accessible 
to  originating  operators  at  keys  and  are  connected  directly  and  per- 
manently to  the  telephone  sets  of  receiving  operators.  One  call 
circuit  can  handle  the  orders  for  a  large  number  of  actual  conver- 
sation circuits.  The  operator  at  the  receiving  end  designates  the 
conversation  circuit  which  shall  be  used,  the  originating  operator 
following  that  instruction. 

By  Telegraph.  Where  traffic  and  distance  are  large,  conver- 
sation lines  cost  more  than  in  the  case  last  assumed.  It  then  is  of 
greater  importance  to  use  all  the  possible  talking  circuits  for  actual 
conversations  in  order  that  the  revenue  may  be  as  high  as  possible. 
A  phantom  circuit  good  enough  for  call  circuit  purposes  would  be 
good  enough  for  actual  commercial  messages,  therefore,  it  is 
customary  to  furnish  such  originating  and  receiving  operators  with 
Morse  telegraph  sets.  The  lines  are  obtained  by  applying  compos- 
ite apparatus  to  the  conversation  circuits.  Two  Morse  circuits  can 
be  had  from  each  long-distance  line  without  impairing  any  quality 
of  that  line  except  the  ability  to  ring  over  it.  As  one  Morse  circuit 
can  carry  information  enough  between  two  operators  to  enable  them 
to  keep  many  telephone  circuits  busy,  they  do  not  need  to  ring  upon 
the  composited  lines,  so  that  nothing  is  lost  while  revenue  is  gained. 
Two=Number  Calls.  In  cases  where  the  traffic  between  com- 
munities is  large,  where  the  rate  is  small,  and  where  the  conversa- 
tions are  short  and  more  on  the  general  order  of  local  calls,  it  is  usual 
to  handle  the  switches  exactly  as  local  calls  are  trunked  between 
central  offices  of  the  same  exchange.  That  is,  the  subscriber's 
operator  who  answers  the  call  trunks  it,  by  the  assistance  of  a  call 
circuit  and  an  incoming  trunk  operator.  The  subscriber's  operator 
records  only  the  numbers  of  the  calling  and  called  subscribers. 


LONG-DISTANCE  SWITCHING  661 

No  long-distance  operators  at  all  assist  in  these  connections.  They 
are  known  as  "two-number  calls."  The  calling  subscriber  remains 
at  his  telephone  until  the  conversation  is  finished. 

Particular=Party=Calls.  In  cases  where  the  traffic  is  smaller, 
and  where  the  rate  is  large,  it  is  customary  to  handle  the  calls  through 
long-distance  operators.  The  ticket  records  the  particular  party 
wished,  and  the  calls  are  named  "particular  party"  calls.  In  such 
connections  the  calling  pa.tron  is  allowed  to  hang  up  his  receiver, 
after  his  call  is  recorded,  and  is  called  again  when  his  correspondent 
is  found  and  is  ready  to  talk.  This  makes  all  calls  for  conversations 
outgoing  ones.  Only  recording  operators  receive  calls  from  patrons. 
Line  operators  make  calls  to  patrons. 

Trunking.  Long-distance  lines  entering  a  city  usually  termi- 
nate in  one  office  only,  no  matter  how  many  offices  the  local  exchange 
may  have.  It  is  possible  to  terminate  these  long-distance  lines  on 
a  position  of  the  multiple  switchboard  for  local  lines.  For  a  variety 
of  reasons  this  is  not  practiced  except  in  special  cases.  The  usual 
method  is  to  terminate  them  in  a  special  long-distance  board  and 
to  provide  trunk  lines  from  this  board  to  the  one  or  more  local  switch- 
boards of  the  exchange.  In  common-battery  systems  these  toll 
trunks  are  so  arranged  that  the  called  local  subscriber  receives  trans- 
mitter current  from  the  office  nearest  to  him,  yet  is  able  to  show  the 
long-distance  operator  the  position  of  his  switch  hook  and  is  able  to 
be  called  by  the  long-distance  operator  without  the  intervention  of 
the  switching  operator  in  the  local  office,  even  though  two  repeating 
coils  may  be  in  the  trunk  circuit. 

Through  Ringing.  There  is  a  distinct  traffic  advantage  in 
having  the  ringing  of  the  subscriber  under  the  control  of  the  long- 
distance operator.  The  latter  may  call  for  the  subscriber  by  stating 
her  wish  over  the  call  circuit  associated  with  the  long-distance  trunk. 
The  connection  having  been  made  by  the  switching  operator,  the 
long-distance  operator  may  withhold  ringing  the  subscriber's  bell 
until  all  is  in  readiness  for  the  conversation. 

High-Voltage  Toll  Trunks.  In  some  systems,  the  long-dis- 
tance trunks  are  further  specialized  by  being  enabled  to  furnish 
transmitter  current  to  subscribers  at  a  higher  voltage  than  is  used  in 
local  conversations,  With  a  given  construction  of  transmitters 
there  is  a  critical  maximum  current  which  can  be  carried  by  the 


662  TELEPHONY 

granular  carbon  of  the  instrument  without  excessive  heating,  con- 
sequent noises,  and  permanent  damage.  The  shortest  lines  and  the 
longest  lines  of  an  exchange  district  being  served  by  a  source  of 
current  common  to  all,  the  standard  potential  of  this  source  must  be 
such  as  to  give  the  longest  lines  current  enough  without  giving  the 
shortest  lines  too  much.  The  very  longest  local  lines,  however,  do 
not  receive  current  enough  from  the  standard  potential  to  give  max- 
imum efficiency  when  talking  over  long  distances,  though  they  get 
enough  for  local  conversations.  By  providing  a  battery  with  a  volt- 
age twice  that  used  for  local  conversations  and  connecting  it  into  the 
current  supply  element  of  the  toll  trunk  through  non-inductive  re- 
sistances, not  too  much  current  may  be  given  to  the  shortest  lines 
and  considerably  more  than  normal  current  to  the  longest  lines. 

Ticket  Passing.  When  only  one  operator  is  necessary  in  a  town, 
her  duty  being  to  switch  both  local  and  long-distance  lines,  she  may 
write  her  own  tickets  and  execute  them  entire.  In  larger  communi- 
ties with  larger  long-distance  traffic,  the  duties  need  to  be  special- 
ized. The  subscribers'  wants  as  to  long-distance  connections  are 
given  by  themselves  to  recording  long-distance  operators,  who  write 
them  on  tickets  and  pass  these  to  operators  who  get  the  parties 
together.  The  problem  of  ticket-passing  becomes  important  and 
many  mechanical  carriers  have  been  tried,  culminating  in  the  system 
which  utilizes  vacuum  tubes.  This  is  in  some  ways  similar  to  vacu- 
um or  compressed-air  tube  systems  for  carrying  cash  in  retail  stores. 
The  ticket  is  carried,  however,  without  any  enclosing  case  and  the 
tubes  are  flat  instead  of  round,  i.  e.,  they  are  rectangular  in  sec- 
tion. By  suitable  means  a  vacuum  is  maintained  in  a  large  common 
tube  having  a  tap  to  a  box-like  valve  at  each  line  operator's  position. 
A  ticket  tube  connects  this  valve  with  a  distributing  table  at  or  near 
which  the  tickets  are  written.  The  tickets  are  of  uniform  size  and 
are  so  made  as  to  enable  a  flap  to  be  bent  up  easily  along  one  edge. 
The  distributing  operator  has  merely  to  insert  the  ticket,  bent  edge 
foremost,  in  the  open  end  of  the  tube,  whereupon  the  air  pressure 
behind  it  will  drive  it  through  to  its  destination,  near  by  or  far  away. 
The  tickets  travel  thirty  feet  a  second.  The  tube  may  be  bent  into 
almost  any  required  form.  The  ticket,  on  arriving  at  a  line  opera- 
tor's position,  slides  between  two  springs,  breaking  a  shunt  around 
a  relay  and  allowing  the  latter  to  light  the  lamp. 


LONG-DISTANCE  SWITCHING  663 

Waystations.  Waystations  on  long-distance  lines  may  be 
equipped  in  several  ways.  Most  of  them  have  magneto  sets  and 
can  ring  each  other.  Some  are  equipped  with  common-battery  sets 
and  get  all  current  for  signaling  and  transmission  from  a  terminal 
central  office.  In  the  latter  case,  there  is  the  advantage  that  the 
ringers  are  in  series  with  condensers,  assisting  great-ly  in  tests  for 
fault  locations.  Such  tests  are  hindered  by  the  presence  of  ringer 
bridges  across  the  line,  as  in  magneto  practice.  Condensers  can  be 
inserted  in  series  with  ringers  of  magneto  sets  if  the  testing  advan- 
tage is  valued  highly  enough.  A  disadvantage  of  the  use  of  common- 
battery  sets  in  waystations  on  long-distance  lines  is  the  lessened 
transmission  volume  of  the  stations  farthest  from  the  current  source. 

Center  Checking.  An  operating  advantage  of  common-battery 
sets  on  long-distance  lines  is  that  all  calls  are  forced  to  be  answered 
by  the  terminal  station.  Waystations  can  not  call  each  other,  as 
they  have  no  calling  means.  With  magneto  sets,  waystation  agents 
sometimes  call  each  other  direct  and  neglect  to  record  the  call  and 
to  remit  its  price.  WTien  they  can  not  call  each  other  direct,  the 
revenues  of  the  company  increase. 

A  traffic  method  which  requires  all  calls  from  waystations  to 
be  made  to  a  central  switching  office  is  called  a  center-checking 
system.  It  is  so  called  because  all  checking  for  stations  so  switched 
is  done  at  the  central  point  instead  of  each  waystation  keeping  its 
own  records  of  calls  sent  and  received.  In  such  practice  it  is  usual 
to  bill  each  station  once  a  month  for  the  messages  it  sent.  Where 
center  checking  is  not  practiced,  the  agent  makes  a  report  and  sends 
a  remittance.  Center  checking  comes  about  naturally  for  waysta- 
tions having  no  ringing  equipment. 

Center  checking  originated  long  before  the  invention  of  common- 
battery  systems.  It  requires  merely  that  no  waystation  shall  have 
a  generator  which  can  ring  a  bell.  The  method  most  widely  used 
is  to  equip  the  waystations  with  magneto  generators  which  produce 
direct  currents  only;  such  a  generator  cannot  operate  a  polarized 
ringer.  It  is  not  usual  to  produce  the  direct  current  by  actually 
rectifying  the  alternating  current,  but  merely  by  omitting  half  the  im- 
pulses, sending  to  the  line  only  alternate  half-cycles  of  the  current 
generated.  Any  drop  or  relay  adapted  to  respond  to  regular  ringing 
current  will  respond  to  this  modified  form  of  generator. 


CHAPTER   XXXVII 

TELEPHONE  TRAFFIC 

The  term  "traffic,"  with  reference  to  telephone  service,  has 
come  to  mean  the  gross  transaction  of  communication  between 
telephone  users.  This  traffic  may  be  expressed  in  whatever  terms 
are  found  convenient  for  the  particular  phase  considered. 

Unit  of  Traffic.  With  reference  to  payment  for  local  telephone 
service,  the  conversation  is  the  unit  of  traffic.  In  the  daily  operations 
of  telephone  systems  there  are  fewer  conversations  than  there  are 
connections  and  fewer  connections  than  there  are  calls,  because 
lines  are  found  busy  and  all  calls  to  subscribers  are  not  answered. 

For  these  reasons,  in  traffic  inquiries  which  have  to  do  with 
the  amount  of  business  which  subscribers  attempt  to  transact,  the 
total  traffic  in  a  given  time  usually  is  considered  as  so  many  calls 
originated  by  the  subscribers  in  the  community.  From  this  condition 
arises  the  term  "originating  calls." 

For  the  reason  that  the  purpose  of  the  switching  equipment 
in  a  central  office  is  to  make  connections,  the  abilities  of  operators 
and  of  equipments  frequently  are  measured  in  terms  of  connections 
per  hour  or  per  other  unit  of  time. 

For  the  reason  that  in  charging  for  service  all  unavailing  calls 
are  omitted,  the  conversation  is  the  unit  of  traffic. 

Traffic  Variations.  Telephone-exchange  traffic  is  subject  to 
such  general  variations  as  are  noted  in  the  way  a  compass  needle 
points  north,  the  migrations  of  birds,  the  blowing  of  the  trade  winds, 
and  other  natural  phenomena.  There  are  variations  in  traffic 
which  occur  each  day,  others  which  change  with  the  seasons,  and 
still  others  which  are  related  to  holidays  and  other  special  commer- 
cial and  social  events.  For  instance,  the  day  before  Thanksgiving 
Day,  in  many  regions,  is  the  busiest  telephone  traffic  day  in  the  year. 

The  daily  variations  in  telephone  traffic  are  closely  related  to 
commercial  activities  and  certain  general  features  of  this  daily 


TELEPHONE  TRAFFIC 


665 


variation  are  common  to  all  telephone  systems  everywhere.  Fig. 
452  is  a  typical  graphic  record  of  the  traffic  of  a  telephone  exchange 
and  represents  what  happens  in  almost  every  town  or  city.  The 
total  calls  in  this  figure  are  not  given  as  absolute  units  but  would 
vary  to  adapt  the  figure  to  a  particular  case.  The  figure  shows 
principally  that  the  traffic  in  the  night  is  light;  that  it  rises  to  its  max- 
imum height  somewhere  between  10  o'clock  A.  M.  and  noon;  that 
though  it  is  never  as  high  again  during  that  day,  the  afternoon  peak 
is  over  80  per  cent  as  great;  and  that  two  minor  peaks  appear  about 
the  dinner  hour  and  after  evening  entertainments. 

Busy-Hour  Ratio.    If  the  story  told  by  Fig.  452  were  to  be  turned 
into  a  table  of  calls  per  hour,  the  busiest  hour  of  the  day  would  be 


/0 


/Z     Z     4      6     8     /O    12    Z    +     6      8    /<?   /? 
W0MGHT  NOOM  MOtttOMT 

Fig.  452.     Load  Curve 

found  to  correspond  to  the  highest  portion  of  the  figure,  and  in  that 
busiest  hour  of  the  day,  if  a  number  of  selected  days  were  to  be  com- 
pared, would  be  found  a  very  constant  traffic.  The  number  of  calls 
made,  or  the  number  of  connections  completed,  in  that  particular 
hour,  day  by  day,  would  be  found  to  be  much  the  same.  The  ratio 
of  the  number  of  units  in  that  hour  to  the  number  of  units  in  that 


666  TELEPHONY 

entire  day  would  be  found  to  be  practically  the  same  ratio  day  by 
day.  This  ratio  of  busy  hour  to  total  day  would  be  found  to  be 
much  more  nearly  constant  than  the  gross  number  of  calls  per  hour 
or  per  day. 

In  a  large,  busy  city,  about  one-eighth  of  the  total  daily  calls  are 
in  some  one  hour;  in  a  smaller,  less  active  city,  probably  one-tenth 
are  so  congested.  This  is  reasonable  when  one  remembers  that  in  the 
larger  city  the  active  business  of  the  day  begins  later  and  ends  earlier. 

Importance  of  Traffic  Study.  A  knowledge  of  the  amount  of 
traffic  in  an  exchange,  and  its  distribution  as  to  time  and  as  to  the 
divisions  of  the  exchange,  is  important  for  a  number  of  reasons. 
Traffic  knowledge  is  essential  in  order  that  the  equipment  may  be 
designed  and  placed  in  the  proper  way  and  the  total  load  distributed 
properly  on  that  apparatus  and  its  operators. 

For  example,  in  an  office  equipped  with  a  manual  multiple 
switchboard,  the  length  of  the  switchboard  is  governed  entirely  by 
the  number  of  operators  who  must  work  before  it.  It  is  mechanically 
possible  to  make  a  switchboard  for  ten  thousand  lines  only  15  feet 
long,  seating  seven  operators.  The  entire  multiple  of  ten  thousand 
lines  could  appear  three  times  in  such  a  switchboard.  The  seven 
operators  could  not  handle  the  traffic  we  know  would  be  originated 
by  ten  thousand  lines,  with  any  present  system  of  charging  for  serv- 
ice. Even  a  rough  knowledge  of  the  probable  traffic  would  enable 
us  to  approximate  the  number  of  operators  needed  and  to  equip  each 
position,  not  only  with  access  to  the  ten  thousand  lines  to  be  called, 
but  also  with  just  enough  keyboard  equipment,  serving  as  tools, 
and  just  enough  answering  jacks,  serving  as  means  of  bringing  the 
traffic  to  her.  It  is  foreknowledge  of  traffic  which  enables  a  switch- 
board to  fit  the  task  it  is  to  perform. 

Rates  of  Calling.  The  rates  of  calling  of  different  kinds  of  lines 
vary.  The  lines  of  business  stations  originate  more  calls  than  do 
the  lines  of  residences.  Some  kinds  of  business  originate  more  calls 
than  others.  Some  kinds  of  business  have  a  higher  rate  of  calling 
in  one  season  than  in  others.  Flat-rate  lines  originate  more  calls 
than  do  message-rate  lines.  When  a  line  changes  from  a  flat  rate  to  a 
message  rate,  the  number  of  originating  calls  per  day  decreases.  An 
operator's  position,  handling  message-rate  lines  only,  can  serve  more 
lines  than  if  all  of  them  were  at  flat  rates.  The  number  of  message- 


TELEPHONE  TRAFFIC  667 

rate  or  coin-prepayment  lines  which  an  operator's  position  can  care 
for  depends  not  only  on  the  traffic  but  on  the  method  of  charging  for 
service,  whether  by  tickets  or  meters  and  upon  the  kind  of  meters;  or 
it  depends  on  the  method  of  collecting  the  coins.  In  some  regions, 
the  rate  of  calling,  on  the  introduction  of  a  complete  measured-serv- 
ice plan,  has  been  reduced  to  one-fourth  of  what  it  was  on  the  flat- 
rate  plan. 

In  manual  switchboards  of  early  types,  wherein  the  position  of 
the  subscriber's  answering  jack  was  fixed  by  his  telephone  number, 
the  inequality  of  traffic  became  a  serious  problem.  Most  of  the  sub- 
scribers who  first  installed  telephones  when  the  exchange  was  small, 
retained  their  telephones  and  numbers;  as  their  use  of  the  telephone 
grew  with  their  business,  it  was  customary  to  find  the  positions 
answering  the  lower  numbers  much  more  busy  than  the  positions 
answering  the  higher  numbers,  the  latter  belonging  to  later  and  usually 
less  active  business  places. 

Functions  of  Intermediate  Distributing  Frame.  The  interme- 
diate distributing  board  was  invented  to  meet  these  conditions  of 
unequal  traffic  upon  lines  and  of  variations  in  traffic  with  changes  of 
seasons  and  of  charges.  The  intermediate  distributing  board  en- 
ables a  line  to  retain  its  number  and  its  position  in  the  multiple,  but 
to  keep  its  answering  jack  and  lamp  signal  in  any  desired  position. 
If  a  flat-rate  subscriber  changes  to  a  message  rate,  his  line  may  be 
moved  to  a  message-rate  position  and  be  answered,  in  company 
with  others  like  it,  by  an  operator  serving  many  more  lines  than  she 
could  serve  if  all  of  them  werje  flat  rate. 

Methods  of  Traffic  Study.  The  best  way  to  learn  traffic 
facts  for  the  purposes  of  designing  and  operating  equipment  is  to 
conduct  systematic  series  of  observations  in  all  exchanges;  to  record 
them  in  company  with  all  related  facts;  and  to  compare  them  from 
time  to  time,  recording  the  results  of  the  comparisons.  Then  when 
it  is  required  to  solve  a  new  problem,  the  traffic  data  will  enable  the 
probable  future  conditions  to  be  known  with  as  great  exactness  as  is 
possible  in  studies  with  relation  to  transportation  or  any  other  human 
activity. 

There  are  three  general  ways  of  observing  traffic.  A  record  of 
originating  calls  is  known  as  a  "peg  count,"  because  the  counting 
formerly  was  done  by  moving  a  peg  from  place  to  place  in  a  series  of 


•568 


TELEPHONY 


TABLE  XIII 
Calling  Rates 


KIND  OF   SERVICE 

CALLS  PER  DAY  1 
METHODS 

>VITH  DIFFERENT 
3F  CHARGE 

FLAT  RATE 

MESSAGE  RATE 

Residence  

8 

4 

Business  

12  to  20 

8  to  14 

Private  Exchange  Trunk 

40 

25 

Hotel  Exchange  Trunk.    .. 

50 

30 

Apartment  House  Trunk  

30 

18 

holes.  The  simplest  exact  way  is  to  provide  each  operator  with  a 
small  mechanical  counter,  the  key  of  which  she  can  depress  once  for 
each  call  to  be  counted.  A  second  way  is  to  determine  a  ratio  which 
exists,  for  the  particular  time  and  place,  between  the  number  of  calls 
in  a  given  period  and  the  average  number  of  cord  circuits  in  use. 
Knowing  this  ratio,  the  cord  circuits  can  be  counted,  the  ratio  ap- 
plied, and  the  probable  total  known.  The  third  method,  which  is 
applicable  to  offices  having  service  meters  on  all  lines,  is  to  associate 
one  master  meter  per  position  or  group  of  lines  with  all  the  meters 
of  that  position  or  group,  so  that  each  time  any  service  meter  of  that 
position  is  operated,  the  master  meter  will  count  one  unit.  This 
method  applies  to  either  manual  or  automatic  equipments. 

Representative  Traffic  Data.  For  purposes  of  comparison,  the 
following  are  representative  facts  as  to  certain  traffic  conditions. 

Calling  Rates.  The  number  of  calls  originated  per  day  by  differ- 
ent kinds  of  lines  with  different  methods  of  charge  are  shown  in 
Table  XIII. 

Operators'  Loads.  The  abilities  of  subscribers'  operators  to 
switch  these  calls  depend  on  the  type  of  equipment  used,  on  the 
kind  of  management  exercised,  and  on  the  individual  skill  of  oper- 
ators. With  manual  multiple  equipment  of  the  common-battery  type, 
and  good  management,  the  numbers  of  originating  calls  per  busy 
hour  given  in  Table  XIV  can  be  handled  by  an  average  operator.  The 
number  of  calls  per  operator  per  busy  hour  depends  upon  the  amount 
of  trunking  to  other  offices  which  that  operator  is  required  to  do.  In 
a  small  city,  for  example,  where  all  the  lines  are  handled  by  one 
switchboard,  there  is  no  local  switching  problem  except  to  complete 


TELEPHONE  TRAFFIC  669 

TABLE  XIV 
Effect  of  Out=Trunking  on  Operator's  Capacity 


PER  CENT 
TRUNKED 

ORIGINATING  CALLS 
TO  OTHER  OFFICES 

CAPACITY  OF  SUBSCRIBERS'  OPERATOR'S 
POSITION  IN  CALLS  PER  BUSY  HOUR 

0 

240 

10 

230 

30 

200 

50 

185 

75 

170 

90 

165 

the  connection  in  the  multiple  before  each  position.  In  a  large  city, 
where  wire  economy  and  mechanical  considerations  compel  the  lines 
to  be  handled  by  a  number  of  offices  with  manual  equipment,  some 
portion  of  the  total  originating  load  of  each  office  must  be  trunked  to 
others.  Table  XIV  shows  that  an  increase  of  90  per  cent  in  the 
amount  of  out-trunking  has  decreased  the  operator's  ability  to  less 
than  70  per  cent  of  the  possible  maximum. 

Trunking  Factor.  In  providing  the  system  of  trunks  inter- 
connecting the  offices,  whether  the  equipment  be  manual  or  auto- 
matic, it  is  essential  to  know  not  only  how  much  traffic  originates 
in  each  office,  but  how  much  of  it  will  be  trunked  to  each  other  office 
and  how  many  trunks  will  be  required.  An  interesting  phase  of 
telephone  traffic  studies  is  that  it  is  possible  to  determine  in  advance 
the  amount  of  traffic  which  can  be  completed  directly  in  the  multiple 
of  that  office  and  how  much  must  be  trunked  elsewhere.  Theoret- 
ical considerations  would  indicate  that  if  the  local  multiple  contains 
one-eighth  of  the  total  lines  of  the  city,  one-eighth  of  the  calls  orig- 
inating in  that  office  could  be  completed  locally  and  seven-eighths 
would  be  trunked  out.  In  almost  all  cases,  however,  it  is  found  that 
more  than  the  theoretical  percentage  of  originating  calls  are  for  the 
neighborhood  of  that  office  and  can  be  completed  in  the  multiple. 
This  results  in  the  determination  of  a  factor  by  which  the  theoretical 
out-trunking  can  be  multiplied  to  determine  the  probable  real  out- 
trunking.  In  most  cases,  the  ratio  of  actual  to  theoretical  out- 
trunking  is  75  per  cent,  or  approximately  that.  In  special  cases,  it 
may  be  far  from  75  per  cent. 

Trunk  Efficiency.  The  capacities  of  trunks  vary  with  their 
methods  of  operation  and  with  the  number  of  trunks  in  a  group. 


670 


TELEPHONY 


TABLE  XV 
Messages  per  Trunk  In  Manual  System 


NUMBER  OF  TRUNKS  IN  GRODP, 
MANUAL  SYSTEM 

MESSAQKS  PER  TRUNK  PER 
BUSY  HOUR 

5 

7 

10 

9 

20 

12 

40 

15 

60 

18 

For  example,  in  the  manual  system  where  trunk  operators  in  distant 
offices  are  instructed  over  call  circuits  and  make  disconnections  in 
response  to  lamp  signals,  such  an  incoming  trunk  operator  can  com- 
plete from  250  to  500  connections  per  busy  hour.  The  actual  ability 
depends  upon  the  number  of  distant  offices  served  by  that  operator 
and  upon  the  amount  of  work  she  has  to  perform  on  each  call. 

The  number  of  messages  which  can  be  handled  by  one  trunk  in 
the  busy  hour  will  depend  upon  the  number  of  trunks  in  the  group 
and  upon  the  system  employed.  It  appears  that  the  ability  of  trunks 
in  this  regard  is  higher  in  the  automatic  system  than  in  the  manual 
system.  For  the  latter,  Table  XV  gives  representative  facts. 

Some  of  the  reasons  for  the  higher  efficiencies  of  trunks  in  the 
automatic  system  are  not  well  defined,  but  unquestionably  exist. 
They  have  to  do  partly  with  the  prompter  answering  observable  in 
automatic  systems.  The  operation  of  calling  being  simple,  a  called 
subscriber  seems  to  fear  that  unless  he  answers  promptly  the  calling 
party  will  disconnect  and  perhaps  may  call  a  competitor.  The  in- 
troduction of  machine-ringing  on  automatic  lines,  where  existing  in 
competition  with  manual  ringing  on  manual  lines,  seems  to  encourage 
subscribers  to  answer  even  more  promptly.  The  length  of  conversa- 
tion in  automatic  systems  seems  to  be  shorter  than  in  manual  systems. 
Still  more  important,  disconnection  in  automatic  systems  is  instan- 
taneous during  all  hours,  whereas  in  manual  systems  it  is  less  prompt 
in  the  busiest  and  least  busy  hours  than  in  the  hours  of  intermediate 
congestion.  The  practical  results  of  trunk  efficiencies  in  automatic 
systems  are  given  in  Table  XVI. 

Toll  Traffic.  Toll  or  long-distance  traffic  follows  the  general 
laws  of  local  or  exchange  traffic.  Conversations  are  of  greater 


TELEPHONE  TRAFFIC 


671 


TABLE   XVI 
Messages  per  Trunk  in  Automatic  System 


NUMBEK  OF  TRUNKS  IN  GROUP, 
AUTOMATIC  SYSTEM 

MESSAGES  PER  TRUNK  PER 
BUSY  HOUR 

5 

15 

10 

22 

20 

28 

40 

32 

60 

34 

average  length  in  long-distance  traffic.  The  long-distance  line  is 
held  longer  for  an  average  conversation  than  is  a  local-exchange  line. 
The  local  trunks  which  connect  long-distance  lines  with  exchange 
lines  for  conversation  are  held  longer  than  are  the  actual  long-distance 
trunks  between  cities.  Knowing  the  probable  traffic  to  be  brought 
to  the  long-distance  switching  center  by  the  long-distance  trunks 
from  exchange  centers,  the  number  of  trunks  required  may  be  deter- 
mined by  knowing  the  capacity  of  each  trunk.  These  trunk  capac- 
ities vary  with  the  method  of  handling  the  traffic  and  they  vary  as 
do  local  trunks  with  the  number  of  trunks  in  a  group.  Table  XVII 
illustrates  this  variation  of  capacity  with  sizes  of  groups. 

TABLE  XVII 
Messages  per  Trunk  in  Lon£=Distance  Groups 


NUMBER  OF  LONG-DISTANCE 
TRUNKS  IN  GROUP 

MESSAGES  PER  TRUNK  PER 
BUSY  HOUH 

5 

2 

10 

3 

20 

3.2 

40 

3.5 

60 

4 

100 

4.6 

Quality  of  Service.  The  quality  of  telephone  service  rendered 
by  a  particular  equipment  managed  in  a  particular  way  depends  on 
a  great  variety  of  elements.  The  handling  of  the  traffic  presented 
by  patrons  is  a  true  manufacturing  problem.  The  quality  of  the  serv- 
ice rendered  requires  continuous  testing  in  order  that  the  manage- 
ment may  know  whether  the  service  is  reaching  the  standard ;  whether 
the  standard  is  high  enough ;  whether  the  cost  of  producing  it  can  be 


672  TELEPHONY 

reduced  without  lowering  the  quality;  and  whether  the  patrons  are 
getting  from  it  as  much  value  as  they  might. 

In  manual  systems,  the  quality  of  telephone  service  depends 
upon  a  number  of  elements.  The  following  are  some  principal  ones : 

1.  Prompt  answering. 

2.  Prompt  disconnection. 

3.  Freedom  from  errors  in  connecting  with  the  called  line. 

4.  Promptness  in  connecting  with  the  called  line. 

5.  Courtesy  and  the  use  of  form. 

6.  Freedom  from  failure  by  busy  lines  and  failure  to  answer. 

7.  Clear  enunciation. 

8.  Team  work. 

Answering  Time.  There  is  an  interrelation  between  these 
elements.  Team  work  assists  both  answering  and  prompt  discon- 
nection. The  quality  of  telephone  service  can  not  be  measured 
alone  in  terms  of  prompt  answering.  Formerly  telephone  service 
was  boasted  of  as  being  "three-second  service"  if  most  of  the  orig- 
inating calls  were  answered  in  three  seconds.  Often  such  prompt 
answering  reacts  to  prevent  prompt  disconnecting.  Patient,  system- 
atic work  is  required  to  learn  the  real  quality  of  the  service. 

As  to  answering,  the  clearest,  truest  statement  concerning 
manual  service  is  found  by  making  test  calls  to  each  position,  dividing 
them  into  groups  of  various  numbers  of  whole  seconds  each,  and 
comparing  the  percentage  of  these  groups  to  the  whole  number  of 
telephones  to  that  position.  For  example,  assume  each  of  the  calls 
to  a  given  position  to  have  been  answered  in  ten  seconds  or  less,  in 
which 

1 00  per  cent  are  answered  in  ten  seconds  or  less ; 
80  per  cent  in  eight  seconds  or  less 
60  per  cent  in  six  seconds  or  less. 

It  is  probable  that  a  reasonably  uniform  manual  service  will  show 
only  a  small  percentage  answered  in  three  seconds  or  under.  Such 
percentages  may  be  drawn  in  the  form  of  curves,  so  that  at  a  glance 
one  may  learn  efficiency  in  terms  of  prompt  answering. 

Disconnecting  Time.  Prompt  disconnection  was  improved  enor- 
mously by  the  introduction  of  relay  manual  boards.  Just  before 
the  installation  of  relay  boards  in  New  York  City,  the  average  dis- 
connecting time  was  over  seventeen  seconds.  On  the  completion 


TELEPHONE  TRAFFIC  673 

of  an  entire  relay  equipment,  the  average  disconnecting  time  was 
found  to  be  under  three  seconds.  The  introduction  of  relay  manual 
apparatus  has  led  subscribers  to  a  larger  traffic  and  to  the  making 
of  calls  which  succeed  each  other  very  closely.  A  most  important 
rule  is,  that  disconnect  signals  shall  be  given  prompt  attention  either 
by  the  operator  who  made  the  connection,  by  an  operator  adjacent, 
or  by  a  monitor  who  may  be  assisting;  and  another,  still  more  impor- 
tant one  is,  that  a  flashing  keyboard  lamp  indicating  a  recall  shall  be 
given  precedence  over  all  originating  and  all  other  disconnect  signals. 

Accuracy  and  Promptness.  Promptness  and  accuracy  in  con- 
necting with  the  called  line  are  vital,  and  yet  a  large  percentage  of 
errors  in  these  elements  might  exist  in  an  exchange  having  a  very 
high  average  speed  of  answering  the  originating  call.  Indeed,  it 
seems  quite  the  rule  that  where  the  effort  of  the  management  is  de- 
voted toward  securing  and  maintaining  extreme  speed  of  original 
answering,  all  the  other  elements  suffer  in  due  proportion. 

Courtesy  and  Form.  It  goes  without  saying  that  operators  should 
be  courteous;  but  it  is  necessary  to  say  it,  and  keep  saying  it  in  the 
most  effective  form,  in  order  to  prevent  human  nature  under  the 
most  exasperating  circumstances  from  lapsing  a  little  from  the  stand- 
ard, however  high.  The  use  of  form  assists  both  the  operators  and 
the  subscribers,  because  in  all  matters  of  strict  routine  it  is  much 
easier  to  secure  high  speed  and  great  accuracy  by  making  as  many 
as  possible  of  the  operations  automatic.  The  use  of  the  word  "num- 
ber" and  other  well-accepted  formalities  has  assisted  greatly  in  se- 
curing speed,  clear  understanding,  and  accurate  performance.  The 
simple  expedient  of  spelling  numbers  by  repeating  the  figures  in  a 
detached  form — as  "1-2-5"  for  125— has  taught  subscribers  the  same 
expedient,  and  the  percentage  of  possible  error  is  materially  reduced 
by  going  one  step  further  and  having  the  operator,  in  repeating,  use 
always  the  opposite  form  from  that  spoken  by  the  calling  subscriber. 

Busy  and  Don't  Answer  Calls.  Notwithstanding  the  old  im- 
pression of  the  public  to  the  contrary,  the  operator  has  no  control  over 
the  "busy  line"  and  "don't  answer"  situation.  It  is,  however,  of 
high  importance  that  the  management  should  know,  by  the  analysis  of 
repeated  and  exhaustive  tests  of  the  service,  to  what  extent  these 
troubles  are  degrading  it.  In  addition  to  improving  the  service  by 
the  elimination  of  busy  reports,  there  is  no  means  of  increasing  rev- 


674  TELEPHONY 

enue  which  is  so  easy  and  so  certain  as  that  which  comes  from  follow- 
ing up  the  tabulated  results  of  busy  calls. 

Enunciation.  It  must  be  remembered  that  clear  enunciation 
for  telephone  purposes  is  a  matter  wholly  relative,  and  the  ability  of 
an  operator  in  this  regard  can  be  determined  only  by  a  close  analysis 
of  many  observations  from  the  standpoint  of  a  subscriber.  A  trick 
of  speech  rather  than  a  pleasant  voice  and  an  easy  address  has  made 
the  answering  ability  of  many  an  operator  captivating  to  a  group  of 
satisfied  subscribers. 

Team  Work.  By  team  work  is  meant  the  ability  of  a  group  of 
operators,  seated  side  by  side,  to  work  together  as  a  unit  in  caring  for 
the  service  brought  to  them  by  the  answering  jacks  within  their  reach. 
In  switchboards  of  the  construction  usual  today,  a  call  before  any 
operator  may  be  answered  by  her,  or  by  the  operator  at  either  the 
right  or  the  left  of  her  position.  In  many  exchanges  this  advantage 
is  wholly  overlooked.  In  the  period  of  general  re-design  of  central- 
office  equipments  about  fourteen  years  ago,  a  switchboard  was  in- 
stalled with  mechanical  visual  signals  and  answering-jacks  on  a 
flat-top  board,  and  an  arrangement  of  operators  such  that  the  signal 
of  any  call  was  extremely  prominent  and  in  easy  reach  of  each  one  of 
four  or  possibly  five  operators.  Associated  with  the  line  signals  with- 
in the  reach  of  such  a  group  was  an  auxiliary  lamp  signal  which  would 
light  when  a  call  was  made  by  any  of  the  lines  so  terminating.  It  was 
found  that  with  this  arrangement  the  calls  were  answered  in  a  strictly 
even  manner,  special  rushes  being  cared  for  by  the  joint  efforts  of 
the  group  rather  than  serving  to  swamp  the  operator  who  happened 
to  be  in  charge  of  the  particular  section  affected  by  the  rush. 

This  principle  has  been  tried  out  in  so  many  ways  that  it  is  as- 
tonishing that  it  is  not  recognized  as  being  a  vital  one.  The  whole 
matter  is  accomplished  by  impressing  upon  each  operator  that  her 
duty  is,  not  to  answer  the  calls  of  a  specific  number  of  lines  before  her, 
but  to  answer,  with  such  promptness  as  is  possible,  any  call  which 
is  within  the  reach  of  her  answering  equipment. 

Observation  of  Service.  All  that  is  required  to  be  known  con- 
cerning the  form  of  address  and  courtesy  may  be  learned  by  a  close 
observation  of  the  operators'  work  by  the  chief  operators  and  moni- 
tors, and  by  the  use  of  listening  circuits  permanently  connected  to  the 
operators'  sets.  It  is  naturally  necessary  that  the  use  of  these  listen- 


TELEPHONE  TRAFFIC  675 

ing  circuits  by  the  chief  operator  or  her  assistants  must  not  be  known 
to  the  operators  at  the  times  of  use,  even  though  they  may  know  of 
the  existence  of  such  facilities. 

With  a  well-designed  and  properly  maintained  automatic  equip- 
ment, the  eight  elements  of  good  manual  service  reduce  themselves 
to  only  one  or  two.  Freedom  from  failure  by  busy  lines  and  failure  to 
answer  are  service-qualities  independent  of  the  kind  of  switching 
apparatus.  Too  great  a  percentage  of  busy  calls  for  a  given  line  in- 
dicates that  the  telephone  facilities  for  calls  incoming  to  that  sub- 
scriber are  inadequate.  The  best  condition  would  be  for  each  sub- 
scriber to  have  lines  enough  so  that  none  of  them  ever  would  be  found 
busy.  This  is  the  condition  the  telephone  company  tries  to  establish 
between  its  various  offices. 

In  manual  practice  it  is  possible  to  keep  such  records  as  will  en- 
able the  traffic  department  to  know  when  the  lines  to  a  subscriber 
are  insufficient  for  the  traffic  trying  to  reach  him.  As  soon  as  such 
facts  are  known,  they  can  be  laid  before  the  subscriber  so  that  he  may 
arrange  for  additional  incoming  lines.  In  automatic  practice  this  is 
not  so  simple,  as  the  source  and  destination  of  traffic  in  general  is 
not  so  clearly  known  to  the  traffic  department.  Automatic  record- 
ers of  busy  calls  are  necessary  to  enable  the  facts  to  be  tabulated. 


CHAPTER  XXXVIII 
MEASURED  SERVICE 

In  the  commercial  relation  between  the  public  and  a  telephone 
system,  the  commodity  which  is  produced  by  the  latter  and  consumed 
by  the  former  is  telephone  service.  Users  often  consider  that  pay- 
ment is  made  for  rental  of  telephone  apparatus  and  to  some  persons 
the  payment  per  month  seems  large  for  the  rental  of  a  mere  telephone 
which  could  be  bought  outright  for  a  few  dollars. 

The  telephone  instrument  is  but  a  small  part  of  the  physical 
property  used  by  a  patron  of  a  telephone  system.  Even  the  entire 
group  of  property  elements  used  by  a  patron  in  receiving  telephone 
service  represents  much  less  than  what  really  is  his  proportion  of  the 
service-rendering  effort.  What  the  patron  receives  is  service  and 
its  value  during  a  time  depends  largely  on  how  much  of  it  he  uses 
in  that  time,  and  less  on  the  number  of  telephones  he  can  call. 

The  cost  of  telephone  service  varies  as  the  amount  of  use.  It  is 
just,  therefore,  that  the  selling  price  should  vary  as  the  amount  of  use. 

Rates.  There  are  two  general  methods  of  charging  for  telephone 
service  and  of  naming  rates  for  this  charge.  These  are  called  flat  rates 
and  measured-service  rates.  The  latter  are  also  known  as  message 
rates,  because  the  message  or  conversation  is  the  unit.  Flat  rates 
are  those  which  are  also  known  as  rentals.  The  service  furnished 
under  flat  rates  is  also  known  as  unlimited  service,  for  the  reason  that 
under  it  a  patron  pays  the  same  amount  each  month  and  is  entitled  to 
hold  as  many  conversations — send  as  many  messages  and  make  as 
many  calls — as  he  wishes,  without  any  additional  payment.  In  the 
measured-service  plan,  the  amount  of  payment  in  a  month  varies  in 
some  way  with  the  amount  of  use,  depending  on  the  plan  adopted. 
The  patron  may  pay  a  fixed  base  amount  per  month,  entitling  him 
to  have  equipment  for  telephone  service  and  to  receive  messages,  but 
being  required  to  pay,  in  addition  to  this  base  amount,  a  sum  which 
is  determined  by  the  number  of  messages  which  he  sends.  Or  he 


MEASURED  SERVICE  677 

may  pay  a  base  amount  per  month  and  be  entitled  to  have  the  equip- 
ment, to  receive  calls,  and  to  send  a  certain  number  of  messages, 
paying  specifically  in  addition  only  for  messages  exceeding  that  cer- 
tain number. 

Whether  flat  rates  or  measured-service  rates  are  practiced,  the 
general  tendency  is  to  establish  lower  rates  for  service  in  homes  than 
in  business  places.  This  is  another  recognition  of  the  justice  of 
graduating  the  rates  in  accordance  with  the  amount  of  use. 

Units  of  Charging.  While  both  the  flat-rate  and  the  measured- 
rate  methods  of  charging  for  unlimited  and  measured  service  are 
practiced  in  local  exchanges,  long-distance  service  universally  is  sold 
at  message  rates.  The  unit  of  message  rates  in  long-distance  serv- 
ice is  time.  The  charge  for  a  message  between  two  points  joined 
by  long-distance  lines  usually  is  a  certain  sum  for  a  conversation 
three  minutes  long  plus  a  certain  sum  for  each  additional  minute  or 
fraction  of  a  minute.  In  local  service,  the  message-rate  time  charge 
per  message  takes  less  account  of  the  time  unit.  The  conversation  is 
almost  universally  the  unit  in  exchanges.  Some  managements  re- 
strict messages  of  multi-party  lines  to  five  minutes  per  conversation, 
because  of  the  desire  to  avoid  withholding  the  line  from  other  parties 
upon  it  for  too  long  periods.  Service  sold  at  public  stations  similarly 
is  restricted  as  to  time,  even  though  the  message  be  local  to  the  ex- 
change. Three  to  five  minutes  local  conversation  is  sold  generally 
for  five  cents  in  the  United  States.  The  time  of  the  average  local 
message,  counting  actual  conversation  time  only,  is 'one  hundred  sec- 
onds. 

Toll  Service.  Long  Haul.  In  long-distance  service,  there  are 
two  general  methods  of  handling  traffic,  as  to  the  relations  between 
the  calling  and  the  called  stations.  For  the  greater  distances,  as 
between  cities  not  closely  related  because  not  belonging  to  one 
general  community,  the  calling  patron  calls  a  particular  person  and 
pays  nothing  unless  he  holds  conversation  with  that  person.  In 
this  method,  the  operator  records  the  name  of  the  person  called 
for;  the  name,  telephone  number,  or  both,  of  the  person  calling;  the 
names  of  the  towns  where  the  message  originated  and  ended;  the 
date,  the  time  conversation  began,  and  the  length  of  time  it  lasted. 

Short  Haul.  Where  towns  are  closely  related  in  commercial 
and  social  ways  and  where  the  traffic  is  large  and  approaches  local 


678  TELEPHONY 

service  in  character,  and  yet  where  conversations  between  them 
are  charged  at  different  rates  than  are  local  calls  within  them,  a 
more  rapid  system  of  toll  charging  than  that  just  described  is  of  ad- 
vantage. In  these  conditions,  patrons  are  not  sold  a  service  which 
allows  a  particular  party  to  be  named  and  found,  nor  is  the  identity 
of  the  calling  person  required.  The  operator  needs  to  know  merely 
of  these  calls  that  they  originate  at  a  certain  telephone  and  are  for 
a  certain  other.  The  facts  she  must  record  are  fewer  and  her  work 
is  simpler.  Therefore,  the  cost  of  such  switching  is  less  than  for 
true  long-distance  calls  and  it  can  be  learned  by  careful  auditing 
just  when  traffic  between  points  becomes  great  enough  to  warrant 
switching  them  in  this  way.  Such  switching,  for  example,  exists 
between  New  York  and  Brooklyn,  between  Chicago  and  suburbs 
around  it  which  have  names  of  their  own  but  really  are  part  of  the 
community  of  Chicago,  and  between  San  Francisco  and  other  cities 
which  cluster  around  Sari  Francisco  Bay. 

Calls  of  the  "long-haul"  class  are  known  as  "particular  person" 
or  "particular  party"  calls,  while  "short-haul"  calls  are  known  as 
"two-number"  long-distance  calls.  It  is  customary  to  handle  par- 
ticular party  calls  on  long-distance  switchboards  and  to  handle  two- 
number  calls  in  manual  systems  on  subscribers'  switchboards  exactly 
like  local  calls,  except  that  the  two  number  calls  are  ticketed.  It  is 
customary  in  automatic  systems  to  handle  two-number  calls  by 
means  of  the  regular  automatic  equipment  plus  ticketing  by  a  subur- 
ban or  two-number  operator. 

Timing  Toll  Connections.  It  formerly  was  customary  to  meas- 
ure the  time  of  long-distance  conversations  by  noting  on  the  ticket 
the  time  of  its  beginning  and  the  time  of  its  ending,  the  operator 
reading  the  time  from  a  clock.  For  human  and  physical  reasons, 
such  timing  seems  not  to  be  considered  infallible  by  the  patron 
who  pays  the  charge,  and  in  cases  of  dispute  concerning  overtime 
charges  so  timed,  telephone  companies  find  it  wisest  to  make  con- 
cessions. The  physical  cause  of  error  in  reading  time  from  a  clock 
is  that  of  parallax;  that  is,  the  error  which  arises  from  the  fact  that 
the  minute  hand  of  a  clock  is  some  distance  from  the  surface  of  the 
dial  so  that  one  can  "look  under  it."  On  an  ordinary  clock  hav- 
ing a  large  face  and  its  minute  hand  pointing  upward  or  downward, 
five  people  standing  in  a  row  could  read  five  different  times  from  it 


MEASURED  SERVICE  679 

at  the  same  instant.  The  middle  person  might  see  the  minute  hand 
pointing  at  6,  indicating  the  time  to  be  half-past  something;  whereas, 
person  No.  1  and  person  No.  5  in  the  row  might  read  the  time  re- 
spectively 29  and  31  minutes  past  something.  Operators  far  to 
the  right  or  to  the  left  of  a  clock  will  get  different  readings,  and  an 
operator  below  a  clock  will  get  different  kinds  of  readings  at  dif- 
ferent times  and  correct  readings  at  few  times. 

Timing  Machines: — Machines  which  record  time  directly  on 
long-distance  tickets  are  of  value  and  machines  which  automatically 
compute  the  time  elapsing  during  a  conversation  are  of  much  greater 
value.  The  calculagraph  is  a  machine  of  the  latter  class.  The 


3      f,°  4  RK 


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5/o  ^  RM. 

O,4  /N    '      &> 


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Ml 

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0, 


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Fig.  453.     Calculagraph  Records 

use  of  some  such  machine  uniformly  reduces  controversy  as  to  time 
which  really  elapsed.  Parallax  errors  are  avoided.  The  record 
possesses  a  dignity  which  carries  conviction. 

Calculagraph  records  are  shown  in  Fig.  453.  In  the  one 
shown  in  the  upper  portion  of  this  figure,  the  conversation  began 
at  10.44 1  P.  M.  This  is  shown  by  the  right-hand  dial  of  the 
three  which  constitute  the  record.  The  minutes  past  10  o'clock 
are  shown  by  the  hand  within  the  dial  and  the  hour  10  is  shown 
by  the  triangular  mark  just  outside  the  dial  between  X  and  XI. 

The  duration  of  the  conversation  is  shown  by  the  middle  and 
the  left-hand  dials.  The  figures  on  both  these  dials  indicate  min- 
utes. The  middle  dial  indicates  roughly  that  the  conversation  lasted 


680  TELEPHONY 

for  a  time  between  0  and  5  minutes.  The  left-hand  dial  indicates 
with  greater  exactness  that  the  conversation  lasted  one  and  one-quarter 
minutes. 

The  hand  of  the  left-hand  dial  makes  one  revolution  in  five 
minutes;  of  the  middle  dial,  one  revolution  in  an  hour.  The  mid- 
dle dial  tells  how  many  full  periods  of  five  minutes  have  elapsed 
and  the  left-hand  dial  shows  tke  excess  over  the  five-minute  in- 
terval. 

The  lower  portion  of  Fig.  453  is  a  similar  record  beginning  at 
the  same  time  of  day,  but  lasting  about  five  and  one-half  minutes. 
As  before,  the  readings  of  the  two  dials  are  added  to  get  the 
elapsed  time. 

The  right-hand  dial,  showing  merely  time  of  day,  stands  still 
while  its  hands  revolve.  The  dies  which  print  the  dials  and  hands 
of  the  middle  and  the  left-hand  records  rotate  together.  Examin- 
ing the  machine,  one  finds  that  the  hands  of  these  dials  always 

P.M. 


if  V 


I     \ 

I 


Fig.  454.     Relative  Position  of  Hands  and  Dials 


point  to  zero.  The  middle  dial  and  hand  make  one  complete  revo- 
lution in  an  hour;  the  left-hand  dial  and  hand,  one  in  five  minutes. 
In  making  the  records,  the  dials  are  printed  at  the  beginning  and 
the  hands  at  the  end  of  the  conversation.  Therefore,  the  hands 
will  have  moved  forward  during  the  conversation  —  still  pointing  to 
zero  in  both  cases  —  but  when  printed  the  hands  will  point  to  some 
other  place  than  they  were  pointing  when  the  dials  were  printed. 
In  this  way,  their  angular  distances  truly  indicate  the  lapse  of  time. 
Fig.  454  shows  the  relative  position  of  the  hands  and  dials  within 
the  machine  at  all  times.  It  will  be  noted  that  the  arrow  of  the 
left-hand  dial  does  not  point  exactly  to  zero.  This  is  due  to  the 
fact  that  the  dials  and  hands  are  printed  by  separate  operations 
and  cannot  be  printed  simultaneously. 

Another  method  of  timing  toll  connections  has  been  developed 


MEASURED  SERVICE 


681 


by  the  Monarch  Telephone  Manufacturing  Company.  This  em- 
ploys a  master  clock  of  great  accuracy,  which  may  be  mounted  on 
the  wall  anywhere  in  the  building  or  another  building  if  desired.  A 
circuit  leads  from  this  clock  to  a  time-stamp  device  on  the  operator's 
key  shelf,  and  the  clock  closes  this  circuit  every  quarter  minute. 
The  impulses  thus  sent  over  the  circuit  energize  the  magnet  of  the 
time  stamp,  which  steps  a  train  of  printing  wheels  around  so  as  always 
to  keep  them  set  in  such  position  as  to  properly  print  the  correct 
time  on  a  ticket  whenever  the  head  of  the  stamp  is  moved  by  the 
operator  into  contact  with  the  ticket.  A  large  number  of  such  stamps 
may  be  operated  from  the  same  master  clock.  By  printing  the  start- 
ing time  of  a  connection  below  the  fin- 
ishing time  the  computation  of  lapsed 
time  becomes  a  matter  of  subtraction.  A 
typical  toll  ticket  with  the  beginning  and 
ending  time  printed  by  the  time  stamp 
in  the  upper  left-hand  corner  and  the 
elapsed  time  recorded  by  hand  in  the  up- 
per right-hand  corner  is  shown  in  Fig.  455. 
It  is  seen  that  this  stamp  records  in  the 
order  mentioned  the  month,  the  day,  the 
hour,  the  minute  and  quarter  minute,  the 
A.  M.  and  P.  M.  division  of  the  day,  and  the 
year. 

An  interesting  feature  of  this  system 
is  that  the  same  master  clock  may  be 
made  in  a  similar  manner  to  actuate  sec- 
ondary clocks  placed  at  subscribers'  stations,  the  impulses  being 
sent  over  wires  in  the  same  cables  as  those  containing  the  subscrib- 
ers' lines.  This  system,  therefore,  serves  not  only  as  a  means  for 
timing  the  toll  tickets  and  operating  time  stamps  wherever  they  are 
required  in  the  business  of  the  telephone  company,  but  also  to  supply 
a  general  clock  and  time-stamp  service  to  the  patrons  of  the  tele- 
phone company  as  a  "by-product"  of  the  general  telephone  busi- 
ness. 

Exchange  service  is  measured  in  terms  of  conversations  with- 
out much  regard  to  their  length.  The  payment  for  the  service 
may  be  made  at  the  time  it  is  received,  as  in  public  stations  and 


a 

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12  23  11-  5*  50  * 

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TOT., 

Fig.  455.     Toll  Ticket  Used 
with  Monarch  System 


682  TELEPHONY 

at  telephones  equipped  with  coin  prepayment  devices;  or  the  calls 
from  a  telephone  may  be  recorded  and  collection  for  them  made  at 
agreed  intervals.  In  the  prepayment  method  the  price  per  call 
is  uniform.  In  the  deferred  payment  method  the  calls  are  recorded 
as  they  are  made,  their  number  summed  up  at  intervals,  and  the 
amount  due  determined  by  the  price  per  call.  The  price  per  call 
may  vary  with  the  number  of  calls  sold.  A  large  user  may  have 
a  lower  rate  per  call  than  a  small  user. 

Local  Service.  Ticket  Method.  Measured  local  service  some- 
times is  recorded  by  means  of  tickets,  similarly  to  the  described 
method  of  charging  long-distance  calls,  except  that  the  time  of  day 
and  the  duration  of  conversation  are  not  so  important.  Where 
local  ticketing  is  practiced,  it  is  usual  to  write  on  the  ticket  only 
the  number  of  the  calling  telephone  and  the  date,  and  to  pass  into 
the  records  only  those  tickets  which  represent  actual  conversations, 
keeping  out  tickets  representing  calls  for  busy  lines  and  calls  which 
were  not  answered. 

Meter  Method.  The  requirements  of  speed  in  good  local  serv- 
ice are  opposed  to  the  ticketing  method.  Where  measured  service 
is  supplied  to  a  substantial  proportion  of  the  lines  of  a  large  exchange, 
electro-mechanical  service  meters  are  attached  to  the  lines.  These 
service  meters  register  as  a  consequence  of  some  act  on  the  part  of 
the  switchboard  operator,  or  may  be  caused  to  register  by  the  answer- 
ing of  the  called  subscriber. 

In  manual  practice,  meters  of  the  type  shown  in  Fig.  456  are 
associated  with  the  lines  as  in  Fig.  457.  The  meters  are  mounted 

separately  from  the  switchboard, 
needing  only  to  be  connected  to 
the  test-strand  of  the  line  by 
cabled  wires.  If  desired,  the  me- 
ter may  be  mounted  on  racks  in 

quarters    especially    devoted    to 
Pig.  456.     Connection  Meter  .         1-1,1 

them,  and  me  cases  in  which  the 

racks  are  mounted  may  be  kept  locked.  In  such  an  arrangement 
the  meters  are  read  from  time  to  time  through  the  glass  doors  of 
the  cases. 

The  meters  are  caused  to  operate  by  pressure  on  the  meter 
key  MK,  associated  with  the  answering  cord  as  in  Fig.  458.  This 


MEASURED  SERVICE 


683 


increases  the  normal  potential  to  30  volts.  When  the  armature  of 
the  meter  has  made  a  part  of  its  stroke,  it  closes  a  contact  which 
places  its  40-ohm  winding  in  shunt  with  its  500-ohm  winding,  thus 
furnishing  ample  power  for  turning  the  meter  wheels. 

Such  meters  are  in  common  use  in  large  exchanges,  notable 
examples  being  the  cities  of  NewT  York  and  London.  In  London, 
there  is  a  zone  within  which  the  price  per  call  is  one  penny  and 
between  which  and  other  zones  the  price  is  twopence.  Calls  within 
the  zone  either  are  completed  by  the  answering  operator  directly 
in  the  multiple  before  her  or  are  trunked  to  other  offices  in  that 
zone.  Calls  for  points  outside  of  that  zone  are  trunked  to  other 


Fig.  457.     Western  Electric  Line  Circuit  and  Service  Meter 

offices  and  in  giving  the  order  the  operator  finds  that  the  call  circuit 
key  lights  a  special  signal  lamp  before  her.  This  reminds  her  that 
the  call  is  at  a  twopence  price,  so  in  recording  it  she  presses  the 
meter  key  twice.  This  counts  two  units  on  the  meter  and  the  units 
are  billed  at  a  penny  each. 

In  automatic  systems  it  is  not  possible  to  operate  a  meter  sys- 
tem in  which  the  operator  will  press  a  key  for  each  call  to  be  charged, 
because  there  is  no  operator.  In  such  systems — a  notable  example 
being  the  measured-service  automatic  system  in  San  Francisco — 
the  meter  registers  only  upon  the  answering  of  the  called  subscriber. 
Calls  for  lines  found  busy  and  calls  which  are  not  answered  do  not 
register.  Calls  for  long-distance  recording  operators,  two-number 
ticket  operators,  information,  complaint,  and  other  company  de- 
partments are  not  registered.  In  the  Chinatown  quarter  of  San 
Francisco,  where  most  calls  begin  and  end  in  the  neighborhood, 
service  is  sold  at  an  unlimited  flat  rate  for  neighborhood  calls  and 


684 


TELEPHONY 


at  a  message  rate  for  other  calls.  The  meter  system  recognizes  this 
condition  and  does  not  register  calls  from  Chinese  subscribers  for 
Chinese  subscribers,  though  it  does  register  calls  from  Chinese 
subscribers  to  Caucasian  subscribers.  The  nature  of  the  system 
is  such  as  to  enable  it  to  discriminate  as  to  races,  localities,  or  other 
peculiarities  as  may  be  desired. 

In  the  manual  meter  circuits  of  Figs.  457  and  458,  the  meter 
windings  have  no  relation  to  the  line  conductors.  In  the  auto- 
matic arrangement  just  described,  there  are  meter  windings  in  the 
line  during  times  of  calling,  but  none  in  the  line  during  times  of  con- 


Fig.  458.     Western  Electric  Cord  Circuit  and 
Service  Meter  Key 

versation.  The  balance  of  the  line,  therefore,  is  undisturbed  at  all 
times  wherein  balance  is  of  any  importance. 

In  both  systems  just  described,  the  meters  of  all  lines  are  in 
their  respective  central  offices.  Meters  for  use  at  subscribers' 
stations  have  been  devised  and  there  is  no  fundamental  reason  why 
the  record  might  not  be  made  at  the  subscriber's  station  instead  of, 
or  in  addition  to,  a  central-office  record.  Experience  has  shown 
that  confidence  in  a  meter  system  can  be  secured  if  the  meters  be 
positive,  accurate,  and  reliable.  The  labor  of  reading  the  meters 
is  much  less  when  they  are  kept  in  central  offices.  Subscribers 
may  have  access  to  them  if  they  wish. 

Prepayment  Method,  Prepayment  measured-service  mechan- 
isms permit  a  coin  or  token  to  be  dropped  into  a  machine  at  the 
subscriber's  telephone  at  the  time  the  conversation  is  held.  A 
variety  of  forms  of  telephone  coin  collectors  are  in  use,  their  opera- 
tions being  fundamentally  either  electrical  or  mechanical. 


MEASURED  SERVICE 


685 


Electrically  operated  coin  collectors  require  either  that  the 
coin  be  dropped  into  the  machine  in  order  to  enable  the  central  office 
to  be  signaled  in  manual  systems,  or  the  switches  to  be  operated  in 
automatic  systems,  or  they  require  that  the  coin  be  dropped  into 
the  machine  after  calling,  but  before  the  conversation  is  permitted. 

Western  Electric  Company  coin  collectors,  shown  in  Fig.  459, 
may  be  operated  in  either  way  in  connection  with  manual  systems. 
The  usual  way  is  to  require  the  coin  to  be  dropped  before  the  cen- 
tral-office line  lamp  can  glow.  The  operator  then  rings  the  called 
subscriber  and  upon  his  answering  places  a  sufficient  potential  upon 
the  calling  line  to  operate  the  polarized  relay  and  to  drop  the  coin 
into  the  cash  box.  If  the  called  subscriber  does  not  answer  or  his 
line  is  busy,  potential  is  placed  on  the  calling  line,  moving  the  polar- 
ized relay  in  the  other  direction  and  dropping  the  coin  into  a  return 
chute  so  that  the  subscriber  may  take  it.  If  it  is  preferred  that 
the  coin  be  paid  only  on  the  request  of  the  operator,  the  return  feature 
need  not  be  provided. 

In  both  forms  of  operation,  the  Western  Electric  coin  collector 
is  adapted  to  bridge  its  polarized  relay  between  one  limb  of  the  line 
and  ground  during  the  time  a 

coin  rests  on  the  pins,  as  shown  HI  I!'    ilHH-^ 

in  Fig.  459.  When  no  coin  is  on 
the  pins — i.  e.,  before  calling  and 
after  the  called  station  responds 
— the  relay  is  not  so  bridged. 

The  armature  of  the  relay 
responds  only  to  a  high  potential 
and  this  is  applied  by  the  oper- 
ator. If  the  coin  is  to  be  taken 
by  the  company,  one  polarity  is 
sent;  if  it  is  to  be  returned  to  the 
patron,  the  other  polarity  is  sent. 
These  polarities  are  applied  to  a 
limb  of  the  line  proper.  It  will 
be  recalled  that  pressures  to  actuate  service  meters  are  applied  to 
the  test-strand.  If  wished,  keys  may  be  arranged  so  as  to  apply 
30  volts  to  the  test-strand  and  the  collecting  potential  to  the  line  at 
the  same  operation.  This  enables  the  service  meter  to  count  the 


TO 


Fig.  459. 


BOX 


Principle  of  Western  Electric 
Coin  Collector 


686  TELEPHONY 

tokens  placed  in  the  cash  box  of  the  coin  collector,  and  serves  as  a 
valuable  check. 

In  automatic  systems,  in  one  arrangement,  coin  collectors 
are  arranged  so  that  no  impulses  can  be  sent  unless  a  coin  has  been 
deposited,  the  coin  automatically  passing  to  the  cash  box  when  the 
called  subscriber  answers,  or  to  the  patron  if  it  is  not  answered. 
In  another  arrangement,  calls  are  made  exactly  as  in  unlimited 
service,  but  a  coin  must  be  deposited  before  a  conversation  can 
be  held.  The  calling  person  can  hear  the  called  party  speak  and 
may  speak  himself  but  can  not  be  heard  until  the  coin  is  depos- 
ited. No  coin-return  mechanism  is  required  in  this  method. 

Coin  collectors  of  these  types  usually  are  adapted  to  receive 
only  one  kind  of  coin,  these,  in  the  United  States,  being  either  nickels 
or  dimes.  For  long-distance  service,  where  the  charges  vary,  it 
is  necessary  to  signal  to  an  operator  just  what  coins  are  paid.  It 
is  uniformly  customary  to  send  these  signals  by  sound,  the  col- 
lector being  so  arranged  that  the  coins  strike  gongs.  In  coin  col- 
lectors of  the  Gray  Telephone  Paystation  Company,  the  coins  strike 
these  gongs  by  their  own  weight  in  falling  through  chutes.  In  coin 
collectors  of  the  Baird  Electric  Company,  the  power  for  the  signals 
is  provided  by  hand  power,  a  lever  being  pulled  for  each  coin 
deposited,  Both  methods  are  in  wide  use. 


CHAPTER   XXXIX 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS 


Definitions.  Phantom  circuits  are  arrangements  of  telephone 
wires  whereby  more  working,  non-interfering  telephone  lines  exist 
than  there  are  sets  of  actual  wires.  When  four  wires  are  arranged 
to  provide  three  metallic  circuits  for  telephone  purposes,  two  of  the 
lines  are  physical  circuits  and  one  is  a  phantom  circuit. 

Simplex  and  composite  circuits  are  arrangements  of  wires 
whereby  telephony  and  telegraphy  can  take  place  at  the  same  time 
over  the  same  wires  without  interference. 

Phantom.  In  Fig.  460  four  wires  join  two  offices.  RR  are 
repeating  coils,  designed  for  efficient  transforming  of  both  talking 


Fig.  460.     Phantom  Circuit 

and  ringing  currents.  The  devices  marked  A  in  this  and  the  fol- 
lowing figures  are  air-gap  arresters.  Currents  how  the  telephones 
connected  to  either  physical  pair  of  wires  pass,  «<•  *-Ti',  .'stani,  in  op- 
posite directions  in  the  two  wires  of  the  pair.  The  phc.:r>tom  circuit 
uses  one  of  the  physical  pairs  as  a  wire  of  its  line.  It  does  this  by 
tapping  the  middle  point  of  the  line  side  of  each  of  the  repeating 
coils.  The  impedance  of  the  repeating-coil  winding  is  lowered  be- 


688 


TELEPHONY 


cause,  all  the  windings  being  on  the  same  core,  the  phantom  line 
currents  pass  from  the  middle  to  the  outer  connections  so  as  to 
neutralize  each  other's  influence.  The  currents  of  the  phantom  cir- 
cuit, unlike  those  of  the  physical  circuits,  are  in  the  same  direction 
in  both  wires  of  a  pair  at  any  instant.  Their  potentials,  therefore, 
are  equal  and  simultaneous. 

A  phantom  circuit  is  formed  most  simply  when  both  physical 
lines  end  in  the  same  two  offices.     If  one  physical  line  is  longer  than 


Fig.  461.     Phantom  from  Two  Physical  Circuits  of  Unequal  Length 

the  other,  a  phantom  circuit  may  be  formed  as  in  Fig.  461,  wherein 
the  repeating  coil  is  inserted  in  the  longer  line  where  it  passes  through 
a  terminal  station  of  the  shorter. 


W~~L 


Fig.  462.     Phantom  Extended  by  Physical  Circuit 

A  circuit  may  be  built  up  by  adding  a  physical  circuit  to  a 
phantom.  A  circuit  may  be  made  up  of  two  or  more  phantom 
circuits,  joined  by  physical  ones.  In  Fig.  462  a  phantom  circuit  is 
extended  by  the  use  of  a  physical  circuit,  while  in  Fig.  463,  two 
phantom  circuits  are  joined  by  placing  between  them  a  physical 
circuit. 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS  689 

Transpositions.  In  phantom  circuits  formed  merely  by  in- 
serting repeating  coils  in  physical  circuits  and  doing  nothing  else, 
an  exact  balance  of  the  sides  of  the  phantom  circuit  is  lacking.  The 


1     • — w  H          R  w — I     3 


Fig.  463.     Two  Phantoms  Joined  by  Physical  Circuit 

resistances,  insulations,  and  capacities  to  earth  of  the  sides  may  be 
equal,  but  the  exposures  to  adjacent  telephone  and  telegraph  circuits 
and  to  power  circuits  will  not  be  equal  unless  the  phantom  circuits 
are  transposed. 

To  transpose  a  set  of  lines  of  two  physical  wires  each,  is  not 
complicated,  though  it  must  be  done  with  care  and  in  accordance 


/300\/300\: 

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Pig.  464.     Transposition  of  Phantom  Circuits 

with  a  definite,  foreknown  plan.  Transposing  phantom  circuits 
is  less  simple,  however,  as  four  wires  per  circuit  have  to  be  trans- 
posed, instead  of  two. 

In  Fig.   464,   the  general  spacing  of  transposition  sections  is 


690  TELEPHONY 

the  usual  one,  1,300  feet,  of  the  ABCB  system  widely  in  use.  The 
pole  circuit,  on  pins  5  and  6  of  the  upper  arm,  is  transposed  once 
each  two  miles.  The  pole  circuit  of  the  second  arm  transposes 
either  once  or  twice  a  mile.  But  neither  pole  circuit  differs  in  trans- 
position from  any  other  regular  scheme  except  in  the  frequency  of 
transposition.  All  the  other  wires  of  each  arm,  however,  are  so 
arranged  that  each  wire  on  either  side  of  the  pole  circuit  moves 
from  pin  to  pin  at  section-ends,  till  it  has  completed  a  cycle  of  changes 
over  all  .four  of  the  pins  on  its  side.  In  doing  so,  each  phantom  cir- 
cuit is  transposed  with  proper  regard  to  each  of  the  other  three  on 
that  twenty-wire  line. 

The  "new  transposition"  lettering  in  Fig.  464  is  for  the  pur- 
pose of  identifying  the  exact  scheme  of  wiring  each  transposition 
pole.  The  complication  of  wiring  at  each  transposition  pole  is 
increased  by  the  adoption  of  phantom  circuits.  Maintenance  of 
all  the  circuits  is  made  more  costly  arid  less  easy  unless  the  work 
at  points  of  transposition  is  done  with  care  and  skill.  Phantom 
circuits,  to  be  always  successful,  require  that  the  physical  circuits 
be  balanced  and  kept  so. 

Transmission  over  Phantom  Circuits.  Under  proper  condi- 
tions phantom  circuits  are  better  than  physical  circuits,  and  in  this 
respect  it  may  be  noted  that  some  long-distance  operating  compa- 
nies instruct  their  operators  always  to  give  preference  to  phantom 
circuits,  because  of  the  better  transmission  over  them.  The  use  of 
phantom  circuits  is  confined  almost  wholly  to  open- wire  circuits; 
and  while  the  capacity  of  the  phantom  circuit  is  somewhat  greater 
than  that  of  the  physical  circuit,  its  resistance  is  considerably  smaller. 
In  the  actual  wire  the  phantom  loop  is  only  half  the  resistance  of 
either  of  the  physical  lines  from  which  it  is  made,  for  it  contains 
twice  as  much  copper.  The  resistance  of  the  repeating  coils,  how- 
ever, is  to  be  added. 

Simplex.  Simplex  telegraph  circuits  are  made  from  metallic 
circuit  telephone  lines,  as  shown  in  Fig.  465.  The  principle  is 
identical  with  that  of  phantom  telephone  circuits.  The  potentials 
placed  on  the  telephone  line  by  the  telegraph  operations  are  equal 
and  simultaneous.  They  cause  no  current  to  flow  around  the  tele- 
phone loop,  only  along  it.  If  all  qualities  of  the  loop  are  balanced, 
the  telephones  will  not  overhear  the  telegraph  impulses.  In  the 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS    691 

figure,  A  A  are  arresters,  as  before,  GC  are  Morse  relays;  a  2-micro- 
farad  condenser  is  shunted  around  the  contact  of  each  Morse  key 
F  to  quench  the  noises  due  to  the  sudden  changes  on  opening  the 
keys  between  dots  and  dashes. 

A  simplex    arrangement    even    more  simple   substitutes  impe- 
dance coils  for  the  repeating  coils  of  Fig.  465.     The  operation  of 


Fig.  465.     Simplex  Telegraph  Circuit 

the  Morse  circuit  is  the  same.  An  advantage  of  such  a  circuit, 
as  shown  in  Fig.  466,  is  that  the  telephone  circuit  does  not.  suffer 
from  the  two  repeating-coil  losses  in  series.  A  disadvantage  is,  that 


Fig.  466.     Simplex  Telegraph  Circuit 

in  ringing  on  such  a  line  with  a  grounded  generator,  the  Morse  relays 
are  caused  to  chatter. 

The  circuit  of  Fig.  465  may  be  made  to  fit  the  condition  of  a 
through  telephone  line  and  a  way  telegraph  station.  The  midway 
Morse  apparatus  of  Fig.  467  is  looped  in  by  a  combination  of  im- 


692 


pedance  coils  and  condensers.  The  plans  of  Figs.  465  and  466 
here  are  combined,  with  the  further  idea  of  stopping  direct  and 
passing  alternating  currents,  as  is  so  well  accomplished  by  the  use 
of  condensers. 


Fig.  467.     Simplex  Circuit  with  Waystation 

Composite.  Composite  circuits  depend  on  another  principle 
than  that  of  producing  equal  and  simultaneous  potentials  on  the 
two  wires  of  the  telephone  loop.  The  opposition  of  impedance 
coils  to  alternating  currents  and  of  condensers  to  direct  currents 
are  the  fundamentals.  The  early  work  in  this  art  was  done  by 
Van  Rysselberghe,  of  Belgium.  In  Fig.  468,  one  telephone  circuit 


Fig.  468.     Composite  Circuit 


forms  two  Morse  circuits,  two  wires  carrying  three  services.  Each 
Morse  circuit  will  be  seen  to  include,  serially,  two  50-ohm  impedance 
coils,  and  to  have  shunts  through  condensers  to  ground.  The  50- 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS  693 

ohm  coils  are  connected  differentially,  offering  low  consequent  im- 
pedance to  Morse  impulses,  whose  frequency  of  interruption  is  not 
great.  As  the  impedance  coils  are  large,  have  cores  of  considerable 
length,  and  are  wound  with  two  separate  though  serially  connected 
windings  each,  their  impedance  to  voice  currents  is  great.  They 
act  as  though  they  were  not  connected  differentially,  so  far  as  voice 
currents  are  concerned. 

Because  of  the  condensers  serially  in  the  telephone  line,  voice 
currents  can  pass  through  it,  but  direct  currents  can  not.  Im- 
pulses due  to  discharges  of  cores,  coils,  and  capacities  in  the  Morse 
circuit  could  make  sounds  in  the  telephones,  but  these  are  choked 
out,  or  led  to  earth  by  the  30-ohm  impedance  coils  and  the  heavy 
Morse  condensers. 

Ringing.  Ringing  over  simplex  circuits  is  done  in  the  way 
usual  where  no  telegraph  service  is  added.  Both  telegraphy  and 
telephony  over  simplex  circuits  follow  their  usual  practice  in  the 
way  of  calling  and  conversing.  In  composite  working,  however, 
ringing  by  usual  methods  either  is  impossible  because  of  heavy 
grounds  and  shunts,  or  if  it  is  possible  to  get  ringing  signals  through 
at  all,  trie  relays  of  the  Morse  apparatus  will  chatter,  interfering 
with  the  proper  use  of  the  telegraph  portion  of  the  service. 

It  is  customary,  therefore,  either  to  equip  composite  circuits 
with  special  signaling  devices  by  which  high-frequency  currents 
pass  over  the  telephone  circuits,  operating  relays  which  in  turn  oper- 
ate local  ringing  signals;  or  to  refrain  from  ringing  on  composite 
circuits  and  to  transmit  orders  for  connections  by  telegraph.  The 
latter  is  wholly  satisfactory  over  composite  lines  between  points 
having  heavy  telegraph  traffic,  and  it  is  between  such  points  as  these 
that  composite  practice  is  most  general. 

Phantoms  from  Simplex  and  Composite  Circuits.  Phantom  and 
simplex  principles  are  identical,  and  by  adding  the  composite  prin- 
ciple, two  simplex  circuits  may  have  a  phantom  superadded,  as  in 
Fig.  469.  Similarly,  as  in  Fig.  470,  two  composite  circuits  can  be 
phantomed.  This  case  gives  seven  distinct  services  over  four  wires : 
three  telephone  loops — two  physical  and  one  phantom — and  four 
Morse  lines. 

Railway  Composite.  The  foregoing  are  problems  of  making 
telegraphy  a  by-product  of  telephony.  With  so  many  telegraph 


694 


TELEPHONY 


Fig.  469.     Phantom  of  Two  Simplex  Circuits 


J.M    !i£^        ^^&    ^"i 

Fig.  470.     Phantom  of  Two  Composite  Circuits 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS    695 


wires  on  poles  over  the  country,  it  has  seemed  a  pity  riot  to  turn  the 
thing  around  and  provide  for  telephony  as  a  by-product  of  telegraphy. 
This  has  been  accomplished,  and  the  result  is  called  a  railway  com- 
posite system.  For  the  reason  that  the  telegraph  circuits  are  not 
in  pairs,  accurately  matched  one  wire  against  another,  and  are  not 
always  uniform  as  to  material,  it  has  not  been  possible  to  secure 
as  good  telephone  circuits  from  telegraph  wires  as  telegraph  circuits 
from  telephone  wires. 

Practical  results  are  secured  by  adaptation  of  the  original  prin- 
ciple of  different  frequencies.  A  study  of  Fig.  468  shows  that  over 
such  a  composite  circuit  the  usual  method  of  ringing  from  station 
to  station  over  the  telephone  cir- 
cuit by  an  alternating  current  of 
a  frequency  of  about  sixteen  per 
second  is  practically  impossible. 
This  is  because  of  the  heavy  short- 
circuit  provided  by  the  two  30- 
ohm  choke  coils  at  each  of  the  sta- 
tions, the  heavy  shunt  of  the  large 
condensers,  and  the  grounding 
through  the  50-ohm  choke  coils. 
If  high-frequency  speech  currents 
can  pass  over  these  circuits  with 
a  very  small  loss,  other  high- 
frequency  circuits  should  find  a 

good  path.  There  are  many  easy  ways  of  making  such  currents, 
but  formerly  none  very  simple  for  receiving  them.  Fig  471  shows 
one  simple  observer  of  such  high-frequency  currents,  it  being  merely 
an  adaptation  of  the  familiar  polarized  ringer  used  in  every  sub- 
scriber's telephone.  In  either  position  of  the  armature  it  makes 
contact  with  one  or  the  other  of  two  studs  connected  to  the  bat- 
tery, so  that  in  all  times  of  rest  the  relay  A  is  energized  When  a 
high-frequency  current  passes  through  this  polarized  relay,  however, 
there  is  enough  time  in  which  the  armature  is  out  of  contact  with 
either  stud  to  reduce  the  total  energy  through  the  relay  A  and  allow 
its  armature  to  fall  away,  ringing  a  vibrating  bell  or  giving  some 
other  signal. 

Fig.  472  shows  a  form  of  apparatus  for  producing  the  high- 


Fig.  471.     Ringing  Device  for 
Composite  Circuits 


696 


TELEPHONY 


/NT£R- 


frequency  current  necessary  for  signaling.  It  is  evident  that  if  a 
magneto  generator,  such  as  is  used  in  ordinary  magneto  telephones, 
could  be  made  to  drive  its  armature  fast  enough,  it  also  might  fur- 
nish the  high-frequency  current  necessary  for  signaling  through 

condensers   and   past   heavy  im- 
pedances. 

Applying  these  principles  of 
high-frequency  signals  sent  and 
received  to  a  single-wire  telegraph 
circuit,  the  arrangement  shown 
in  Fig.  473  results,  this  being  a 
type  of  railway  composite  circuit. 
The  principal  points  of  interest  herein  are  the  insertion  of  impedances 
in  series  with  the  telegraph  lines,  the  shunting  of  the  telegraph  relays 
by  small  condensers,  the  further  shunting  of  the  whole  telegraph 
mechanism  of  a  station  by  another  condenser,  and  thus  keeping  out 
of  the  line  circuit  changes  in  current  values  which  would  be  heard 
in  the  telephones  if  violent,  and  might  be  inaudible  if  otherwise. 


Fig.  472.     Ringing  Current  Device 


Fig.   473.     Railway  Composite  Circuit 

A  further  interesting  element  is  the  very  heavy  shunting  of  the 
telephone  receiver  by  means  of  an  inductive  coil.  This  shunt  is 
applied  for  by-path  purposes  so  that  heavy  disturbing  currents 
may  be  kept  out  of  the  receiver  while  a  sufficient  amount  of  voice 
current  is  diverted  through  the  receiver  It  is  well  to  have  the  induct- 
ance of  this  shunt  made  adjustable  by  providing  a  movable  iron 


PHANTOM,  SIMPLEX,  AND  COMPOSITE  CIRCUITS    697 

core  for  the  shunt  winding.  When  the  core  is  drawn  out  of  the 
coil,  its  impedance  is  diminished  because  the  inductance  is  dimin- 
ished. This  reduces  the  amount  of  disturbing  noise  in  the  receiver. 
The  core  should  be  withdrawn  as  little  as  the  amount  of  disturbance 
permits,  as  this  also  diminishes  the  loudness  of  the  received  speech. 

Because  the  signaling  over  lines  equipped  with  this  form  of  com- 
posite working  results  in  the  ringing  of  a  bell  by  means  of  local  cur- 
rent, it  is  of  particular  advantage  in  cases  where  the  bell  needs  to 
ring  loudly.  Switch  stations,  crossings,  and  similar  places  where  the 
attendant  is  not  constantly  near  the  telephone  can  be  equipped  with 
this  type  of  composite  apparatus  and  it  so  offers  a  valuable  substi- 
tute for  regular  railway  telegraph  equipment,  with  which  the  at- 
tendant may  not  be  familiar.  The  success  of  the  local  bell-ringing 
arrangement,  however,  depends  on  accurate  relay  adjustment  and 
on  the  maintenance  of  a  primary  battery.  The  drain  on  the  ringing 
battery  is  greater  than  on  the  talking  battery. 

A  good  substitute  for  the  bell  signal  on  railway  composite  cir- 
cuits is  a  telephone  receiver  responding  directly  to  high-frequency 
currents  over  the  line.  The  receiver  is  designed  specially  for  the 
purpose  and  is  known  as  a  "howler."  Its  signal  can  be  easily  heard 
through  a  large  room.  The  condenser  in  series  with  it  is  of  small 
capacity,  limiting  the  drain  upon  the  line.  Usually  the  howler  is 
detached  by  the  switch  hook  during  conversation  from  a  station. 

Railway  Composite  Set.  The  circuit  of  a  set  utilizing  such  an 
arrangement  together  with  other  details  of  a  complete  railway  com- 
posite set  is  shown  in  Fig.  474.  The  drawing  is  arranged  thus,  in 
the  hope  of  simplifying  the  understanding  of  its  principles.  It  will  be 
seen  that  the  induction  coil  serves  as  an  interrupter  as  well  as  for  trans- 
mission. All  of  the  contacts  are  shown  in  the  position  they  have 
during  conversation.  The  letters  Hcl,  He  2,  etc.,  and  Kc  1,  Kc  °3, 
etc.,  refer  to  hook  contacts  and  key  contacts,  respectively,  of  the  num- 
bers given.  The  arrangements  of  the  hook  and  key  springs  are  shown 
at  the  right  of  the  figure.  RR  represent  impedance  coils  connected 
serially  in  the  line  and  placed  at  terminal  stations.  The  composite 
telephone  sets  are  bridged  from  the  line  to  ground  at  any  points 
between  the  terminal  impedance  coils. 

The  direct  currents  of  telegraphy  are  prevented  from  passing 
to  ground  through  the  telephone  set  during  conversation  by  the  2- 


698 


TELEPHONY 


microfarad  condenser  which  is  in  series  with  the  receiver.  They  are 
prevented  from  passing  to  ground  through  the  telephone  set  when  the 
receiver  is  on  the  hook  by  a  .05  microfarad  condenser  in  series  with  the 
howler.  The  alternating  currents  of  speech  and  interrupter  signaling 
are  kept  from  passing  to  ground  at  terminals  by  the  impedance  coils. 
Signals  are  sent  from  the  set  by  pressing  the  key  K.  This  oper- 
ates the  vibrator  by  closing  contacts  Kc  6  and  Kc  7.  The  howler 
is  cut  off  and  the  receiver  is  short-circuited  by  the  same  operation  of 
the  key.  The  impedance  of  the  coil  /  is  changed  by  moving  its 
adjustable  core. 


Ac./ 


Ac.  .5 


Fig.  474.     Railway  Composite  Set 

Applications.  A  chief  use  of  composite  and  simplex  circuits 
is  for  ticket  wire  purposes.  These  are  circuits  over  which  long- 
distance operators  instruct  each  other  as  to  connecting  and  discon- 
necting lines,  the  routing  of  calls,  and  the  making  of  appointments. 
One  such  wire  will  care  for  all  the  business  of  many  long-distance 
trunks.  The  public  also  absorbs  the  telegraph  product  of  telephone 
lines.  Such  telegraph  service  is  leased  to  brokers,  manufacturers, 
merchants,  and  newspapers.  Railway  companies  use  portable  tele- 
phone adjuncts  to  telegraph  circuits  on  trains  for  service  from 
stations  not  able  to  support  telegraph  attendants,  and  in  a  limited 
degree  for  the  dispatching  of  trains.  Telephone  train  dispatching, 
however,  merits  better  equipment  than  a  railway  composite  system 
affords. 


CHAPTER   XL/ 
*TELEPHONE  TRAIN  DISPATCHING 

It  has  been  only  within  the  past  few  years  that  the  telephone 
has  begun  to  replace  the  telegraph  for  handling  train  movements. 
The  telegraph  and  the  railroads  have  grown  up  together  in  this  coun- 
try since  1850,  and  in  view  of  the  excellent  results  that  the  telegraph 
has  given  in  train  dispatching  and  of  the  close  alliance  that  has 
always  naturally  existed  between  the  railway  and  the  telegraph,  it  has 
been  difficult  for  the  telephone,  which  came  much  later,  to  enter  the 
field. 

Rapid  Growth.  The  telephone  has  been  in  general  use  among 
the  railroads  for  many  years,  but  only  on  a  few  short  lines  has  it  been 
used  for  dispatching  trains.  In  these  cases  the  ordinary  magneto 
circuit  and  instruments  have  been  employed,  differing  in  no  respect 
from  those  used  in  commercial  service  at  the  present  time.  Code 
ringing  was  used  and  the  number  of  stations  on  a  circuit  was  limited 
by  the  same  causes  that  limit  the  telephones  on  commercial  party 
lines  at  present. 

The  present  type  of  telephone  dispatching  systems,  however, 
differs  essentially  from  the  systems  used  in  commercial  work,  and  is, 
in  fact,  a  highly  specialized  party-line  system,  arranged  for  selective 
ringing  and  many  stations.  The  first  of  the  present  type  was  in- 
stalled by  the  New  York  Central  and  Hudson  River  Railroad  in  Oc- 
tober, 1907,  between  Albany  and  Fonda,  New  York,  a  distance  of 
40  miles.  This  section  of  the  road  is  on  them  ain  line  and  has  four 
tracks  controlled  by  block  signals. 

The  Chicago,  Burlington,  and  Quincy  Railroad  was  the  second  to 
install  train-dispatching  circuits.  In  December,  1907,  a  portion  of  the 
main  line  from  Aurora  to  Mendota,  Illinois,  a  distance  of  46  miles, 
was  equipped.  This  was  followed  in  quick  succession  by  various 

*We  wish  particularly  to  acknowledge  the  courtesy  of  the  Western  Electric  Com- 
pany in  their  generous  assistance  in  the  preparation  of  this  chapter. 


700  TELEPHONY 

other  circuits  ranging,  in  general,  in  lengths  over  100  miles.  At  the 
present  time  there  are  over  20  train-dispatching  circuits  on  the  Chi- 
cago, Burlington,  and  Quincy  Railroad  covering  125  miles  of  double 
track,  28  miles  of  multi-track,  and  1,381  miles  of  single  track,  and 
connecting  with  286  stations. 

Other  railroads  entered  this  field  in  quick  order  after  the  initial 
installations,  and  at  the  present  time  nearly  every  large  railroad  sys- 
tem in  the  United  States  is  equipped  with  several  telephone  train- 
dispatching  circuits  and  all  of  these  seem  to  be  extending  their  systems. 

In  1910,  several  railroads,  including  the  Delaware,  Lackawanna, 
and  Western,  had  their  total  mileage  equipped  with  telephone  dis- 
patching circuits.  The  Atchison,  Topeka,  and  Santa  Fe  Railroad 
is  equipping  its  whole  system  as  rapidly  as  possible  and  already  is 
the  largest  user  of  this  equipment  in  this  country.  From  latest 
information,  over  55  railroads  have  entered  this  field,  with  the  re- 
sult that  the  telephone  is  now  in  use  in  railroad  service  on  over 
29,000  miles  of  line. 

Causes  of  Its  Introduction.  The  reasons  leading  to  the  intro- 
duction of  the  telephone  into  the  dispatching  field  were  of  this  nature: 
First,  and  most  important,  was  the  enactment  of  State  and  Federal 
Laws  limiting  to  nine  hours  the  working  day  of  railroad  employes 
transmitting  or  receiving  orders  pertaining  to  the  movement  of  trains. 
The  second,  which  is  directly  dependent  upon  the  first,  was  the  in- 
ability of  the  railroads  to  obtain  the  additional  number  of  telegraph 
operators  which  were  required  under  the  provisions  of  the  new  laws. 
It  was  estimated  that  15,000  additional  operators  would  be  required 
to  maintain  service  in  the  same  fashion  after  the  new  laws  went  into 
effect  in  1907.  The  increased  annual  expense  occasioned  by  the 
employment  of  these  additional  operators  was  roughly  estimated 
at  $10,000,000.  A  third  reason  is  found  in  the  decreased  efficiency 
of  the  average  railway  and  commercial  telegraph  operator.  There 
is  a  very  general  complaint  among  the  railroads  today  regarding  this 
particular  point,  and  many  of  them  welcome  the  telephone,  because, 
if  for  no  other  reason,  it  renders  them  independent  of  the  telegrapher. 
What  has  occasioned  this  decrease  in  efficiency  it  is  not  easy  to  say, 
but  there  is  a  strong  tendency  to  lay  it,  in  part,  to  the  attitude  of  the 
telegraphers'  organization  toward  the  student  operator.  It  is  a  fact, 
too,  that  the  limits  which  these  organizations  have  placed  on  stu- 


TELEPHONE  TRAIN  DISPATCHING  701 

dent  operators  were  directly  responsible  for  the  lack  of  available  men 
when  they  were  needed. 

Advantages.  In  making  this  radical  change,  railroad  officials 
were  most  cautious,  and  yet  we  know  of  no  case  where  the  introduc- 
tion of  the  telephone  has  been  followed  by  its  abandonment,  the 
tendency  having  been  in  all  cases  toward  further  installations  and 
more  equipment  of  the  modern  type.  The  reasons  for  this  are  clear, 
for  where  the  telephone  is  used  it  does  not  require  a  highly  spec  al- 
ized  man  as  station  operator  and  consequently  a  much  broader  field 
is  open  to  the  railroads  from  which  to  draw  operators.  This,  we 
think,  is  the  most  far-reaching  advantage. 

The  telephone  method  also  is  faster.  On  an  ordinary  train- 
dispatching  circuit  it  now  requires  from  0.1  of  a  second  to  5  seconds 
to  call  any  station.  In  case  a  plurality  of  calls  is  desired,  the  dis- 
patcher calls  one  station  after  another,  getting  the  answer  from  one 
while  the  next  is  being  called,  and  so  on.  By  speaking  into  a  tele- 
phone many  more  words  may  be  transmitted  in  a  given  time  than 
by  Morse  telegraphy.  It  is  possible  to  send  fifty  words  a  minute  by 
Morse,  but  such  speed  is  exceptional.  Less  than  half  that  is  the  rule. 
The  gain  in  high  speed,  therefore,  which  is  obtained  is  obvious  and 
it  has  been  found  that  this  is  a  most  important  feature  on  busy  di- 
visions. It  is  true  that  in  the  issuance  of  "orders,"  the  speed,  in 
telephonic  train  dispatching,  is  limited  to  that  required  to  write  the 
words  in  longhand.  But  all  directions  of  a  collateral  character, 
the  receipt  of  important  information,  and  the  instantaneous  descrip- 
tions of  emergency  situations  can  be  given  and  received  at  a  speed 
limited  only  by  that  of  human  speech. 

The  dispatcher  is  also  brought  into  a  closer  personal  relation 
with  the  station  men  and  trainmen,  and  this  feature  of  direct  per- 
sonal communication  has  been  found  to  be  of  importance  in  bring- 
ing about  a  higher  degree  of  co-operation  and  better  discipline  in 
the  service. 

Telephone  dispatching  has  features  peculiar  to  itself  which 
are  important  in  improving  the  class  of  service.  One  of  these  is  the 
"answer-back"  automatically  given  to  the  dispatcher  by  the  way- 
station  bell.  This  informs  the  dispatcher  whether  or  not  the  bell 
at  the  station  rang,  and  excuses  by  the  operators  that  it  did  not,  are 
eliminated. 


702  TELEPHONY 

Anyone  can  answer  a  telephone  call  in  an  emergency.  The 
station  operator  is  frequently  agent  also,  and  his  duties  often  take 
him  out  of  hearing  of  the  telegraph  sounder.  The  selector  bell 
used  with  the  telephone  can  be  heard  for  a  distance  of  several  hun- 
dred feet.  In  addition,  it  is  quite  likely  that  anyone  in  the  neighbor- 
hood would  recognize  that  the  station  was  wanted  and  either  notify 
the  operator  or  answer  the  call. 

In  cases  of  emergency  the  train  crews  can  get  into  direct  com- 
munication with  the  dispatcher  immediately,  by  means  of  portable 
telephone  sets  which  are  carried  on  the  trains.  It  is  a  well-known 
fact  that  every  minute  a  main  line  is  blocked  by  a  wreck  can  be  reck- 
oned as  great  loss  to  the  railroad. 

It  is  also  possible  to  install  siding  telephone  sets  located  either  in 
booths  or  on  poles  along  the  right-of-way.  These  are  in  general  serv- 
ice today  at  sidings,  crossings,  drawbridges,  water  tanks,  and  such 
places,  where  it  may  be  essential  for  a  train  crew  to  reach  the  nearest 
waystation  to  give  or  receive  information. 

The  advantage  of  these  siding  sets  is  coming  more  and  more  to  be 
realized.  With  the  telegraph  method  of  dispatching,  a  train  is  ordered 
to  pass  another  train  at  a  certain  siding,  let  us  say.  It  reaches  this 
point,  and  to  use  a  railroad  expression,  "goes  into  the  hole."  Now,  if 
anything  happens  to  the  second  train  whereby  it  is  delayed,  the  first 
train  remains  tied  up  at  that  siding  without  the  possibility  of  either 
reaching  the  dispatcher  or  being  reached  by  him.  With  the  telephone 
station  at  the  siding,  which  requires  no  operator,  this  is  avoided.  If  a 
train  finds  itself  waiting  too  long,  the  conductor  goes  to  the  siding 
telephone  and  talks  to  the  dispatcher,  possibly  getting  orders  which 
will  advance  him  many  miles  that  would  otherwise  have  been  lost. 

It  is  no  longer  necessary  for  a  waystation  operator  to  call  the 
dispatcher.  When  one  of  these  operators  wishes  to  talk  to  the  dis- 
patcher, he  merely  takes  his  telephone  receiver  off  the  hook,  presses 
a  button,  and  speaks  to  the  dispatcher. 

With  the  telephone  it  is  a  simple  matter  to  arrange  for  pro- 
vision so  that  the  chief  dispatcher,  the  superintendent,  or  any  other 
official  may  listen  in  at  will  upon  a  train  circuit  to  observe  the  char- 
acter of  the  service.  The  fact  that  this  can  be  done  and  that  the 
operators  know  it  can  be  done  has  a  very  strong  tendency  to  im- 
prove the  discipline. 


TELEPHONE  TRAIN  DISPATCHING  703 

The  dispatchers  are  so  relieved,  by  the  elimination  of  the  strain 
of  continuous  telegraphing,  and  can  handle  their  work  so  much  more 
quickly  with  the  telephone,  that  in  many  cases  it  has  been  found 
possible  to  increase  the  length  of  their  divisions  from  30  to  50  per 
cent. 

Railroad  Conditions.  One  of  the  main  reasons  that  delayed 
the  telephone  for  so  many  years  in  its  entrance  to  the  dispatching 
field  is  that  the  conditions  in  this  field  are  like  nothing  which  has  yet 
been  met  with  in  commercial  telephony.  There  was  no  system  de- 
veloped for  meeting  them,  although  the  elements  were  at  hand.  A 
railroad  is  divided  up  into  a  number  of  divisions  or  dispatchers' 
districts  of  varying  lengths.  These  lengths  are  dependent  on  the 
density  of  the  traffic  over  the  division.  In  some  cases  a  dispatcher 
will  handle  not  more  than  25  miles  of  line.  In  other  cases  this  dis- 
trict may  be  300  miles  long.  Over  the  length  of  one  of  these  divi- 
sions the  telephone  circuit  extends,  and  this  circuit  may  have  upon 
it  5  or  50  stations,  all  of  which  may  be  required  to  listen  upon  the  line 
at  the  same  time. 

It  will  be  seen  from  this  that  the  telephone  dispatching  circuit 
partakes  somewhat  of  the  nature  of  a  long-distance  commercial 
circuit  in  its  length,  and  it  also  resembles  a  rural  line  in  that  it  has  a 
large  number  of  telephones  upon  it.  Regarding  three  other  char- 
acteristics, namely,  that  many  of  these  stations  may  be  required  to 
be  in  on  the  circuit  simultaneously,  that  they  must  all  be  signaled 
selectively,  and  that  it  must  also  be  possible  to  talk  and  signal  on 
the  circuit  simultaneously,  a  telephone  train-dispatching  circuit 
resembles  nothing  in  the  commercial  field.  These  requirements 
are  the  ones  which  have  necessitated  the  development  of  special 
equipment. 

Transmitting  Orders.  The  method  of  giving  orders  is  the  same 
as  that  followed  with  the  telegraph,  with  one  important,  exception. 
When  the  dispatcher  transmits  a  train  order  by  telephone,  he  writes 
out  the  order  as  he  speaks  it  into  his  transmitter.  In  this  way  the 
speed  at  which  the  order  is  given  is  regulated  so  that  everyone  re- 
ceiving it  can  easily  get  it  all  down,  and  a  copy  of  the  transmitted 
order  is  retained  by  the  dispatcher.  All  figures  and  proper  names 
are  spelled  out.  Then  after  an  order  has  been  given,  it  is  repeated 
to  the  dispatcher  by  each  man  receiving  it,  and  he  underlines  each 


704  TELEPHONY 

word  as  it  comes  in.  This  is  now  done  so  rapidly  that  a  man  can 
repeat  an  order  more  quickly  than  the  dispatcher  can  underline. 
The  doubt  as  to  the  accuracy  with  which  it  is  possible  to  transmit 
information  by  telephone  has  been  dispelled  by  this  method  of  pro- 
cedure, and  the  safety  of  telephone  dispatching  has  been  fully  estab- 
lished. 

Apparatus.  The  apparatus  which  is  employed  at  waystations 
may  be  divided  into  two  groups — the  selector  equipment  and  the  tele- 
phone equipment.  The  selector  is 
an  electro-mechanical  device  for 
ringing  a  bell  at  awaystation  when 
the  dispatcher  operates  a  key 
corresponding  to  that  station.  At 
first,  as  in  telegraphy,  the  selector 
magnets  were  connected  in  series 
in  the  line,  but  today  all  systems 
bridge  the  selectors  across  the 
telephone  circuit  in  the  same  way 
and  for  the  same  reasons  that  it 

Fig.  475.     Western  Electric  Selector  .  . 

is  done  in  bridging  party-line 

work.  There  are  at  the  present  time  three  types  of  selectors  in 
general  use,  and  the  mileage  operated  by  means  of  these  is  probably 
considerably  over  95  per  cent  of  the  total  mileage  so  operated  in  the 
country. 

The  Western  Electric  Selector.  This  selector  is  the  latest  and 
perhaps  the  simplest.  Fig.  475  shows  it  with  its  glass  dust-proof 
cover  on,  and  Fig.  476  shows  it  with  the  cover  removed.  This  se- 
lector is  adapted  for  operating  at  high  speed,  stations  being  called 
at  the  rate  of  ten  per  second. 

The  operating  mechanism,  which  is  mounted  on  the  front  of 
the  selector  so  as  to  be  readily  accessible,  works  on  the  central-en- 
ergy principle — the  battery  for  its  operation,  as  well  as  for  the  oper- 
ation of  the  bell  used  in  connection  with  it,  both  being  located  at  the 
dispatcher's  office.  The  bell  battery  may,  however,  be  placed  at  the 
waystation  if  this  is  desired. 

The  selector  consists  of  two  electromagnets  which  are  bridged 
in  series  across  the  telephone  circuit  and  are  of  very  high  impedance. 
It  is  possible  to  place  as  many  of  these  selectors  as  may  be  desired 


TELEPHONE  TRAIN  DISPATCHING 


705 


across  a  circuit  without  seriously  affecting  the  telephonic  trans- 
mission. Direct-current  impulses  sent  out  by  the  dispatcher  operate 
these  magnets,  one  of  which  is  slow  and  the  other  quick-acting. 
The  first  impulse  sent  out  is  a  long  impulse  and  pulls  up  both  arma- 


Fig    476.     Western  Electric  Selector 


tures,  thereby  causing  the  pawls  above  and  below  the  small  ratchet 
wheel,  shown  in  Fig  476,  to  engage  with  this  wheel.  The  remain- 
ing impulses  operate  the  quick-acting  magnet  and  step  the  wheel 


Fig.  477.     Dispatcher's  Keys 


around  the  proper  number  of  teeth,  but  do  not  affect  the  slow-acting 
magnet  which  remains  held  up  by  them.  The  pawl  connected  to 
the  slow-acting  magnet  merely  serves  to  prevent  the  ratchet  wheel 
from  turning  back,  Attached  to  the  ratchet;  wheel  is  a  contact  whose 


706  TELEPHONY 

position  can  be  varied  in  relation  to  the  stationary  contact  on  the 
left  of  the  selector  with  which  this  engages.  This  contact  is  set  so 
that  when  the  wheel  has  been  rotated  the  desired  number  of  teeth, 
the  two  contacts  will  make  and  the  bell  be  rung.  Any  selector 
may  thus  be  adjusted  for  any  station,  and  the  selectors  are  thus  in- 
terchangeable. When  the  current  is  removed  from  the  line  at  the 
dispatcher's  office,  the  armatures  fall  back  and  everything  is  restored 
to  normal.  An  "answer-back"  signal  is  provided  with  this  selector 
dependent  upon  the  operation  of  the  bell.  When  the  selector  at  a 


Fig.  478.     Dispatcher's  Key  Mechanism 

station  operates,  the  bell  normally  rings  for  a  few  seconds.  The 
dispatcher,  however,  can  hold  this  ring  for  any  length  of  time  desired. 
The  keys  employed  at  the  dispatcher's  office  for  operating 
selectors  are  shown  in  Fig.  477.  There  is  one  key  for  each  way- 
station  on  the  line  and  the  dispatcher  calls  any  station  by  merely 
giving  the  corresponding  key  a  quarter  turn  to  the  right.  Fig.  478 
shows  the  mechanism  of  one  of  these  keys  and  the  means  employed 
for  sending  out  current  impulses  over  the  circuit.  The  key  is  ad- 
justable and  may  be  arranged  for  any  station  desired  by  means  of 
the  movable  cams  shown  on  the  rear  in  Fig.  478,  these  cams,  when 
occupying  different  positions,  serving  to  cover  different  numbers 
of  the  teeth  of  the  impulse  wheel  which  operate  the  impulse  contacts. 


TELEPHONE  TRAIN  DISPATCHING  707 

The  Gill  Selector.  The  second  type  of  selector  in  extensive 
use  throughout  the  country  today  is  known  as  the  Gill,  after  its  in- 
ventor. It  is  manufactured  for  both  local-battery  and  central- 
energy  types,  the  latter  being  the  latest  development  of  this  selector. 
With  the  local-battery  type,  the  waystation  bell  rings  until  stopped 
by  the  dispatcher.  With  the  central-energy  type  it  rings  a  definite 
length  of  time  and  can  be  held  for  a 
longer  period  as  is  the  case  with  the 
Western  Electric  selector.  The  se- 
lector is  operated  by  combinations  of 
direct-current  impulses  which  are  sent 
out  over  the  line  by  keys  in  the  dis- 
patcher's office. 

The  dispatcher  has  a  key  cab- 
inet, and  calls  in  the  same  way  as 
already  described,  but  these  keys  in- 
stead of  sending  a  series  of  quick 
impulses,  send  a  succession  of  im- 

.  ,     .  Fig.  479.     Gill  selector      • 

pulses  with  intervals  between  corre- 
sponding to  the  particular  arrangement  of  teeth  in  the  correspond- 
ing waystation  selector  wheel.      Each    key,    therefore,  belongs  def- 
initely with  a  certain  selector  and  can  be  used  in  connection  with 
no  other. 

A  concrete  example  may  make  this  clearer.  The  dispatcher 
may  operate  key  No.  1421.  This  key  starts  a  clockwork  mechanism 
which  impresses  at  regular  intervals,  on  the  telephone  line,  direct- 
current  impulses,  with  intervals  between  as  follows:  1-4-2-1.  There 
is  on  the  line  one  selector  corresponding  to  this  combination  and 
it  alone,  of  all  the  selectors  on  the  circuit,  will  step  its  wheel  clear 
around  so  that  contact  is  made  and  the  bell  is  rung.  In  all 
the  others,  the  pawls  will  have  slipped  out  at  some  point  of  the 
revolution  and  the  wheels  will  have  returned  to  their  normal  po- 
sitions. 

The  Gill  selector  is  shown  in  Fig.  479.  It  contains  a  double- 
wound  relay  which  is  bridged  across  the  telephone  circuit  and  operates 
the  selector.  This  relay  has  a  resistance  of  4,500  ohms  and  a  high 
impedance,  and  operates  the  selector  mechanism  which  is  a  special 
modification  of  the  ratchet  and  pawl  principle.  The  essential  fea- 


708  TELEPHONY 

tures   of   this   selector  are  the  "step-up"  selector  wheel  and  a  time 
wheel,  normally  held  at  the  bottom  of  an  inclined  track. 

The  operation  of  the  selector  magnet  pushes  the  time  wheel 
up  the  track  and  allows  it  to  roll  down.  If  the  magnet  is  operated 
rapidly,  the  wheel  does  not  get  clear  down  before  being  pushed 
back  again.  A  small  pin  on  the  side  of  the  pawl,  engaging  the  selector 
wheel  normally,  opposes  the  selector  wheel  teeth  near  their  outer 
points.  When  the  time  wheel  rolls  to  the  bottom  of  the  track,  how- 
ever, the  pawl  is  allowed  to  drop  to  the  bottom  of  the  tooth.  Some 
of  the  teeth  on  the  selector  wheel  are  formed  so  that  they  will  effect- 
ually engage  with  the  pawl  only  when  the  latter  is  in  normal  posi- 
tion, while  others  will  engage  only  while  the  pawl  is  at  the  bottom 
position;  thus  innumerable  combinations  can  be  made  which  will 
respond  to  certain  combinations  of  rapid  impulses  with  intervals 
between.  The  correct  combination  of  impulses  and  intervals  steps 


Fig.  480.     Cummings-Wray  Dispatcher's  Sender 

the  selector  wheel  clear  around  so  that  a  contact  is  made.  The 
selector  wheels  at  all  other  stations  fail  to  reach  their  contact  po- 
sition because  at  some  point  or  points  in  their  revolution  the  pawls 
have  slipped  out,  allowing  the  selector  wheels  to  return  "home." 

The  "answer-back"  is  provided  in  this  selector  by  means  of 
a  few  inductive  turns  of  the  bell  circuit  which  are  wound  on  the 
selector  relay.  The  operation  of  the  bell  through  these  turns  in- 
duces an  alternating  current  in  the  selector  winding  which  flows 
out  on  the  line  and  is  heard  as  a  distinctive  buzzing  noise  by  the 
dispatcher. 

The  Cummings-Wray  Selector.  Both  of  the  selectors  already 
described  are  of  a  type  known  as  the  individual-call  selectors,  mean- 


TELEPHONE  TRAIN  DISPATCHING  709 

ing  that  only  one  station  at  a  time  can  be  called.  If  a  plurality  of 
calls  is  desired,  the  dispatcher  calls  one  station  after  another.  The 
third  type  of  selector  in  use  today  is  of  a  type  known  as  the  multiple- 
call,  in  which  the  dispatcher  can  call  simultaneously  as  many  station? 
as  he  desires. 

The  Cummings-Wray  selector  and  that  of  the  Kellogg  Switch- 
board and  Supply  Company  are  of  this  type  and  operate  on  the 
principle  of  synchronous 
clocks.  When  the  dispatch- 
er wishes  to  put  through  a 
call,  he  throws  the  keys  of 
all  the  stations  that  he  de- 
sires and  then  operates  a 
starting  key.  The  bells  at 
all  these  stations  are  rung 
by  one  operation. 

The  dispatcher's  send- 
ing equipment  of  the  Cum- 

Fig.  481.     Cummings-Wray  Selector 

mmgs-Wray  system  is  shown 

in  Fig.  480,  and  the  waystation  selector  in  Fig.  481.  It  is  necessary 
with  this  system  for  the  clocks  at  all  stations  to  be  wound  every 
eight  days. 

In  the  dispatcher's  master  sender  the  clock-work  mechanism 
operates  a  contact  arm  which  shows  on  the  face  of  the  sender  in 
Fig.  480.  There  is  one  contact  for  every  station  on  the  line.  The 
clock  at  this  office  and  the  clocks  at  all  the  waystation  offices  start 
together,  and  it  is  by  this  means  that  the  stations  are  signaled, 
as  will  be  described  later,  when  the  detailed  operation  of  the  circuits 
is  taken  up. 

Telephone  Equipment.  Of  no  less  importance  than  the  selective 
devices  is  the  telephone  apparatus.  That  which  is  here  illustrated 
is  the  product  of  the  Western  Electric  Company,  to  whom  we  are 
indebted  for  all  the  illustrations  in  this  chapter. 

Dispatcher's  Transmitter.  The  dispatcher,  in  most  cases,  uses 
the  chest  transmitter  similar  to  that  employed  by  switchboard  oper- 
ators in  every-day  service.  He  is  connected  at  all  times  to  the  tele- 
phone circuit,  and  for  this  reason  equipment  easy  for  him  to  wear  is 
essential.  In  very  noisy  locations  he  is  equipped  with  a  double  head 


710 


TELEPHONY 


receiver.  On  account  of  the  dispatcher  being  connected  across 
the  line  permanently  and  of  his  being  required  to  talk  a  large  part  of  the 
time,  there  is  a  severe  drain  on  the  transmitter  battery.  For  this 
reason  storage  batteries  are  generally  used. 


Fig.  482.     Waystation  Desk  Telephone 

Waystation  Telephones.  At  the  waystations  various  types  of 
telephone  equipment  may  be  used.  Perhaps  the  most  common  is 
the  familiar  desk  stand  shown  in  Fig.  482,  which,  for  railroad  service, 
is  arranged  with  a  special 
hook-switch  lever  for  use 
with  a  head  receiver. 

Often  some  of  the  fa- 
miliar swinging-arm  tele- 
phone supports  are  used,  in 

connection  with  head  receivers,  but  certain  special 
types  developed  particularly  for  railway  use  are  ad- 
vantageous, because  in  many  cases  the  operator  who 
handles  train  orders  is  located  in  a  tower  where  he 
must  also  attend  to  the  interlocking  signals,  and  for 
such  service  it  is  necessary  for  him  to  be  able  to  get 
away  from  the  telephone  and  back  to  it  quickly.  The  Fi  483  Tele_ 
Western  Electric  telephone  arm  developed  for  this  use  Ph°ne  Arm 
is  shown  in  Fig.  483.  In  this  the  transmitter  and  the  receiver  are 
so  disposed  as  to  conform  approximately  to  the  shape  of  the  oper- 
ator's head.  When  the  arm  is  thrown  back  out  of  the  way  it  opens 
the  transmitter  circuit  bv  means  of  a  commutator  in  its  base. 


TELEPHONE  TRAIN  DISPATCHING 


711 


Siding  Telephones.  Two  types  of  sets  are  employed  for  siding 
purposes.  The  first  is  an  ordinary  magneto  wall  instrument,  which 
embodies  the  special  apparatus  and  circuit  features  employed  in  the 
standard  waystation  sets.  These  are  used  only  where  it  is  possible  to 


Fig.  484.     Weather- Proof  Telephone  Set 

locate  them  indoors  or  in  booths  along  the  line.  These  sets  are  per- 
manently connected  to  the  train  wire,  and  since  the  chances  are 
small  that  more  than  one  of  them  will  be  in  use  at  a  time,  they  are 
rung  by  the  dispatcher,  by  means 
of  a  regular  hand  generator,  when 
it  is  necessary  for  him  to  signal  a 
switching. 

In  certain  cases  it  is  not  feasi- 
ble to  locate  these  siding  telephone 
sets  indoors,  and  to  meet  these  con- 
ditions an  iron  weather-proof  set 
is  employed,  as  shown  in  Figs.  484 
arid  485.  The  apparatus  in  this  set 
is  treated  with  a  moisture-proofing 
compound,  and  the  casing  itself  is 
impervious  to  weather  conditions. 

Portable  Train  Sets.  Portable 
telephone  sets  are  being  carried 
regularly  on  wrecking  trains  and  their  use  is  coming  into  more  and 


Fig.  485.    Weather-Proof  Telephone 
Set 


712 


TELEPHONY 


more  general  acceptance  on  freight  and  passenger  trains.     Fig.  486 
shows  one  of  these  sets  equipped  with  a  five-bar  generator  for  calling 


Fig.  486.     Portable  Telephone  Set 

the  dispatcher.  Fig.  487  shows  a  small  set  without  generator  for 
conductors'  and  inspectors'  use  on  lines  where  the  dispatcher  is  at  all 
times  connected  in  the  circuit. 


Fig.  487.     Portable  Telephone  Set 


These  sets  are  connected  to  the  telephone  circuit  at  any  point 
on  the  line  by  means  of  a  light  portable  pole  arranged  with  terminals 
at  its  outer  extremity  for  hooking  over  the  line  wires,  and  with  flexible 


TELEPHONE  TRAIN  DISPATCHING  713 

conducting  cords  leading  to  the  portable  set.  The  use  of  these  sets 
among  officials  on  their  private  cars,  among  construction  and  bridge 
gangs  working  on  the  line,  and  among  telephone  inspectors  and  re- 
pairmen for  reporting  trouble,  is  becoming  more  and  more  general. 

Western  Electric  Circuits.  As  already  stated,  a  telephone  train- 
dispatching  circuit  may  be  from  25  to  300  miles  in  length,  and  upon 
this  may  be  as  many  stations  as  can  be  handled  by  one  dispatcher. 
The  largest  known  number  of  stations  upon  an  existing  circuit  of 
this  character  is  65. 

Dispatcher's  Circuit  Arrangement.  The  circuits  of  the  dis- 
patcher's station  in  the  Western  Electric  system  are  shown  in  Fig. 
488,  the  operation  of  which  is  briefly  as  follows :  When  the  dispatcher 
wishes  to  call  any  particular  station,  he  gives  the  key  corresponding 
to  that  station  a  quarter  turn.  This  sends  out  a  series  of  rapid 


TO   r£L£PHOME -SET 

Fig.  488.     Dispatcher's  Station — Western  Electric  System 

direct-current  impulses  on  the  telephone  line  through  the  contact 
of  a  special  telegraph  relay  which  is  operated  by  the  key  in  a  local 
circuit.  The  telegraph  relay  is  equipped  with  spark-eliminating 
condensers  around  its  contacts  and  is  of  heavy  construction  through- 
out in  order  to  carry  properly  the  sending  current. 

Voltage.  The  voltage  of  the  sending  battery  is  dependent  on 
the  length  of  the  line  arid  the  number  of  stations  upon  it.  It  ranges 
from  100  to  300  volts  in  most  cases.  When  higher  voltages  are 
required  in  order  successfully  to  operate  the  circuit,  it  is  generally 
customary  to  install  a  telegraph  repeater  circuit  at  the  center  of  the 
line,  in  order  to  keep  the  voltage  within  safe  limits.  One  reason  fcr 
limiting  the  voltage  employed  is  that  the  condensers  used  in  the 
circuit  will  not  stand  much  higher  potentials  without  danger  of  burn- 
ing out.  It  is  also  possible  to  halve  the  voltage  by  placing  the 
dispatcher  in  the  center  "of  the  line,  from  which  position  he  may 
signal  in  two  directions  instead  of  from  one  end. 


714  TELEPHONY 

Simultaneous  Talking  and  Signaling.  Retardation  coils  and 
condensers  will  be  noticed  in  series  with  the  circuit  through  which 
the  signaling  current  must  pass  before  going  out  on  the  line. 
These  are  for  the  purpose  of  absorbing  the  noise  which  is  caused 
by  high-voltage  battery,  thus  enabling  the  dispatcher  to  talk  and 
signal  simultaneously.  The  250-ohm  resistance  connected  across 
the  circuit  through  one  back  contact  of  the  telegraph  relay  absorbs 
the  discharge  of  the  6-rnicrofarad  condenser. 

Waystation  Circuit.  The  complete  selector  set  for  the  way- 
stations  is  shown  in  Fig.  489,  and  the  wiring  diagram  of  its  appara- 
tus in  Fig.  490.  The  first  impulse  sent  out  by  the  key  in  the  dispatch- 


rig.  489.     Selector  Set — Western  Electric 
System 

er's  office  is  a  long  direct-current  impulse,  the.  first  tooth  being  three 
or  four  times  as  wide  as  the  other  teeth.  This  impulse  operates 
both  magnets  of  the  selector  and  attracts  their  armatures,  which, 
in  turn,  cause  two  pawls  to  engage  with  the  ratchet  wheel,  while 
the  remaining  quick  impulses  operate  the  "stepping-up"  pawl  and 
rotate  the  wheel  the  requisite  number  of  teeth.  Retardation  coils 
are  placed  in  series  with  the  selector  in  order  to  choke  back  any 
lightning  discharges  which  might  come  in  over  the  line.  The  selec- 
tor contact,  when  operated,  closes  a  bell  circuit,  and  it  will  be  noted 
that  both  the  selector  and  the  bell  are  operated  from  battery  cur- 
rent coming  over  the  main  line  through  variable  resistances.  There 
are,  of  course,  a  number  of  selectors  bridged  across  the  circuit,  and 
the  variable  resistance  at  each  station  is  so  adjusted  as  to  give  each 


TELEPHONE  TRAIN  DISPATCHING 


715 


approximately  10  milliamperes,  which  allows  a  large  factor  of  safety 
for  line  leakage  in  wet  weather.  The  drop  across  the  coils  at  10 
milliamperes  is  38  volts.  If  these  coils  were  not  employed,  it  is  clear 


Fig.  490.     Selector  Set — Western  Electric  System 

that  the  selectors  nearer  the  dispatcher  would  get  most  of  the  current 
and  those  further  away  very  little. 

A  time-signal  contact  is  also  indicated  on  the  selector-circuit 
diagram  of  Fig.  490.  This  is  common  to  all  offices  and  may  be  oper- 
ated by  a  special  key  in  "the  dispatcher's  office,  thereby  enabling  him 
to  send  out  time  signals  over  the  telephone  circuit. 

Gill  Circuits.  The  circuit  arrangement  for  the  dispatcher's 
outfit  of  the  Gill  system  is  shown  in  Fig.  491.  This  is  similar  to 
that  of  the  Western  Electric  system  just  described.  The  method 
of  operation  also  is  similar,  the  mechanical  means  of  accomplishing 


Fig.  491.     Gill  Dispatcher's  Station 

the  selection  being  the  main  point  of  difference.  In  Fig.  492  the 
wiring  of  the  Gill  selector  at  a  waystation  for  local-battery  service 
is  shown.  The  selector  contact  closes  the  bell  circuit  in  the  station 
arid  a  few  windings  of  this  circuit  are  located  on  the  selector  magnets, 
as  shown.  These  provide  the  "answer-back"  by  inductive  means. 


716 


TELEPHONY 


Fig.  493  shows  the  wiring  of  the  waystation,  central-energy  Gill 
selector.  In  this  case,  the  local  battery  for  the  operation  of  the  bell 
is  omitted  and  the  bell  is  rung,  as  is  the  case  of  the  Western  Electric 
selector,  by  the  main  sending  battery  in  the  dispatcher's  office. 


SELECTOR  CONTACT 


Fig.  492.    GUI  Selector — Local  Battery 

The  sending  keys  of  these  two  types  of  circuits  differ,  in  that 
with  the  local-battery  selector  the  key  contact  is  open  after  the 
selector  has  operated,  and  the  ringing  of  the  bell  must  be  stopped  by 
the  dispatcher  pressing  a  button  or  calling  another  station.  Either 
of  these  operations  sends  out  a  new  current  impulse  which  releases 
the  selector  and  opens  its  circuit. 

With  the.  central-energy  selector,  however,  the  contacts  of  the 
sending  key  at  the  dispatcher's  office  remain  closed  after  operation  for 
a  definite  length  of  time.  This  is  obviously  necessary  in  order  that 
battery  may  be  kept  on  the  line  for  the  operation  of  the  bell.  In  this 
case  the  contacts  remain  closed  during  a  certain  portion  of  the  revo- 
lution of  the  key,  and  the  bell  stops  ringing  when  that  portion  of  the 


S£i£CT0/f  COHTACT 

Fig.  493.     Gill  Selector — Central  Energy 

revolution  is  completed.  If,  however,  the  dispatcher  desires  to  give 
any  station  a  longer  ring,  he  may  do  so  by  keeping  the  key  contacts 
closed  through  an  auxiliary  strap  key  as  soon  as  he  hears  the  "answer- 
back" signal  from  the  called  station. 

Cummings=Wray  Circuits.  The  Cummings-Wray  system,  as  pre- 
viously stated,  is  of  the  multiple-call  type,  operating  with  synchro- 
nous clocks.  Instead  of  operating  one  key  after  another  in  order 


TELEPHONE  TRAIN  DISPATCHING 


717 


to  call  a  number  of  stations,  all  the  keys  are  operated  at  once  and 
a  starting  key  sets  the  mechanism  in  motion  which  calls  all  these 
stations  with  one  operation.  Fig.  494  shows  the  circuit  arrangement 
of  this  system. 

In  order  to  ring  one  or  more  stations,  the  dispatcher  presses  the 
corresponding  key  or  keys  and  then  operates  the  starting  key.  This 
starting  key  maintains  its  contact  for  an  appreciable  length  of  time 
to  allow  the  clock  mechanism  to  get  under  way  and  get  clear  of  the 
releasing  magnet  clutch.  Closing  the  starting  key  operates  the 
clock-releasing  magnet  and  also  operates  the  two  telegraph-line 
relays.  These  send  out  an  impulse  of  battery  on  the  line  operating 


WAY:  STAT/OH 

SELECTOR 


Fig.  494.     Cummings-Wray  System 

the  bridged  2,500-ohm  line  relays  and,  in  turn,  the  selector  releasing 
magnets;  thus,  all  the  waystation  clocks  start  in  unison  with  the 
master  clock.  The  second  hand  arbor  of  each  clock  carries  an  arm, 
which  at  each  waystation  is  set  at  a  different  angle  with  the  normal 
position  than  that  at  any  other  station.  Each  of  these  arms  makes 
contact  precisely  at  the  moment  the  master-clock  arm  is  passing  over 
the  contact  corresponding  to  that  station. 

If,  now,  a  given  station  key  is  pressed  in  the  master  sender,  the 
telegraph-line  relays  will  again  operate  when  the  master-clock  arm 
reaches  that  point,  sending  out  another  impulse  of  battery  over  the 
line.  The  selector  contact  at  the  waystation  is  closed  at  this  moment; 


718 


TELEPHONY 


TCHER'S 


therefore,  the  closing  of  the  relay  contact  operates  the  ringing  relay 
through  a  local  circuit,  as  shown.  The  ringing  relay  is  immediately 
locked  through  its  own  contact,  thus  maintaining  the  bell  circuit 
closed  until  it  is  opened  by  the  key  and  the  ringing  is  stopped. 

As  the  master-clock  arm  passes  the  last  point  on  the  contact 
dial,  the  current  flows  through  the  restoring  relay  operating  the  re- 
storing magnet  which  releases  all  the  keys.  A  push  button  is  provided 
by  means  of  which  the  keys  may  be  manually  released,  if  desired. 
This  is  used  in  case  the  dispatcher  presses  a  key  by  mistake.  Re- 
tardation coils  and  variable  resistances  are  provided  at  the  waysta- 
tion  just  as  with  the  other  selector  systems  which  have  been  described 

and  for  the  same  reasons. 
The  circuits  of  the  oper- 
ator's telephone  equipment 
shown  in  Fig.  495,  are  also 
bridged  across  the  line.  This 
apparatus  is  of  high  im- 
pedance and  of  a  special 
design  adapted  to  railroad 
C\  sendee.  There  may  be  any 
number  of  telephones  listen- 
ing in  upon  a  railroad  train 
wire  at  the  same  time,  and 
often  a  dispatcher  calls  in 
five  or  six  at  once  to  give 
orders.  These  conditions  have  necessitated  the  special  circuit  ar- 
rangement shown  in  Fig.  495. 

The  receivers  used  at  the  waystations  are  of  high  impedance 
and  are  normally  connected,  through  the  hook  switch,  directly  across 
the  line  in  series  with  a  condenser.  When  the  operator,  at  a  way- 
station  wishes  to  talk,  however,  he  presses  the  key  shown.  This 
puts  the  receiver  across  the  line  in  series  with  the  retardation  coil 
and  in  parallel  with  the  secondary  of  the  induction  coil.  It  closes 
the  transmitter  battery  circuit  at  the  same  time  through  the  primary 
of  t'he  induction  coil. 

The  retardation  coil  is  for  the  purpose  of  preventing  excessive 
side  tone,  and  it  also  increases  the  impedance  of  the  receiver  cir- 
cuit, which  is  a  shunt  on  the  induction  coil.  This  latter  coil,  how- 


Fig.  495.     Telephone  Circuits 


TELEPHONE  TRAIN  DISPATCHING 


719 


ever,  is  of  a  special  design  which  permits  just  enough  current  to  flow 
through  the  receiver  to  allow  the  dispatcher  to  interrupt  a  way- 
station  operator  when  he  is  talking. 

The  key  used  to  close  the  transmitter  battery  is  operated  by 
hand  and  is  of  a  non-locking  type.  In  some  cases,  where  the  operators 
are  very  busy,  a  foot  switch  is  used  in  place  of  this  key.  The  use  of 
such  a  key  or  switch  in  practical  operation  has  been  found  perfectly 
satisfactory,  and  it  takes  the  operators  but  a  short  time  to  become 
used  to  it. 

The  circuits  of  the  dispatcher's  office  are  similarly  arranged, 
Fig.  495,  being  designed  especially  to  facilitate  their  operation.  In 
other  words,  as  the  dispatcher  is  doing  most  of  the  work  on  the  cir- 
cuit, his  receiver  is  of  a  low-impedance  type,  which 
gives  him  slightly  better  transmission  than  the  way- 
stations  obtain.  The  key  in  his  transmitter  circuit 
is  of  the  locking  type,  so  that  he  does  not  have  to 
hold  it  in  while  talking.  This  is  for  the  reason  that 
the  dispatcher  does  most  of  the  talking  on  this  cir- 
cuit. Foot  switches  are  also  employed  in  some  cases 
by  the  dispatchers. 

Test  Boards.  It  is  becoming  quite  a  general 
practice  among  the  railroads  to  install  more  than 
one  telephone  circuit  along  their  rights-of-way.  In 
many  cases  in  addition  to  the  train  wire,  a  message 
circuit  is  also  equipped,  and  quite  frequently  a  block 
wire  also  operated  by  telephone,  parallels  these  two. 
It  is  desirable  on  these  circuits  to  be  able  to  make 
simple  tests  and  also  to  be  able  to  patch  one  circuit 
with  another  in  cases  of  emergency. 

Test  boards  have  been  designed  for  facilitating  this  work.  These 
consist  of  simple  plug  and  jack  boxes,  the  general  appearance  of 
which  is  shown  in  Fig.  496.  The  circuit  arrangement  of  one  of  these 
is  shown  in  Fig.  497.  Each  wire  comes  into  an  individual  jack  as 
will  be  noted  on  one  side  of  the  board,  arid  passes  through  the  inside 
contact  of  this  jack,  out  through  a  similar  jack  on  the  opposite  side. 
The  selector  and  telephone  set  at  an  office  are  taken  off  these  inside 
contacts  through  a  key,  as  shown.  The  outside,  contacts  of  this  key 
are  wired  across  two  pairs  of  cords.  Now,  assume  the  train  wire 


Fig.  496.     Test 
Board 


720 


TELEPHONY 


comes  in  on  jacks  1  and  3,  and  the  message  wire  on  jacks  9  and  11. 
In  case  of  an  accident  to  the  train  wire  between  two  stations,  it  is 
desirable  to  patch  this  connection  with  a  message  wire  in  order  to 
keep  the  all-important  train  wire  working.  The  dispatcher  instructs 
the  operator  at  the  last  station  which  he  can  obtain,  to  insert  plugs 
1  and  2  in  jacks  1  and  10,  and  plugs  3  and  4  m  jacks  3  and  12,  at 
the  same  time  throwing  the  left-hand  key.  Then,  obtaining  an 
operator  beyond  the  break  by  any  available  means,  he  instructs 


TO  SELECTOff 
A  HO  TEL. 


Fig.  497.     Circuits  of  Test  Board 

him  likewise  to  insert  plugs  1  and  2  in  jacks  9  and  2,  and 
plugs  3  and  4  m  jacks  11  and  4,  similarly  throwing  the  left-hand 
key.  By  tracing  this  out,  it  will  be  observed  that  the  train  wire 
is  patched  over  the  disabled  section  by  means  of  the  message  cir- 
cuit, and  that  the  selector  and  the  telephone  equipment  are  cut  over 
on  to  the  patched  connections;  in  other  words,  bridged  across  the 
patching  cords. 

It  will  also  be  seen  that  with  this  board  it  is  possible  to  open  any 
circuit  merely  by  plugging  into  a  jack.  Two  wires  can  be  short- 
circuited  or  a  loop  made  by  plugging  two  cords  of  corresponding 


TELEPHONE  TRAIN  DISPATCHING  721 

colors  into  the  two  jacks.  A  ground  jack  is  provided  for  grounding 
any  wire.  In  this  way,  a  very  flexible  arrangement  of  circuits  is 
obtained,  and  it  is  possible  to  make  any  of  the  simple  tests  which 
are  all  that  are  usually  required  on  this  type  of  circuit. 

Blocking  Sets.  As  was  just  mentioned,  quite  frequently  in 
addition  to  train  wires  and  message  circuits,  block  wires  are  also 
operated  by  telephone.  In  some  cases  separate  telephone  instru- 
ments are  used  for  the  blocking  service,  but  in  others  the  same  man 
handles  all  three  circuits  over  the  same  telephone.  The  block  wire 
is  generally  a  converted  telegraph  wire  between  stations,  usually 
of  iron  and  usually  grounded.  It  seldom  ranges  in  length  over  six 
miles. 

Where  the  block  wires  are  operated  as  individual  units  with 
their  own  instruments,  it  is  unnecessary  to  have  any  auxiliary  ap- 


Fig.   498.     Blocking  Set 

paratus  to  be  used  in  connection  with  them.  Where,  however,  they 
are  operated  as  part  of  a  system  and  the  same  telephone  is  used  on 
these  that  is  used  on  the  train  wire  and  message  wire,  additional 
apparatus,  called  a  blocking  set,  is  required.  This  blocking  set, 
shown  in  Figs.  498  and-  499,  was  developed  especially  for  this  service 
by  the  Western  Electric  Company.  As  will  be  noted,  a  repeating 
coil  at  the  top  and  a  key  on  the  front  of  the  set  are  wired  in  con- 
nection with  a  pair  of  train  wire  cords.  This  repeating  coil  is  for 
use  in  connecting  a  grounded  circuit  to  a  metallic  circuit,  as,  for 
instance,  connecting  a  block  wire  to  the  train  wire,  and  is,  of 


course,  for  the  purpose  of  eliminating  noise.  Below  the  key  are 
three  combined  jacks  and  signals.  One  block  wire  comes  into  each 
of  these  and  a  private  line  may  be  brought  into  the  middle  one. 
When  the  next  block  rings  up,  a  visual  signal  is  displayed  which 
operates  a  bell  in  the  office  by  means  of  a  local  circuit.  The  oper- 
ator answers  by  plugging  the  telephone  cord  extending  from  the 
bottom  of  the  set  into  the  proper  jack.  This  automatically  restores 
the  signal  and  stops  the  bell. 

Below  these  signals  appear  four  jacks.  One  is  wired  across 
the  train  wire;  one  across  the  message 
wire;  and  the  other  two  are  bridged 
across  the  two  pairs  of  patching  cords 
on  each  side  of  the  set.  The  operator 
answers  a  call  on  any  circuit  by  plug- 
ging his  telephone  cord  into  the  proper 
jack. 

If  a  waystation  is  not  kept  open  in 
the  evening,  or  the  operator  leaves  it 
for  any  reason  and  locks  up,  he  can 
connect  two  blocks  together  by  mean^ 
of  the  block-wire  cords.  These  are 
arranged  simply  for  connecting  two 
grounded  circuits  together  and  serve  to 
join  two  adjacent  blocks,  thereby  elim- 
inating one  station.  A  jack  is  wired 
across  these  cords,  so  that  the  way- 
station  operator  can  listen  in  on  the  connection  if  he  so  desires. 

In  some  cases  not  only  are  the  telephone  circuits  brought  into  the 
test  board,  but  also  two  telegraph  wires  are  looped  through  this 
board  before  going  to  the  peg  switchboard.  This  is  becoming  quite 
a  frequent  practice  and,  in  times  of  great  emergency,  enables 
patches  to 'be  made  to  the  telegraph  wires  as  well  as  to  the  telephone 
wires. 

Dispatching  on  Electric  Railways.  As  interurban  electric  rail- 
ways are  becoming  more  extended,  and  as  their  traffic  is  becoming 
heavier,  they  approximate  more  closely  to  steam  methods  of  operation. 
It  is  not  unusual  for  an  electric  railway  to  dispatch  its  cars  exactly 
as  in  the  case  of  a  steam  road.  There  is  a  tendency,  however,  in 


Fig.  499.     Blocking  Set 


TELEPHONE  TRAIN  DISPATCHING  723 

this  class  of  work,  toward  slightly  different  methods,  and  these  will 
be  briefly  outlined. 

On  those  electric  railways  where  the  traffic  is  not  especially  heavy, 
an  ordinary  magneto  telephone  line  is  frequently  employed  with 
standard  magneto  instruments.  In  some  cases  the  telephone  sets  are 
placed  in  waiting  rooms  or  booths  along  the  line  of  the  road.  In 
other  cases  it  is  not  feasible  to  locate  the  telephone  indoors  and  then 
iron  weatherproof  sets,  such  as  are  shown  in  Figs.  484  and  485, 
are  mounted  directly  on  the  poles  along  the  line  of  railway.  With 
a  line  of  this  character  there  is  usually  some  central  point  from 
which  orders  are  issued  and  the  trainmen  call  this  number  when 
arriving  at  sidings  or  wherever  they  may  need  to  do  so. 

Another  method  of  installing  a  telephone  system  upon  electric 
ys  is  as  follows:  Instead  of  instruments  being  mounted  in 
booths  or  on  poles  along  the  line,  portable  telephone  sets  are  carried 
on  the  cars  and  jacks  are  located  at  regular  intervals  along  the  right- 
of-way  on  the  poles.  The  crew  of  the  car  wishing  to  get  in  touch 
with  the  central  office  or  the  dispatcher,  plugs  into  one  of  these 
jacks  and  uses  the  portable  telephone  set.  At  indoor  stations,  in 
offices  or  buildings  belonging  to  the  railroad,  the  regular  magneto 
sets  may  be  employed,  as  in  the  first  case  outlined. 

On  electric  railway  systems  where  the  traffic  is  heavy,  the  train 
or  car  movements  may  be  handled  by  a  dispatcher  just  as  on  the 
steam  railroad.  There  is  usually  one  difference,  however.  On  a 
steam  road,  the  operators  who  give  the  train  crews  their  orders  and 
manipulate  the  semaphore  signals  are  located  at  regular  intervals 
in  the  different  waystations.  No  such  operators  are  usually  found 
on  electric  railways,  except,  perhaps,  at  very  important  points,  and, 
therefore,  it  is  necessary  for  the  dispatcher  to  be  able  to  signal  cars 
at  any  point  and  to  get  into  communication  with  the  crews  of  these 
cars.  He  does  this  by  means  of  semaphores  operated  by  telephone 
selectors  over  the  telephone  line.  The  telephone  circuit  may  be 
equipped  with  any  number  of  selectors  desired,  and  the  dispatcher 
can  operate  any  particular  one  without  operating  any  other  one  on 
the  circuit.  Each  selector,  when  operated,  closes  a  pair  of  contacts. 
This  completes  a  local  circuit  which  throws  the  semaphore  arm  to  the 
"danger"  position,  at  the  same  time  giving  the  dispatcher  a  distinc- 
tive buzz  in  his  ear,  which  informs  him  that  the  arm  has  actual1  y 


724  TELEPHONY 

moved  to  this  position.  He  can  get  this  signal  only  by  the  operation 
of  the  arm. 

Each  semaphore  is  located  adjacent  to  a  telephone  booth  in  which 
is  also  placed  the  restoring  lever,  by  means  of  which  the  semaphore 
is  set  in  the  "clear"  position  by  the  crew  of  the  car  which  has  been 
signaled.  The  wall-type  telephone  set  is  usually  employed  for  this 
class  of  service,  but  if  desired,  desk  stands  or  any  of  the  various 
transmitter  arms  may  be  used. 

It  is  necessary  for  the  crew  of  the  car  which  first  approaches  a 
semaphore  set  at  "danger,"  to  get  out,  communicate  with  the  dis- 
patcher, and  restore  the  signal  to  the  "clear"  position.  The  dis- 
patcher can  not  restore  the  signal.  The  signal  is  set  only  in  order 
that  the  train  crew  may  get  into  telephonic  communication  with  the 
dispatcher,  and  in  order  to  do  this,  it  is  necessary  for  them  to  go 
into  the  booth  in  any  case* 


CHAPTER   XLI 
TYPES  OF  TELEPHONE  LINES 

Telephone  lines  may  be  underground  or  overhead.  If  the  latter, 
they  are  called  aerial  lines.  Wherever  rJlaced,  the  lines  must  be 
insulated  from  each  other  and  from  other  things,  such  as  the  earth. 
If  aerial,  the  lines  either  are  of  bare  wire  supported  by  solid  in- 
sulators, or  they  are  of  wire  insulated  throughout  its  length  by  some 
covering.  That  covering  in  the  present  practice  is  principally  of 
rubber,  gutta-percha,  or  dry  paper. 

The  mechanical  conditions  of  practice  require  bare  aerial  wires 
to  be  borne  by  poles  or  other  supports  100  to  200  feet  apart;  to  be 
spaced  8  to  12  inches  apart;  to  be  stretched  tightly  enough  not  to  be 
swung  together  by  winds,  and  to  be  strong  enough  to  bear  their  own 
weight  plus  wind  pressure  and  a  load  of  ice.  These  dimensions  cause 
the  total  space  occupied  by  a  line  of  several  hundred  wires  to  be  great. 

The  electrical  conditions  of  practice  require  of  exchange  (city) 
lines  that  the  loop  resistance  be  not  over  about  500  ohms  and  the 
mutual  capacity  not  over  about  .3  microfarad.  These  conditions  are 
not  so  severe  as  to  require  large  wires  far  apart.  Open  wires  on 
poles  must  be  larger  and  farther  apart  for  mechanical  reasons  than 
they  need  to  be  for  electrical  reasons.  Small  wires  close  together 
are  good  enough  for  exchange  lines  if  they  do  not  have  to  be  strung 
on  poles  as  bare  wires. 

The  solution  for  these  requirements  is  the  telephone  cable. 
Many  wires  can  be  put  in  a  small  space  if  cabled.  Cables  may  be 
supported  by  poles,  buildings,  or  fences,  or  may  be  laid  in  the  earth 
or  in  water.  There  is  no  difference  between  the  electrical  operations 
of  cables  in  the  earth,  in  the  water,  and  in  the  air. 

Cabled  wires  have  the  advantage  that  they  are  less  likely  to  in- 
dividual insulation  and  continuity  troubles  than  are  open  wires. 
Insulation  troubles,  when  they  do  happen  to  cable  wires,  usually 
affect  more  circuits  than  in  open-wire  construction ;  cabled  wires  have 


726  TELEPHONY 

higher  mutual  capacity  than  open  wires;  these  are  disadvantages. 
Cost  facts  also  concern  the  question  "cabled  wires  versus  open  wires 
for  exchange  lines";  the  solution  of  all  the  facts  is  favorable  to  cables 
if  the  number  of  exchange  lines  along  a  route  is  more  than  a  dozen 
or  two. 

Feasibility,  first  cost,  owning-and-using  costs,  self-interest, 
and  public  policy  are  the  considerations  which  control  the  decision 
whether  cable  shall  be  overhead  or  underground.  Cities  steadih 
increase  the  areas  in  which  local  laws  prohibit  pole  lines  on  the 
street.  In  cities  having  such  laws  at  all,  and  having  also  adequate 
development  of  telephone  service,  the  area  in  which  it  is  economical 
to  place  wires  underground  usually  is  larger  than  the  area  in  which 
it  is  legally  obligatory  to  place  them  so.  This  is  fortunate  for  all 
concerned. 

In  general  terms,  the  solution  of  the  facts  of  costs  and  policy  us- 
ually shows  that  wires  shall  be  placed  underground  when  there  are 
more  than  500  lines  in  the  route.  Underground  cables  for  electrical 
service  generally  may  be  placed  directly  in  the  earth  as  gas  pipes  are; 
but  gas  pipes  are  self-protecting  against  most  attacks,  such  as  by 
picks  and  shovels.  Cables  are  less  so.  Access  to  cables  for  changes 
in  them  is  necessary;  they  require  to  be  changed  in  position  and  con- 
nection and  to  be  replaced  by  others.  Standard  underground 
telephone-cable  practice  is  to  provide  an  underground  system  of 
ducts  adapted  to  protect  the  cables  against  mechanical  damage  and 
to  allow  them  to  be  placed  and  replaced  without  further  opening  of 
the  earth  than  was  first  required  to  lay  the  system  of  ducts.  Under- 
ground cables  so  placed  cost  less  to  maintain  than  do  aerial  cables. 
The  value  of  a  cable  after  withdrawal  from  a  duct  is  greater  than 
that  of  a  similar  aerial  cable  after  being  taken  down. 

The  open  aerial  wires  of  exchange  lines  have  to  be  larger  and 
further  apart  for  mechanical  reasons  than  they  do  for  electrical  rea- 
sons. This  is  not  true  of  the  open  aerial  wires  of  long-distance  lines. 
Electrical  reasons,  in  the  design  of  long-distance  circuits,  make 
low  resistance  and  low  mutual  capacity  important.  Unless  the 
inductance  be  increased  by  loading  the  line,  cable  circuits  are  limited 
in  their  speaking  distance  from  one-tenth  to  one-fifth  the  speaking 
distance  of  the  same  wires  on  poles  in  open  air. 

In  other  wordc    the  smallest  practical  open  wires  for  a  given 


TYPES  OF  TELEPHONE  LINES  727 

length  of  exchange  line  are  larger  than  their  electrical  requirements 
demand;  the  smallest  practical  open  wires  for  a  given  length  of 
long-distance  line  are  larger  than  their  mechanical  requirements 
demand.  Also,  exchange  wires  would  fulfill  electrical  requirements 
sufficiently  well  if  much  closer  together;  while  long-distance  wires 
would  fulfill  electrical  requirements  much  better  if  much  further 
apart.  Mechanical  reasons  only  control  the  spacing  of  both  kinds 
of  wire. 

When  cables  are  laid  under  water,  it  usually  is  because  there  is 
no  other  practical  way  of  placing  them.  The  solution  as  to  the  best 
way  generally  has  no  alternatives.  A  given  submarine  cable  may  not 
be  the  best  way  of  meeting  the  requirements;  it  may  be  the  only  way. 


CHAPTER   XLII 
OPEN  WIRES 

Wire  for  open  use  on  insulators  is  made  of  copper,  bronze,  steel, 
or  iron,  or  of  steel  covered  with  copper.  Copper  for  all  physical  rea 
sons  is  best;  iron  and  steel  are  cheaper;  bronze  is  little  used.  Copper- 
clad  steel,  combining  strength,  conductivity,  and  freedom  from 
corrosion  with  possible  low  cost,  is  likely  to  become  more  and  more 
useful. 

Iron.  Iron  and  steel  are  poorer  than  copper  in  specific  conduc- 
tivity. Calling  the  specific  conductivity  of  copper  100,  that  of  iron 
is  from  12  to  18  and  of  steel  from  8  to  12,  depending  upon  the  state 
and  purity  of  the  metal.  Iron  and  steel  are  stronger  than  copper. 
That  they  rust  is  a  great  fault.  Galvanizing  is  a  protection,  but  its 
value  varies  widely  in  different  situations;  in  certain  regions  having 
dry  air  and  no  smoke,  galvanized  open  iron  wires  last  twenty-five 
years.  In  most  cities  of  the  temperate  zone,  such  wires  last  six 
to  eight  years.  In  cities  burning  much  soft  coal  and  near  smelters, 
such  wires  often  last  only  three  years.  The  larger  the  wire,  the  longer 
it  will  last,  as  there  is  more  of  it  to  rust  away. 

Galvanizing.  Galvanizing  is  a  name  for  the  coating  of  iron  or 
steel  with  zinc.  The  coating  is  applied,  in  the  "hot"  process,  by 
dipping  the  iron  or  steel  in  molten  zinc  in  the  presence  of  a  flux. 
This  is  a  dipping  process,  not  an  electroplating  process,  though  the 
name  implies  the  latter.  In  the  "cold"  process,  the  iron  or  steel  is 
zinc-plated  by  electrical  means.  The  hot,  or  dipping,  process  is  the 
present  standard  for  wire. 

The  test  for  acceptable  galvanizing  is: 

Immerse  the  sample  in  a  saturated  solution  of  copper  sulphate  for  one 
minute,  then  wipe  it  clean.  Do  this  four  times  in  all.  If  the  sample  appears 
black  after  the  fourth  wiping,  the  galvanizing  is  acceptable;  if  it  has  a  copper 
color,  wholly  or  in  spots,  the  iron  is  exposed.  Reject  such  galvanizing,  for 
the  coating  is  too  thin. 


OPEN  WIRES  729 

Strength.  The  strength  of  iron  wire  is  about  3.1  times  its  weight 
per  mile;  of  steel  wire  about  3.7  times  its  weight  per  mile. 

Mile-Ohm.  The  term  "mile-ohm"  sometimes  is  used  to  indicate 
the  resistance  of  wire.  It  is  the  weight  of  a  wire  1  mile  long  and  hav- 
ing a  resistance  of  1  ohm.  The  lower  the  conductivity  of  a  metal  the 
higher  is  its  weight  per  mile-ohm.  For  example,  for  soft  copper  the 
mile-ohm  is  about  860  pounds;  for  hard-drawn  copper,  880  pounds; 
for  the  best  iron,  4,500  pounds ;  for  good  iron,  5,400  pounds,  and  for 
steel  as  high  as  7,000  pounds. 

Copper.  Copper  is  drawn  into  wire  by  pulling  it  successively 
through  smaller  and  smaller  holes.  It  hardens  in  being  so  drawn, 
so  it  is  softened  (annealed)  by  heating  and  cooling.  If  it  is  not  an- 
nealed after  the  last  drawings,  it  has  much  greater  strength.  This 
is  hard-drawn  copper.  Its  uses  are  in  open  wire  lines,  either  bare  or 
insulated.  The  uses  of  soft  copper  are  in  other  circuits.  Even 
though  from  the  earliest  times  it  was  known  of  drawn  copper  wire 
that  after  one  or  two  drawings  the  wire  became  so  hard  as  not  to  be 
successfully  drawn  without  softening,  no  wire  was  furnished  to  the 
market  in  such  a  hardened  state.  The  earliest  uses  of  copper  wire 
for  line  purposes  were  failures  and  it  was  standard  practice  for  many 
years  to  use  copper  only  in  electrical  circuits  where  it  did  not  have 
to  bear  its  own  weight.  Credit  for  the  development  of  the  right 
method  is  due  to  Thomas  B.  Doolittle,  who  developed  the  present 
successful  method  of  producing  hard-drawn  copper  wire  for  use  in 
open  lines. 

Copper  vs.  Iron.  When  high  conductivity  and  long  life  are 
required,  use  copper.  Hard-drawn  copper  is  stronger  than  soft 
(annealed)  copper.  Where  greater  strength  than  that  of  hard- 
drawn  copper  is  required  and  high  conductivity  is  not  of  importance, 
use  iron  or  steel.  This  case  meets  certain  needs  of  long  spans.  Where 
low  cost  is  important,  corrosive  causes  are  not  great,  and  high  con- 
ductivity is  not  essential,  use  iron  or  steel.  This  case  describes  the 
needs  of  many  country  lines  (toll  lines,  rural  lines)  under  15  miles 
long. 

Copper=Clad  Steel.  The  advantages  of  copper  wire,  in  its  su- 
perior conductivity  and  its  freedom  from  corrosion  when  exposed  to 
the  elements,  and  the  advantage  of  steel  wire  in  its  superior  strength 
long  have  added  zest  to  the  search  for  a  wire  combining  the  advantages 


730  TELEPHONY 

of  both.  Many  efforts  have  been  made  to  provide  a  strong  steel 
wire  with  a  good  copper  coating,  but  until  recently  these  efforts  have 
been  unsuccessful.  In  some  of  the  earlier  attempts,  the  copper 
was  applied  to  the  wire  by  electroplating.  It  was  not  found  that  the 
coating  would  cling  to  the  steel  tightly  enough  to  preserve  it  perfectly, 
and  in  time  rust  crept  between  the  metals  and  the  copper  would  fall 
away.  Other  attempts  have  been  in  the  direction  of  fitting  a  billet 
of  steel  tightly  into  a  copper  tube,  then  drawing  the  whole  into  wire. 
In  this  attempt  also  the  lack  of  a  perfect  union  between  the  two 
metals  defeated  the  attempt. 

Monnot  Process.  The  Monnot  process,  named  after  the  inventor, 
is  that  employed  by  the  Duplex  Metals  Company  of  New  York,  and 
consists  of  uniting  the  steel  core  and  copper  shell  while  they  are  hot. 
Under  proper  conditions  actual  welding  and  alloying  take  place 
between  the  two  metals.  Such  a  billet  may  be  drawn  into  wire  with- 
out breaking  the  contact  between  the  two  metals.  The  steel  remains 
centered  through  many  drawings  and  the  experience  which  is  avail- 
able at  the  time  of  this  writing  indicates  that  copper-clad  steel  wire 
may  be  considered  a  practical  element  of  electrical  construction. 

Characteristics.  Compared  with  hard-drawn  copper  wire,  cop- 
per-clad steel  wire  has,  in  general  terms,  higher  tensile  strength 
and  lower  conductivity.  It  is  obvious  that  in  the  manufacture  of 
copper-clad  wire,  its  resulting  tensile  strength  will  depend  both  upon 
the  grade  of  steel  chosen  for  the  core  and  upon  the  relative  amounts 
of  copper  in  a  wire  of  given  diameter.  The  more  steel  in  the  core, 
the  less  copper  there  will  be  in  the  shell,  and,  therefore,  the  greater 
the  strength,  and  the  lower  the  conductivity. 

The  inductance  of  copper-clad  steel  wire  is  less  than  that  of 
iron,  and  yet  it  is  more  than  that  of  copper,  for  the  same  diameter  of 
wire,  for  the  same  distance  between  the  two  sides  of  the  circuit,  and 
for  the  same  permeability  of  iron  and  steel. 

Uses.  Copper-clad  steel  wire  for  electrical  uses  may  be  had 
of  conductivities  30  and  40  per  cent  of  that  of  solid  copper  wire,  but 
with  these  reduced  conductivities  go  an  increase  of  strength.  As  we 
have  shown,  there  are  many  uses  of  wire  for  lines  exposed  to  the 
elements  in  which  a  reasonable  conductivity  only  is  required,  coupled 
with  a  minimum  tensile  strength.  For  these  purposes  a  copper-clad 
steel  wire  is  eminently  suitable,  if  it  is  made  in  such  a  way  as  to  be  as 


OPEN  WIRES  731 

non-corrosive  as  copper  and  if  it  can  be  bought  at  a  lower  price. 
Copper-clad  steel  wire  has  been  used  with  considerable  success  for 
line  wire  in  train-dispatching  work.  These  lines  are  as  a  rule  not  of 
extreme  length,  and  the  freedom  from  corrosion  of  copper-clad  steel 
wire,  increased  conductivity  over  iron,  and  greater  strength  than 
copper  have,  together  with  its  moderate  cost,  made  it  attractive  for 
railroads. 

Insulated  wires  twisted  in  pairs  for  connecting  subscribers' 
premises  to  nearby  cable  terminals  would  be  perfectly  satisfactory, 
so  far  as  conductivity  is  concerned,  if  they  were  as  small  as  No.  22 
B.  &  S.  gauge,  as  the  remainder  of  the  line  in  the  cable  is  of  that 
size.  But  wires  as  small  as  No.  22  B.  &  S.  gauge  can  not  sup- 
port their  own  weight  successfully,  so  that  the  added  weight  of  the 
rubber  insulation  and  braid  would  break  them  down  promptly.  If 
these  were  of  high-grade  steel,  they  might  support  themselves  suc- 
cessfully, if  of  No.  22  B.  &  S.  gauge,  but  wherever  exposed  at 
terminals  or  elsewhere,  the  steel  wire  would  rust.  It  is  in  such 
cases  as  these  and  in  moderate  length  of  bare-wire  lines  that  copper- 
clad  wire  finds  use  if  costs  permit. 

Insulated  Open  Wire.  Even  when  carried  on  glass  or  porce- 
lain insulators  on  poles,  open  telephone  wires  sometimes  require  to 
be  insulated  by  an  actual  wire  covering.  The  circumstances  making 
this  necessary  usually  are  where  foliage  may  touch  the  wires.  The 
best  practice  is  to  insulate  such  conductors  with  rubber  compound 
of  high  insulating  quality  and  to  cover  this  in  turn  with  a  heavy  cotton 
braid  saturated  with  some  weatherproof  compound. 

Drops.  In  some  plans  of  construction,  wires  which  leave 
a  pole  line  to  reach  a  subscriber's  premises  are  both  open  wires.  These 
wires  are  called  "drops"  or  "drop  wires."  Where  two  single  wires 
make  up  such  a  drop  and  the  span  is  long,  it  is  good  practice  to  have 
at  least  one  of  them  insulated,  so  that  if  the  two  wires  swing  together, 
as  is  more  likely  than  in  a  straight  span  of  a  pole  line,  they  will  not 
short-circuit  the  line  and  throw  it  out  of  service. 

The  better  practice  for  drop  wires,  and  that  which  is  becoming 
customary  with  the  wide  use  of  cables  and  the  limited  use  of  open 
wires  in  city  systems,  is  to  use  two  insulated  wires  twisted  into  a  pair 
for  drop  service.  Such  twin  or  paired  wire  is  known  as  drop  wire. 
The  conductor  usually  is  of  No.  14  or  No.  16  B.  &  S.  gauge, 


732 


TELEPHONY 


if  of  hard-drawn  copper,  or  of  No.  17  gauge,  if  of  copper-clad  steel 
wire.  No.  14  wire  is  used  where  the  climate  makes  ice  a  possible 
burden.  No.  16  is  suitable  in  climates  where  ice  does  not  form.  If 
No.  18  copper-clad  steel  wire  of  suitable  quality  can  be  had  with  a 
tensile  strength  as  great  as  that  of  No.  16  hard-drawn  copper,  it  forms 
an  acceptable  substitute  in  regions  where  ice  does  not  form. 

Wall  and  Fence  Wire.  It  is  becoming  a  more  general  practice 
to  terminate  underground  cables  on  the  back  walls  of  buildings  and 
to  carry  twisted  pairs  of  wires  through  rings  along  horizontal  and 
vertical  lines  on  those  back  walls.  As  compared  with  a  distributing 
pole,  the  method  is  more  sightly  and  as  simple  to  maintain.  The 
distance  between  the  rings  being  short,  no  great  tensile  strength  is 
required  of  the  wire.  It  is,  therefore,  good  practice  to  use  No.  18 
B.  &  S.  copper  wire,  with  a  thinner  insulation  than  is  required  of 
drop  wires. 

Braiding.  All  classes  of  insulated  line  wires  for  outdoor  use 
require  a  braiding  saturated  with  a  weatherproof  compound  as  a 


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Fig.  500.     Sizes  and  Weights  of  Line  Wires 

protection  to  the  rubber  covering,  for  rubber  deteriorates  by  exposure 
to  the  air  and  to  the  sunlight,  the  action  of  sunlight  being  a  par- 
ticularly powerful  deteriorating  cause. 

Designation  of  Sizes.  There  are  three  ways  in  which  wires  may 
be  designated  as  to  their  size:  by  their  diameter,  by  arbitrary  gauge 
numbers  indicating  this  diameter,  and  by  their  weight  per  unit  length. 


OPEN  WIRES 


733 


TABLE  XVIII 

Sizes  of  Copper  Wire  Suitable  for  Bare  Line  Construction,  in  Various 

Standard  Gauges 

(Arranged  in  Order  of  Size) 


1 
NUMBER 

DIAMETER 

IN 

MILS 

WEIGHT 
PER  MILE 

IN 

POUNDS 

RESISTANCE 
PER  MILE  OF 
WIRE  IN 
OHMS.  60°  F. 

RESISTANCE 
PER  MILE  OF 
CIRCUIT  IN 
OHMS,  60°  F. 

8  B.  W.  G  

165. 

435 

1  .  9742 

3  .  9484 

6  B   &  S  G 

162 

419 

2.0481 

4.0962 

8  N   B.  S  G  

160 

409 

2.0998 

4  .  1996 

9  B.  W   G  

148. 

350 

2.4541 

4  .  9082 

7  B.  &  S.  G  

144.3 

331 

2  .  5925 

5.1850 

9  N.  B.  S.  G  

144. 

331 

2.5925 

5.1850 

10  B  W.  G  

134 

287 

2.9838 

5.9676 

8  B.  &  S.  G  

128  5 

262 

3.2810 

6  .  5620 

10  N.  B.  S.  G  

128. 

262 

3.2810 

6  .  5620 

11  B.  W.  G  

120. 

230 

3  .  7330 

7  .  4660 

11  N.  B   S.  G  

116 

215 

3.9948 

7  .  9896 

9  B.  &  S.  G  

114  4 

208 

4  1363 

8.2726 

12  B.  W.  G  

109. 

190 

4  .  5244 

9.0488 

12  N.  B.  S.  G  

104. 

173 

4.9701 

9.9402 

10  B.  &  S.  G  

101.9 

166 

5  .  1665 

10.3330 

13  B.  W.  G  

95. 

144 

5  .  9558 

11.9116 

13  N.  B.  S.  G  

92. 

135 

6.3518 

12.7036 

11  B.  &  S  G  

90.74 

132 

6.4891 

12  .9782 

14  B.  W.  G  

83. 

110 

7.8068 

15.6076 

12  B.  &  S.  G  

80.81 

105 

8  .  1946 

16.3892 

14  N.  B.  S.  G  

80. 

102 

8.4005 

16.8010 

All  three  of  these  ways  are  in  common  practice.  Wires  for  use  in 
open  lines  are  frequently  designated  merely  by  their  weight  in  pounds 
per  mile.  As  the  weight  and  conductivity  both  vary  as  functions  of  the 
cross-section  of  the  wire,  speaking  of  the  weight  immediately  suggests 
the  conductivity.  Stating  the  diameter  of  the  wire  in  fractions  of  an 
inch  or  of  a  meter  is  good  practice  and  avoids  errors  which  are  in- 
troduced by  the  use  of  the  third  method,  that  of  arbitrary  gauge  num- 
bers. If  there  were  but  one  wire  gauge  in  use  throughout  the  world, 
these  errors  would  not  arise  as  often  as  they  do.  The  requirements 
of  practice  having  become  more  exact,  it  often  is  found  that  a  wire 
having  exactly  the  right  cross-section  to  meet  a  given  case  falls  be- 
tween two  sizes  of  a  given  wire  gauge  system. 

Wire  Gauges.     For  use  in  actual  telephone  lines,  there  are  three 
principal  wire  gauges.     These  are  the  American  wire  gauge  (also 


734 


TELEPHONY 


known  as  the  Brown  and  Sharpe  gauge,  abbreviated  B.  &  S.),  the 
new  British  standard  gauge  (legal  in  Great  Britain;  also  known 
as  English  Legal  Standard  and  abbreviated  N.  B.  S.  and  E.  L.  S.), 
and  the  Birmingham  wire  gauge  (B.  W.  G.,  also  known  as  Stubs 
gauge). 

All  of  these  gauges  are  in  common  use  in  the  United  States. 
The  Brown  and  Sharpe  gauge  is  only  universal  in  this  country  for 
the  smaller  wires.  Wires  in  windings  of  apparatus,  for  example, 
do  not  follow  any  other  gauge.  In  line  construction,  the  special 
needs  of  a  case  may  make  it  necessary  to  choose  sizes  in  other  than 
the  Brown  and  Sharpe  gauge. 

TABLE  XIX 

Size,  Weight,  Approximate  Elastic  Limit,  Approximate  Breaking 
Weight,  and  Average  Resistance  of  Copper=Clad  Wire 

(40  Per  Cent  Conductivity) 


B.  &S. 

GAUGE 
bio. 

WEIGHT 
PEB  MILE 

APPROXIMATE 

ELASTIC 
LIMIT 

APPROXIMATE 
BREAKING 
WEIGHT 

Av.    RESISTANCE 
IN  OHMS  PER 
MILE  AT60°F. 

0000 

3140. 

8523. 

9470. 

0.634 

000 

2490. 

6660. 

7400. 

0.800 

00 

1975. 

5922. 

6580. 

1.009 

0 

1570. 

4707 

5230. 

1.272 

1 

1240. 

4104 

4560. 

1.605 

2 

985. 

3240. 

3600. 

2.024 

3 

780. 

2970. 

3300. 

2.552 

4 

620. 

2340. 

2600 

3.217 

5 

491. 

1980. 

2200.                      4.060 

6 

390. 

1530. 

1700.                      5.117 

7 

309. 

1305. 

1450. 

6.450 

8 

245. 

035. 

1150. 

8.132 

9 

194. 

855. 

950. 

10.26 

10 

154. 

684. 

760. 

12.93 

11 

122. 

558. 

620. 

16.33 

12 

97. 

441. 

490. 

20.57 

13 

77. 

351. 

390.                    25.90 

14 

61. 

288. 

320. 

32.70 

15 

49.0 

225. 

250. 

41.20 

16 

38.3 

180. 

200. 

52.05 

17 

30.5 

149. 

165. 

65.45 

18 

24.1 

117. 

130. 

82.68 

19 

19.1 

90. 

100. 

104.2 

20 

15.2 

72. 

80. 

131.1 

OPEN  WIRES 


735 


Characteristics  of  Copper  Wire.  The  diameters,  weights,  and 
resistances  of  copper  wires  of  all  the  sizes  in  common  use  in  bare- 
wire  telephone  lines  appear  in  Table  XVIII. 

The  curve  of  Fig.  500  gives  the  weight  per  mile  of  wire  of  the 
sizes  given  in  Table  XVIII.  The  curve  gives  at  a  glance  an  idea 
of  the  similarity  between  certain  sizes  of  the  different  gauges. 

Characteristics  of  Copper=Clad  Wire.  The  mechanical  and 
electrical  characteristics  of  copper-clad  wire,  having  a  conductivity 
of  about  40  per  cent  of  that  of  copper  wire  of  the  same  gauges,  are 
given  in  Table  XIX. 

Characteristics  of  Iron  Wire.  The  mechanical  and  electrical 
characteristics  frequently  specified  for  iron  wire,  of  the  sizes  most 
commonly  used  for  telephone  lines,  are  given  in  Table  XX. 

TABLE  XX 

Size,  Weight,  Tensile  Strength,  and  Approximate  Resistance  of 
Iron  Wire  Commonly  Used  in  Telephone  Lines 


»• 

.  •  o 

£s 
«•« 

DlA.   IN 

MILS 

LENGTH 

OF 

BUNDLES 
MILES 

WEIGHT 
IN  LB. 
PER  MILE 

MIN.  TENSILE 
STRENGTH  IN  LBS. 

APPROX.  RESIS.  IN 
OHMS,  PER  MILE 

E.B.B.    B.  B. 

Steel 

E.B.B. 

B.  B. 

Steel 

6 

203. 

1 

590 

1475 

1652 

1770 

8.0 

9.5 

11.0 

8 

165. 

§ 

390 

975 

1092 

1170 

12.1 

14.4 

16.7 

9 

148. 

i 

314 

785 

879 

942 

15.0 

17.8 

20.7 

10 

134. 

I 

258 

645 

722 

774 

18.2 

21.7 

25.2 

11 

120. 

\ 

206 

515 

577 

618 

22.8 

27.2 

31.6 

12 

109. 

J 

170 

425 

476 

510 

27.7 

32.9 

38.2 

14 

83. 

} 

99 

247 

277 

297 

47.5 

56.6 

65.7 

CHAPTER  XLIII 

CABLES 
t 

Early  Types.  Early  telephone  cables  were  copies  of  telegraph 
cables.  For  outdoor  use,  the  wires  were  insulated  with  rubber; 
for  indoor  use  they  were  insulated  with  cotton.  Rubber  soon  was 
found  unsatisfactory  for  telephone  cables,  principally  on  account  of 
its  high  specific  inductive  capacity.  Cotton  insulation,  as  used  on 
the  wires  of  indoor  cables,  was  found  preferable  in  that  respect,  so 
such  cables,  covered  by  a  lead  sheath  to  keep  the  cotton  dry,  were 
used  somewhat  widely.  The  lead  sheath  was  applied  by  threading 
the  cable  through  successive  lengths  of  lead  pipe,  and  soldering 
together  the  adjacent  ends  of  the  sections  of  pipe.  A  next  step  in  the 
development  of  the  process  was  to  pass  the  cabled  wires  through  a 
machine  to  make  the  lead  pipe  directly  upon  them,  in  a  continuous 
length.  The  cotton  covered  and  cabled  wires  were  saturated  with 
paraffin  or  with  some  hydrocarbon  compound. 

Dry  Paper.  The  search  for  an  insulating  material  of  still  lower 
specific  inductive  capacity  finally  led  to  the  adoption  of  the  present 
standard,  dry  paper,  a  material  much  better  than  others  because  a 
cable  insulated  with  it  contains  so  much  air,  not  only  in  the  paper 
itself,  but  in  the  spaces  between  the  wires,  when  the  core  of  wires 
and  paper  is  not  compressed  too  tightly. 

Manufacture.  The  process  is  roughly  that  of  insulating  the 
untinned  copper  wire  by  loosely  wrapping  paper  ribbon  around  it, 
twisting  two  wires  so  insulated  into  a  pair,  laying  up  the  requisite 
number  of  pairs  into  a  rope,  and  forming  a  lead  sheath  over  it. 

Conductors: — Cables  for  exchange  uses  usually  are  formed  of 
No.  19  or  No.  22  B.  &  S.  gauge  wires.  Paper  ribbon  ^  to  f  inch 
wide,  and  from  .002  to  .004  inch  thick,  is  wrapped  spirally  on  the 
wire.  The  edges  of  the  spirals  of  paper  overlap.  Either  one  or 
two  paper  wrappings  are  applied,  as  required  by  the  fancy  of  the 
engineer  or  the  real  requirements  of  the  cable's  intended  use.  A 


CABLES  737 

single  wrap  suffices  for  all  needs  in  most  cases,  though  repairs  requir- 
ing boiling  out  of  moisture  by  hot  paraffin  are  more  certainly  done 
when  two  wrappings  exist. 

Pairs: — Two  paper-wrapped  wires  are  then  twisted  into  a  pair, 
the  two  wires  having  different  colored  papers,  to  enable  them  to  be 
distinguished  from  each  other  in  splicing  and  terminating.  The 


Pig.  501.     Paper  Cable 

"lay"  of  the  twist,  i.  e.,  the  length  of  one  complete  spiral,  varies  from 
3  to  6  inches,  depending  on  the  size  of  wire  used;  the  smaller  the 
wire,  the  shorter  the  length  of  lay.  The  reason  for  twisting  the 
wires  of  pairs  in  cables  is  the  same  as  that  for  transposing  the  wires 
of  an  open  air  line,  viz,  in  order  to  neutralize  the  effects  of  electro- 
magnetic and  electrostatic  induction  between  adjacent  lines.  In  tele- 
phone cables,  if  the  wires  were  not  twisted  into  pairs,  it  would  be 
possible  for  conversations  which  are  being  carried  on  on  one  line  to 
be  overheard  on  another  line. 

Core : — A  number  of  pairs  are  taken  as  a  beginning,  and  others 
are  wrapped  around  them  in  a  spiral  layer.     Over  this,  other  layers 


Fig.  502.     Paper  Cable 


are  wrapped,  the  direction  of  the  spiral  reversing  from  layer  to  layer. 
When  all  the  pairs  are  in  place,  one  or  more  layers  of  paper  tape  are 
wrapped  over  the  entire  cable-core  to  hold  it  in  form.  Some  makers 
use  a  binding  of  cotton  yarn  instead  of  the  paper  tape,  or  with  it.  The 


738 


TELEPHONY 


wick-like  nature  of  cotton  yarn  is  an  objectionable  quality,  as  cot- 
ton carries  moisture  further  from  a  fault  than  does  paper  tape  alone. 
i  .  ..  Drying: — The  cable-core  now  is  dried,  to  free  the  paper  from 
moisture  absorbed  from  the  air  before  and  during  the  manufacture 
of  the  core.  Heat  is  applied  by  putting  the  reeled  core  into  an  oven, 
and  often  by  exhausting  air  from  the  oven.  The  core  is  then  drawn 
directly  from  its  reel  in  the  oven  into  and  through  a  lead  press  to  apply 
the  sheath.  Figs.  501  and  502  show  finished  cables. 

Forming  Lead  Sheath : — It  is  an  interesting  way  in  which  the 
lead  press  acts,  to  mould  a  lead  or  lead-alloy  sheath  directly  upon  the 
cable-core.  Fig.  503  is  not  a  slavishly  exact  picture  of  a  lead  press 
in  action,  but  is  meant  to  help  show  how  a  lead  press  works.  The 

core  of  the  cable  6  passes  into 
the  press  at  the  right  of  the 
figure,  and  emerges  at  the  left 
with  its  sheath  1  moulded  over 
it.  Let  4  represent,  in  general 
terms,  a  strong  containing  vessel, 
acting  as  the  barrel  of  the  press. 
Note  that  everything  shaded  as 
are  the  areas  2  in  the  figure,  is  a 
part  of  one  mass  of  lead,  hot  but 
not  quite  fluid.  The  piston  5 
presses  downward  on  this  mass 
of  lead,  and  the  lead  is  forced 
out  through  the  opening  in  the 
part  7,  through  which  also  the 
cable  emerges.  This  opening  and 
the  part  3,  taken  as  a  unit,  form 
the  die  of  the  press,  and  it  is  the 
die  which  is  the  unique  feature  of  the  whole  matter.  Obviously 
the  mere  hole  in  the  side  of  the  press  would  squeeze  out  lead  and 
the  cable  together,  by  the  pressure  of  the  piston,  but  this  would 
compel  the  sheath  to  compress  the  core.  The  part  3  cares  for  this 
feature.  The  outer  surface  of  the  part  3  moulds  the  inner  surface 
of  the  sheath,  just  as  the  walls  of  the  hole  in  the  part  7  mould  the 
miter.  If  it  be  noted  that  the  mass  of  lead  is  always  one  mass,  a 
part  always  exuding  from  the  press  in  the  form  of  a  pipe,  carrying 


Pig.  503.     Principle  of  Lead  Press 


CABLES  739 

the  cable  within  it,  the  thought  will  be  complete.  If  the  core  should 
be  omitted,  a  mere  lead  pipe  would  exude.  This  re-suggests  the 
fact  that  the  lead  is  not  molten  but  semi-molten  only,  and  flows 
partly  because  softened  by  heat  and  partly  because  of  the  great 
pressure  upon  it. 

Alloyed  Sheath: — An  alloy  of  97  per  cent  lead  with  3  per  cent 
tin  is  used  as  sheath  material  in  some  cables.  The  tin  originally  was 
adopted  to  lessen  the  corrosive  effect  of  acetic  acid  from  wood  ducts, 
but  that  hazard  having  disappeared  through  the  change  of  methods 
in  duct  construction,  the  tin  was  retained  in  the  belief  that  it  gave  a 
tougher  sheath,  less  likely  to  be  crushed  by  misuse  or  to  be  cracked 
from  vibration  and  flexure.  The  tendency  of  present  belief  is  favor- 
able to  the  alloy  with  tin. 

Capacity.  In  a  cable  formed  of  No.  19  B.  &  S.  gauge  wires  it  is 
possible  to  secure  a  mutual  electrostatic  capacity  as  low  as  .05  micro- 
farad per  mile  of  pair,  the  term  mutual  capacity  meaning  the  ca- 


Pig.  50 1.     Relative  Sizes  of  Cables  Having  Same  Numerical  Capacity 

pacity  from  one  wire  to  another  only.  Capacities  are  also  expressed 
in  terms  of  the  capacity  of  one  wire  with  relation  to  its  mate  and  all 
the  other  wires  together.  Measured  in  the  latter  way,  the  capacity 
is  about  50  per  cent  greater  than  when  measured  in  the  former  way. 
Capacity  by  the  second  method  is  sometimes  known  as  "regular" 
capacity,  although  one  is  no  more  regular  than  the  other. 

Mutual  and  "Regular."  The  two  methods  of  specifying  should 
be  contrasted.  One  says:  "The  electrostatic  capacity  of  the  wire, 
measured  against  its  mate,  the  remaining  wires  being  grounded  to  the 
sheath,  shall  be  not  more  than  x  microfarads  per  mile."  This  is 
mutual  capacity.  The  other  says:  "The  electrostatic  capacity  of 


740 


TELEPHONY 


the  wire,  measured  against  the  remaining  wires  grounded  to  the  sheath, 
shall  be  not  more  than  x  microfarads  per  mile."  This  is  "regular" 
capacity. 

Fig.  504,  which  is  from  a  photograph  loaned  by  the  Standard 
Underground  Cable  Company,  shows  the  relative  sizes  of  cables 
specified  to  have  the  same  numerical  capacity  in  the  two  methods  of 
expression.  In  actual  terms,  the  capacity  of  the  larger  cable  is  only 
.054  microfarad  per  mile,  expressed  in  the  preferable  form,  as  mutual 
capacity.  The  reason  for  calling  the  mutual  expression  the  better 
is  that  it  is  the  amount  of  mutual  or  shunt  capacity  which  determines 
the  influence  of  capacity  on  voice  currents.  For  purposes  of  calcu- 
lation the  capacity  enters  in  that  form.  Therefore,  it  is  rational  to 
speak  of  and  think  of  it  in  those  terms. 

Effect  of  Temperature  on  Capacity.  Cables  should  have  their 
specified  capacities  when  the  core  has  a  temperature  of  60°  or  80°  F., 
or  some  other  known  temperature.  If  the  test  is  not  made  with  the 
core  actually  at  that  temperature,  the  corrective  factors  given  in  Table 
XXI  should  be  used  to  learn  the  capacity  at  60°  F.  To  apply  them, 
merely  multiply  the  observed  capacity  by  the  factor  in  the  table  cor- 
responding to  the  observed  temperature.  The  product  is  the  capac- 
ity at  60°  F.  This  table  is  due  to  the  tests  of  H.  W.  Fisher. 

Insulation.  The  insulation  resistance  of  dry-core  telephone 
cables  should  be  specified  as  not  less  than  500  megohms  per  mile  at 
60°  F.  Each  wire  is  measured  against  all  the  others  grounded  to 

TABLE  XXI 
Corrective  Factors  for  Capacity 


OBSERVED   TEMPERATURE,   FAHRENHEIT 

FACTOR 

30° 

1.065 

40 

1.043 

50 

1.021 

60 

1.000 

70 

0.970 

80 

0.945 

90 

0.918 

100 

0.894 

110 

0.864 

120 

0.836 

130 

0.805 

CABLES 


741 


the  sheath.  If  not  tested  at  that  temperature,  the  corrective  factors 
given  in  Table  XXII  should  be  used.  That  is,  multiply  the  observed 
insulation  resistance  by  the  factor  corresponding  to  the  observed  tem- 
perature. The  data  of  this  table  is  also  due  to  tests  of  H.  W.  Fisher. 

It  will  be  seen,  therefore,  from  a  study  of  Tables  XXI  and 
XXII  that  the  colder  the  cable,  the  better  it  is,  the  insulation  being 
higher  and  the  mutual  capacity  lower. 

Diameters  and  Weights.  The  electrostatic  capacity,  thickness 
of  sheath,  external  diameter,  and  approximate  weight  of  paper  insu- 
lated cable,  of  the  sizes  most  frequently  employed  in  telephone  use, 
are  given  in  Table  XXIII.  It  must  be  understood,  however,  that 
these  figures,  particularly  as  to  diameters  and  weights,  are  subject  to 
considerable  variation. 

Submarine  Cables.  Paper.  Submarine  cables  for  telephone 
lines,  in  present  practice,  are  of  limited  length.  Unless  they  differ 
radically  in  construction,  submarine  cables  have  the  same  general 
characteristics  as  underground  cables.  Present  apparatus  enables 
good  speech  to  be  limited  by  about  35  miles  of  No.  19  B.  &  S.  gauge 
dry-core  cable,  unless  loading  coils  are  inserted.  Under  that  length, 
for  distances  where  unloaded  underground  cables  would  be  practical, 
submarine  cables  are  used  freely. 

Armor: — The  usual  practice,  for  such  reasonable  lengths  of  sub- 
marine cables,  is  to  add  to  the  cable,  over  the  sheath,  a  protecting  ar- 
mor of  some  kind.  No  ducts  being  available  in  submarine  work, 

TABLE  XXII 
Corrective  Factors  for  Insulation 


OBSERVED  TEMPERATURE,  FAHRENHEIT 

FACTOR 

60 

1.00 

65 

1.67 

70 

2.45 

75 

3.33 

80 

4.66 

85 

6.85 

90 

7.66 

95 

9.45 

100 

11.65 

110 

19.40 

120 

39.00 

742 


TELEPHONY 


TABLE  XXIII 
Aerial  and  Underground  Telephone  Cable 


No.  PAIRS 

GAUGE 
B.  &  S. 

ELECTRO- 
STATIC 
CAPACITY 

THICKNESS 

OF 

SHEATH 

APPROXIMATE 
EXTERNAL 
DIAMETER 

APPROXIM'E 
WEIGHT 
PER  FOOT 

5 

22 

High 

A 

0.48 

0.55 

10 

22 

High 

A 

0.59 

0.71 

15 

22 

High 

T2 

0.6G 

0.83 

20 

22 

High 

A 

0.72 

0.93 

25 

22 

High 

i 

0.7,7 

1.02 

50 

22 

High 

i 

0.97 

1.45 

50 

20 

High 

A 

1.10 

1.88 

100 

22 

High 

8 
III 

1.32 

2.36 

100 

22 

Low 

/* 

1.50 

2.63 

100 

20 

High 

i 

1.57 

3  .  60 

100 

20 

Low 

i 

1.81 

4.11 

200 

22 

High 

i 

1.84 

4.43 

200 

22 

Low 

i 

2.11 

4.99 

200 

20 

High 

i 

2.11 

5.47 

200 

20 

Low 

i 

2.46 

6.19 

200 

19 

High 

i 

2.24 

6.08 

200 

19 

Low 

i 

2  .  65 

6.94 

300 

22 

Hi-h 

i 

2.21 

5.71 

300 

22 

Low 

i 

2.51 

6.32 

300 

20 

High 

* 

2.53 

7.09 

300 

20 

Low 

i 

2.96 

7.94 

300 

19 

High 

i 

2.69 

7.95 

300 

19 

Low 

i 

3.20 

9.04 

400 

22 

High 

i 

2.51 

6.84 

400 

22 

Low 

i 

2.86 

7.56 

400 

20 

High 

i 

2.89 

8.56 

400 

20 

Low 

i 

3.43 

9.37 

600 

22 

High 

i 

3.20 

9.21 

90 

16 

Low 

i 

2.88 

7.20 

43 

13 

Low 

i 

2.88 

7.17 

50 

10 

High 

i 

2.88 

8.95 

NOTE.     High    capacity    0.067—0.090     mutual;     0.10—0.12     grounded. 
Low   capacity  0.054—0.067   mutual;     0.080—0.10   grounded. 

the  armor  is  necessary  for  protection  of  the  cable,  lest  its  sheath  be 
torn  or  punctured.  In  lakes  and  seas,  anchors  may  foul  the  cable. 
The  waters  may  chafe  it  against  rocks.  In  streams,  drifting  things 
may  encounter  it. 

Two  kinds  of  armor  are  used,  one  of  steel  tape,  shown  in  Fig. 
505,  and  the  other  of  steel  wires,  shown  in  Fig.  506,  both  of   these 


CABLES 


743 


Fig.  505.    Submarine  Cable  Steel-Tape  Armor 


being  applied  spirally.  In  both  cases  a  cushion  or  bed  of  tarred  jute 
is  laid  over  the  lead  cable  sheath,  then  the  armor  of  wires  or  steel 
tape  is  applied,  then  another  tarred  jute  covering,  finished  by  apply- 
ing lime  and  sand.  The  cable  then  may  be  reeled  and  unreeled  with- 
out danger  of  the  armor  injuring  the  sheath  or  core  by  buckling. 

The  wire  armor  is  the  better  and  has  been  used  since  the  first 
deep-sea  telegraph  cables  were 
made;  it  protects  many  miles 
of  lead-covered  cable  and  very 
many  more  miles  of  gutta- 
percha  cable  without  lead 
sheath. 

Double  sheaths  of  lead 
sometimes  are  used  in  lieu  of 
or  in  conjunction  with  armor.  The  failure  of  one  sheath,  in  this 
construction,  may  still  allow  the  other  to  protect  the  cable  core  from 
the  water. 

Loading.  For  longer  lengths  than  those  just  considered,  loading 
coils  are  essential,  as  the  capacity  can  not  be  kept  low  enough  in  any 
cable  to  allow  it  to  approach  the  speaking  quality  of  an  open  wire 
line.  There  are  only  two  known  ways  of  loading  cables:  by  insert- 
ing distributed  inductance  and  by  inserting  "lumped"  inductance  — 
loading  coils  at  intervals.  Only  the  latter  way  has  been  generally 
employed;  however,  the  former  has  been  used  in  several  instances 
with  submarine  cables.  In 
underground  cables,  these  coils 
are  located  in  manholes.  In 
submarine  cables,  they  have  to 
be  incorporated  in  the  cable 
itself,  a  matter  of  no  great  sim- 
plicity, as  the  cable  needs  to 
be  paid  out  from  a  ship,  and 
such  lumps  as  loading  coils  add  little  to  the  ease  of  the  task.  Such 
a  cable,  however,  has  been  laid  in  Lake  Constance.  It  is  a  lead- 
sheathed,  dry-core  cable,  the  loading  coils  being  within'  the  sheath. 

Rubber  and  Gutta-Percha.  For  uses  where  capacity  is  negli- 
gible, such  as  for  very  short  lengths,  rubber-insulated  wires  may  be 
formed  into  cables  and  armored  with  wires  or  tape  as  in  Figs.  505  and 


Fig.  506.    Submarine  Cable  Steel- Wire  Armor 


744  TELEPHONY 

506,  the  lead  sheath  being  omitted.  The  rubber  compound  on  the 
wires  serves  as  a  sufficient  protection  against  water. 

Gutta-percha  insulated  submarine  cables  have  the  advantage 
that  they  also  require  only  mechanical  protection  by  armor,  and  no 
lead  sheath.  They  have,  however,  the  disadvantage  that  the  spe- 
cific inductive  capacity  of  rubber  is  high.  It  has  been  asserted  by 
competent  cable  engineers,  however,  that  the  contribution  to  human 
knowledge  by  O.  Heaviside  is  not  limited  in  its  application,  and  that 
as  inserting  serial  inductance  in  small  degree  offsets  shunt  capacity  of 
small  degree,  the  same  general  result  would  ensue  if  larger  induc- 
tances were  inserted  to  offset  larger  capacity. 

This  seems  reasonable.  Indeed,  it  would  be  strange  if  it  were 
not  so.  The  proof  is  at  hand,  in  one  of  two  telephone  cables  recently 
laid  between  England  and  France.  An  unloaded  cable  has  been 
in  service  under  those  waters  for  years.  Recently  the  governments 
of  England  and  France  agreed  to  lay  two  new  cables,  each  govern- 


Fig.  507.    Arrangement  of  Loading  Coils  in  Cables 

ment  to  lay  one  of  them.  France  laid  a  duplicate  of  the  old  one. 
England  laid  a  loaded  gutta-percha  cable,  of  vastly  superior  working 
qualities,  and  having  in  it  the  loading  inductances  in  lumps.  This 
cable  contains  two  pairs  of  soft  copper  wires,  each  weighing  160 
pounds  per  mile  of  wire  (about  No.  10  B.  &  S.  gauge).  Each  wire 
is  insulated  with  300  pounds  of  gutta-percha  per  mile.  The 
length  of  the  cable  is  24.2  statute  miles,  and  the  total  loop  resistance 
of  each  pair,  unloaded,  is  302.5  ohms.  Twenty  loading  coils  are  in- 
serted in  each  wire ;  the  two  coils  of  a  pair  of  wires,  at  each  point  of 
loading,  are  wound  on  one  circular  core.  Therefore,  two  cores  carry- 
ing two  coils  each  are  inserted  at  each  point,  the  points  being  1.153 
statute  miles  (one  nautical  mile)  apart.  The  coils  have  an  inductance 
of  .1  henry  and  a  resistance  of  6  ohms  each.  The  mutual  capacity 
these  coils  oppose  is  (for  the  unloaded  cable)  .12  microfarad  per  mile. 
Fig.  507  shows  the  arrangement  of  mounting  the  coils  in  cable. 
The  increase  in  size  of  the  cable  at  the  loading  point  is  marked,  but 
it  was  possible  to  pay  out  the  entire  length  of  cable  without  unusual 
risk  or  difficulty. 


CHAPTER  XLIV 
POLES  AND  POLE  FITTINGS 

Pole  Equipment.  Poles,  The  cheapest  way  to  support  line 
conductors  and  the  way  that  is  nearly  always  practiced,  except  in 
communities  of  dense  congestion,  is  to  place  them  on  poles.  The 
poles  are  usually  of  wood,  although  in  special  cases  structural  iron 
poles  and  reinforced  concrete  poles  have  been  used.  Owing  to  the 
increasing  scarcity  of  timber  in  the  United  States,  it  is  not  unlikely 
that  the  reinforced  concrete  pole  will  find  greater  favor  in  the  fu- 
ture, as  the  cost  of  wooden  ones  increases  and  as  the  methods  of  man- 
ufacturing those  of  concrete  are  bettered  and  cheapened. 

Cedar: — All  things  considered,  the  Michigan,  or  white  cedar,  pole 
is  the  best  adapted  for  telephone  use.  While  cedar  is  not  in  itself 
a  wood  of  very  great  strength,  it  has  several  important  things  in  its 
favor.  Principal  among  these  is  its  long  life.  A  good  cedar  pole, 
properly  cut  and  seasoned,  may  under  ordinary  circumstances  be 
depended  upon  for  a  life  of  from  sixteen  years  up.  Another  thing  in 
its  favor  is  its  shape.  Nature  caused  it  to  grow  in  just  about  the  form 
that  an  engineer  would  have  designed  it  for  strength,  i.  e.,  large  at 
the  butt  and  gently  tapering  toward  the  top.  Of  less  importance, 
it  is  a  light  wood  and,  therefore,  easily  transported  and  erected,  and 
also  presents  a  sightly  appearance,  if  poles  may  ever  be  said  to  be 
sightly. 

Chestnut: — Another  good  wood  is  chestnut.  It  has  a  life  equal 
to  or  greater  than  that  of  white  cedar  and  is  of  stronger  fiber.  It 
is  not  so  well  shaped  as  the  white  cedar  pole,  being  relatively  smaller 
at  the  butt  for  a  given  length  and  top  diameter;  its  greater  inherent 
strength,  however,  in  large  measure  makes  up  for  this  deficiency,  and 
while  it  is  somewhat  less  sightly  than  cedar  and  also  much  heavier, 
there  is  very  little  to  choose  between  them.  Chestnut  is  very  largely 
used  in  the  eastern  states  and  in  the  south,  where  the  cost  of  trans- 
portation of  cedar  poles  is  almost  prohibitive. 


746  TELEPHONY 

Other  Timbers : — In  sections  of  the  country  where  neither  white 
cedar  nor  chestnut  are  available,  cypress,  pine,  tamarack,  and  Idaho 
cedar  are  employed  with  varying  degrees  of  success.  Cypress  under 
certain  conditions  is  said  to  have  excellent  lasting  qualities,  but  the 
writers'  experience  with  it  and  the  experiences  of  others  has  seemed 
to  indicate  that  cypress  is,  to  say  the  least,  a  treacherous  wood, 
and  will  often  rot  away  to  an  astonishing  extent  in  a  very  few  years, 
leaving  only  a  small  core  of  sound  wood  at  the  center  of  the  pole. 
There  are,  however,  well-authenticated  cases  of  cypress  poles  that 
have  shown  good  life,  and  its  very  low  cost  in  some  localities  frequently 
forces  it  into  consideration,  especially  where  the  conditions  for  its  en- 
durance are  known  to  be  favorable. 

Idaho  cedar  is  widely  employed  throughout  the  country  partic- 
ularly where  very  high  poles  are  required.  The  fiber  of  the  wood  is 
good,  but  these  poles  have  a  very  grave  defect  in  their  extreme  slen- 
derness  for  a  given  top  dimension.  They  are  perfectly  smooth  and 
straight,  and  there  is  a  common  joke  about  them  to  the  effect  that 
either  end  of  them  may  be  put  in  the  ground  equally  well. 

In  southern  and  western  districts  pine  poles  are  widely  used  with 
varying  success.  Yellow  pine,  on  account  of  the  amount  of  pitch  it 
contains,  would  lead  one  to  believe  that  it  would  have  good  lasting 
qualities,  but  it  frequently  rots  very  rapidly. 

Cutting: — Poles  should  be  cut  from  live,  growing  timber,  while 
the  sap  is  down,  and  should  be  free  from  knots  and  shakes,  and 
reasonably  sound.  With  cedar  poles  a  certain  amount  of  butt  rot, 
i.  e.,  rot  exposed  at  the  butt  section  of  the  pole,  is  to  be  permitted, 
since  it  is  not  commercially  possible  to  obtain  poles  free  from  this. 
With  cedar  and  chestnut  poles  it  is  not  practicable  always  to  secure 
perfectly  straight  poles  and,  therefore,  a  reasonable  amount  of 
crookedness  is  to  be  permitted. 

Sizes: — Standard  sizes  of  poles  vary  in  length  by  five-foot  steps. 
The  usual  way  of  indicating  the  size  of  a  pole  irrespective  of  its  height 
is  by  the  diameter  in  inches  at  its  top.  Thus,  a  pole  referred  to  as  a 
"7-inch  30"  would  be  7  inches  in  diameter  at  the  top  and  30  feet 
long.  On  account  of  the  great  variations  that  may  occur  in  the  butt 
sizes  of  poles  of  equal  top  diameter  and  length,  it  is  well  that  the 
butt  sizes  be  specified  also,  since  this  is  a  feature  having  most  bearing 
on  the  strength  of  the  pole. 


POLES  AND  POLE  FITTINGS  747 

Northwestern  Cedarmen's  Specifications: — The  latest  specifi- 
cation of  the  Northwestern  Cedarmen's  Association,  which  practi- 
cally governs  the  purchase  of  white  cedar  poles  in  the  United  States, 
is  as  follows : 

STANDARD  TELEGRAPH,  TELEPHONE,  AND  ELECTRIC  POLES. — Sizes,  5-inch 
25  foot,  and  upwards.  Above  poles  must  be  cut  from  live,  growing  timber, 
peeled,  and  reasonably  well  proportioned  for  their  length.  Tops  must  be 
reasonably  sound,  and  when  seasoned  must  measure  as  follows:  5-inch  poles, 
15  inches  in  circumference  at  top  end;  6-inch  poles,  18£  inches  in  circumference 
at  top  end;  7-inch  poles,  22  inches  in  circumference  at  top  end.  If  poles  are 
green,  fresh  cut,  or  water  soaked,  then  5-inch  poles  must  be  16  inches  in  cir- 
cumference at  top  end,  and  6-inch  poles  must  be  19£  inches  in  circumference, 
and  7-inch  poles  must  be  22 f  inches  in  circumference  at  top  end.  One  way 
sweep  allowable  not  exceeding  1  inch  for  every  5  feet;  for  example,  in  a  25- 
foot  pole,  sweep  not  to  exceed  5  inches,  and  in  a  40-foot  pole  not  to  exceed  8 
inches.  Measurement  for  sweep  should  be  taken  as  follows:  That  part  of 
the  pole  when  in  the  ground  (six  feet)  not  being  taken  into  account  in  arriving 
at  sweep,  tightly  stretch  a  tape  line  on  the  side  of  the  pole  where  the  sweep  is 
greatest,  from  a  point  6  feet  from  butt  to  the  upper  surface  at  top,  and  having 
so  done  measure  widest  point  from  tape  to  surface  of  pole  and  if,  for  illustration, 
upon  a  25-foot  pole  said  widest  point  does  not  exceed  5  inches,  said  pole  comes 
within  the  meaning  of  these  specifications.  Butt  rot  in  the  center  including 
small  ring  rot  outside  of  the  center;  total  rot  must  not  exceed  10  per  cent  of  the 
area  of  the  butt.  Butt  rot  of  a  character  which  plainly  seriously  impairs  the 
strength  of  the  pole  above  the  ground  is  a  defect.  Wind  twist  is  not  a  defect 
unless  very  unsightly  and  exaggerated.  Rough  large  knots  if  sound  and 
trimmed  smooth  are  not  a  defect. 

Trimming: — The  knots  on  all  poles  should  be  closely  trimmed 
and  the  bark  removed,  as  the  presence  of  the  bark  induces  rotting. 
It  is  preferable  to  remove  the  bark  by  stripping,  but  if  this  is  not 
feasible  it  should  be  done  by  shaving,  and  the  amount  of  shaving  in 
all  cases  should  be  kept  a  minimum,  since  the  strength  and  life  of  a 
pole  is  reduced  by  too  deep  shaving.  The  poles  should  be  thoroughly 
seasoned  before  setting. 

Treating: — The  constantly  increasing  cost  of  wooden  poles,  due 
to  the  scarcity  of  timber,  has  led  in  some  cases  to  the  practice  of  treat- 
ing the  poles  with  a  preservative.  This  is  not  generally  done  where 
cedar  and  chestnut  are  used,  but  in  the  south  where  the  difficulty  of 
securing  a  long-lived  pole  is  very  great,  there  is  a  growing  tendency 
toward  the  use  of  these  preservative  processes.  The  most  success- 
ful of  these  so  far  is  the  process  of  creosoting,  which  consists  in  the 
impregnation  of  the  pole  with  creosote,  which  is  a  dead  oil  of  coal-tar. 


748 


TELEPHONY 


There  are  a  number  of  methods  by  means  of  which  this  impregnation 
is  accomplished,  some  of  them  securing  a  penetration  of  the  creosote 


500 


/OOO  tfOO  ZOOO 

WE/GMT  //V  POM0S 


Z5M   Z80d 


Fig.  508.     Weights  of  Cedar  Poles 


for  only  a  short  distance  below  the  surface,  and  others  a  penetration 
reaching  almost  or  quite  to  the  center  of  the  pole.     Another  material 

used  for  impregnation  is  chloride 
of  zinc.  The  reports  of  the 
United  States  Department  of  Ag- 
riculture give  much  valuable  in- 
formation and  data  on  the  subject 
of  preservation  of  timber,  partic- 
ularly that  used  for  telephone 
and  telegraph  poles,  railway  ties, 
etc. 

In  localities  where  timber  is 
cheap  and  not  of  good  lasting 
quality  and  where  creosoting 
plants  have  been  established,  it 
is  undoubtedly  economical  to  use 
Fig.  509.  weights  of  cypress  Poles  creosoted  poles.  In  other  locali- 


^ 

IV 

/ 

k  *" 

1 

/ 

\ 

$ 

K 

"f 

/ 

i* 

,(5 

/ 

/ 

i 

/ 

$ 

/ 

\ 

/ 

r. 

1 

i 

50 

0 

/Ol 

;<? 

'5t 

W 

ZOO 

POLES  AND  POLE  FITTINGS 


749 


ties  it  is  found  that  creosoting  an  already  expensive  pole  results  in 
prohibitive  cost. 

It  is  the  practice,  however,  of  the  large  telephone  and  telegraph 
companies  to  treat  the  poles  externally  by  painting  them  for  a  distance 
of  about  three  feet,  above  and  below  the  ground  line,  with  two 
coats  of  carbolineum  aveuarius.  The  roofs  and  gains  of  the  poles 
are  also  painted  with  the  same  material.  In  city  work  the  poles  are 
usually  painted  all  over  with  a  good  oil  paint. 


60 
55 

50 


k 

SJ5 

§ 
^ 

30 

Z5 
ZO 


st 


4 


500 


/OOO 


/500 


ZOOO 


2500 


3000 


3600 


4000 


Fig.  510.     Weights  of  Chestnut  Poles 


Weights  :  —  The  curves  of  Figs.  508,  509  and  510  give  the  approxi- 
mate weights  of  different  sizes  and  lengths  of  cedar,  cypress,  and 
chestnut  poles. 

Table  XXIV  gives  useful  information  concerning  the  loading  of 
cedar  poles  on  cars.  Forty-foot  poles  and  longer  are  usually  loaded 
on  two  cars,  and  the  number  of  poles  in  each  case  as  given  in  this 
table  is  that  constituting  a  single  or  double  load. 

Gaining:  —  Where  a  pole  is  to  carry  more  than  one  or  two  bare 
wires,  cross-arms  are  provided  for  supporting  the  wires,  and  in  order  to 
afford  a  seat  for  these,  gains  or  mortises  are  cut  in  the  side  of  the  pole. 
It  is  a  mistake  to  make  these  gains  too  deep,  as  the  pole  is  greatly 
weakened  and  its  life  is  shortened  thereby.  It  should  merely  be  a 


750 


TELEPHONY 


TABLE  XXIV 

Cedar  Poles 


ON  SINGLE  CARS 

ON  DOUBLE  OAKS 

Size 

Number  in  Load 

Size 

Number  in  Load 

4"  25' 

175  to  225 

7"   40' 

60  to  75 

5"  25' 

150  to  200 

7"  45' 

50  to  65 

6"  25' 

100  to  125 

7"  50' 

40  to  50 

7"  25' 

75  to  100 

7"  55' 

35  to  45 

5"  30' 

100  to  125 

7"  60' 

25  to  35 

6"  30' 

75  to  100 

7"  65' 

20  to  25 

7"  30' 

60  to     80 

5"  35' 

75  to  100 

6"  35' 

60  to     80 

7"  35' 

55  to     75 

rectangular  notch  about  £  inch  deep  and  of  sufficent  height  to  just 
accommodate  the  cross-arm. 

Roofing: — In  order  that  the  tops  of  the  poles  may  drain  as 
rapidly  as  possible,  and  thus  rid  themselves  of  moisture  which  would 
otherwise  tend  to  rot  them,  the  top  is  usually  beveled  in  two  planes 


Fig.  511.     Details  of  Roofing  and  Gaining 


parallel  to  the  direction  of  the  pole  line  so  as  to  form  a  roof.     The 
details  of  the  roofing  and  gaining  of  a  pole  are  shown  in  Fig.  511. 


POLES  AND  POLE  FITTINGS 


751 


Fig.  512.     Insulator  Pin 


The  distance  between  the  centers  of  the  gains  and,  therefore,  between 
the  centers  of  the  cross-arms,  varies  from  18  to  22  inches,  according 
to  the  type  of  line. 

Cross-Arms.  Cross-arms  for  telephone  work  may  be  of  two, 
four,  six,  eight,  and  ten  pins  each,  and  the  best  practice  usually  is  to 
employ  only  ten-pin  arms,  even  though  a  fewer  number  of  wires 
than  ten  are  to  be  strung.  This  pro- 
vides for  growth,  which  nearly  always 
is  greater  than  expected.  Good  cross- 
arms  are  becoming  scarce.  They  are 
most  commonly  of  white  pine,  yellow 
pine,  or  Washington  fir.  The  latter  is  by  far  the  most  expensive  in 
tnost  parts  of  the  United  States  on  account  of  the  transportation 
charge,  but  as  a  rule  it  is  true  economy  to  use  them.  The  life  of 
cross-arms  varies  from  four  to  sixteen  or  more  years,  according  to 
the  kind  of  wood  used  and  the  climatic  conditions.  There  are  two 
sizes  of  cross-arms  employed  in  telephone  work,  one  known  as  the 
telephone  arm  and  having  a  cross-section  of  2f  by  3f  inches.  The 
other,  known  as  the  standard  arm,  has  a  cross-section  of  3^  by  4^ 
inches.  The  saving  in  cost  of  the  smaller  arm  does  not  usually 
warrant  its  use. 

Pins: — The  arms  are  bored  usually  with  l{-inch  holes,  into  which 
the  pins  for  supporting  the  insulators  are  placed.     A  standard  pin 


17    + —   /7 


I  if  >'  I  TT 

4-/e  Mr-/*  —  r-  /*  -t-/*  -r-**r  a  4-/^"H—  '*  —  I*-/*  -+-  /*  -r* 


Fig.  513.     Ten-  Pin  Cross-  Arms 


is  shown  in  Fig.  512.     They  are  made  of  various  woods,  locust,  ca 
talpa,  maple,  Bois  d'Arc,  kalkeen,  or  oak.     A  ten-pin  arm  equipped 
with  pins  is  shown  in  Fig.  513,  the  spacing  between  pins  being  that 
of  standard  practice. 

Hardware.  Through  Bolts. — The  standard  way  of  attaching 
a  cross-arm  to  the  pole  is  by  means  of  a  through  bolt  long  enough 
to  pass  through  the  pole  and  the  cross-arm  and  receive  a  nut  on 


752 


TELEPHONY 


its  screw-threaded  end.  A  large,  flat  iron  washer  about  TV  inch  thick 
and  2^  inches  square  is  placed  under  the  head  of  the  bolt  and  un- 
der the  nut  to  afford  a  large  bearing  surface  on  the  wood  against 


W/TH  ff/AM.  OF  POLE 
t . 


Fig.  514.     Through  Bolt 


which  the  bolt  may  draw.     The  details  of  a  standard  through  bolt 
are  shown  in  Fig.  514. 

Braces : — In  order  to  more  rigidly  support  the  cross-arm  on  the 
pole,  two  braces  are  employed  for  each  arm.     These  consist  usually 


Fig.  515.     Cross- Arms  Attached 


of  rectangular  strips  of  wrought  iron  about  \  by  \\  inches  in 
cross-section  and  from  20  to  30  inches  in  length.  The  details  of  the 
method  of  securing  a  cross-arm  to  a  pole,  including  the  attachment 


Fig.  516.     Carriage  Bolt 


of  the  cross-arm  braces,  is  shown  in  Fig.  515.     Where  the  two  lower 
ends  of  the  braces  meet  at  the  pole  they  are  secured  by  a  single  lag 


POLES  AND  POLE  FITTINGS 


753 


screw  passing  through  both  of  them  at  the  end  of  the  pole,  and  the 
outer  ends  of  the  braces  are  secured  to  the  cross-arm  by  means  cf 
carriage  bolts  passing  through  both  the  brace  and  the  arm  and  held  in 
place  by  a  nut  and  washer. 

Carriage  Bolts  and  Lag  Screws : — The  form  and  dimensions  of  a 


Fig.  517.     Lag  Screws 

carriage  bolt  for  attaching  braces  to  cross-arms  are  shown  in  Fig. 
516.  Fig.  517  shows  three  sizes  of  lag  screws,  the  5-inch  size  usually 
being  employed  to  secure  two  braces  to  the  pole. 

Pole  Steps: — The  standard  iron  pole  step  is  shown  in  Fig.  518 
and  is  made  of  f-inch  stock.  It  is  attached  to  the  pole  by  drilling  a 
£-inch  hole  in  the  pole  to  a  depth  of  from  2  to  3  inches  for  cedar 
and  about  4  inches  for  harder  woods,  and  then  the  step  is  driven  into 
the  pole  to  such  a  depth  that  the  distance  from  the  pole  to  the  out- 
side edge  of  the  step  is  approximately  5|  inches.  Ordinarily  the 


Fig.  518.     Pole  Step 

lower  five  steps  on  a  pole  are  made  of  triangular  pieces  of  wood  se- 
cured to  the  pole  by  one  60d  and  one  20d  nail.  The  purpose  of 
employing  the  wooden  steps  at  the  bottom  is  to  avoid  the  injury 
which  the  projecting  iron  steps  might  cause  to  passing  persons  or 
teams. 

Hardware  Requirements: — The  cross-arm  braces,  bolts,  steps, 
and  other  pieces  of  hardware  employed  in  pole  line  work  are  com- 
monly referred  to  as  pole  hardware.  In  general  the  material 
should  of  course  be  free  from  flaws,  cracks,  and  other  imperfections. 


754 


TELEPHONY 


In  the  case  of  bolts,  rods,  braces,  steps,  and  like  fittings,  the  wrought 
iron  or  mild  steel,  of  which  they  are  necessarily  made,  should  have 
the  properties  which  conform  to  the  standard  specifications  adopted 
by  the  bridge  builders,  as  set  forth  in  the  handbook  on  constructional 
iron,  issued  by  the  Carnegie  Steel  Company  in  1893.  Where  inalle- 


Fig.  519.     Equipped  Pole 

able  castings  are  used,  as  in  clamps,  they  should  be  reasonably 
straight,  smooth,  and  true  to  pattern  and  free  from  imperfections. 
They  should  also  be  capable  of  being  bent  to  a  reasonable  degree 
without  breaking.  All  bolts  and  rods  should  be  capable  of  stand- 
ing a  90-degree  bend  on  a  radius  equal  to  the  diameter  of  the 
bolt  without  fracture  of  the  steel  on  the  outside  of  the  bend. 
The  breaking  strength  of  all  bolts  and  drive  screws  should  be  at 
least  equal  to  the  following: 


POLES  AND  POLE  FITTINGS 


755 


Size  of  Bolt 


f-  inches 
J  inches 
£  inches 


Breaking  Strength 


3400  pounds 

6300  pounds 

10000  pounds 


The  holding  power  of  the  nuts  on  such  bolts  should  not  fall  below 
the  figures  just  given. 

Galvanizing: — Hardware,  including  bolts — threads  and  nuts — 
should  be  thoroughly  galvanized.  A  coating  of  zinc  should  be 
evenly  and  uniformly  applied  and  should  be  capable  of  withstanding 
the  standard  four-immersion  test  in  a  saturated  solution  of  sulphate 
of  copper. 

Equipped  Pole.  The  number  of  cross-arms  on  a  pole  depends, 
of  course,  on  the  number  of  open  wires  to  be  carried,  it  being  under- 
stood that  in  standard  practice  ten  wires  is  the  maximum  that  any 
arm  may  carry.  The  growing  tendency  to  use  cable  is  causing  the 
gradual  disappearance  of  very  heavy  bare  wire  pole  leads,  and  poles 
of  enormous  height  carrying  ten,  fifteen,  and  even  twenty-five  cross- 
arms  are  no  longer  seen.  Few  lines  are  now  to  be  found  with  more 
than  six  cross-arms,  and  these  only  in  heavy  cross-country  lines. 

A  pole  completely  equipped  with  four  cross-arms  is  shown  in 
Fig.  519.  The  steps  shown  on  the  sides  of  this  pole  would  ordinarily 
be  employed  only  in  city  work,  and  not  on  toll  lines  except  at  test 
poles. 

Pole  Setting.  The  distance  to  which  poles  should  be  set  in  the 
ground  depends  on  the  height  of  the  pole,  character  of  the  soil,  and 

TABLE  XXV 
Pole  Setting  Data 


LENGTH  OF  POLE 

DEPTH  IN  SOIL 

DEPTH  IN  ROCK 

20   feet 

4     feet 

3     feet 

25  feet 

5     feet 

3     feet 

30  feet- 

5£  feet 

3J  feet 

35  feet 

6     feet 

4     feet 

40  feet 

6     feet 

4£  feet 

45  feet 

6£  feet 

4£  feet 

50  feet 

7     feet 

4£  feet 

55  feet 

7     feet 

4JI  feet     , 

60  feet 

7*  feet 

5     feet 

65  feet 

7i  feet 

5     feet 

70  feet 

8     feet 

5^V  feet 

756  TELEPHONY 

the  strain  to  which  the  pole  is  to  be  subjected.  In  general,  Table 
XXV  represents  good  practice. 

Where  a  pole  is  set  on  a  sloping  bank,  the  depth  as  given  in  Table 
XXV  should  measure  from  the  lowest  side  of  the  opening  of  the  hole. 

With  this  preliminary  discussion  of  poles  and  pole  fittings  we 
may  divide  the  discussion  of  pole  lines  into  three  principal  headings: 
Toll  Lines,  Rural  Lines,  and  City  or  Exchange  Lines. 

Toll  Lines.  The  term  toll  lines  is  rather  loosely  applied  to  lines 
extending  across  country  between  cities  or  towns  and  carrying  the 
wires  which  serve  as  interurban  trunks.  A  governing  factor  in  the 
construction  of  such  a  line  is  the  number  of  wires  that  it  will  ultimately 
carry,  since  this  determines  the  number  of  cross-arms  and,  in  large 
measure,  the  height  and  the  strength  of  the  pole. 

Sizes  of  Poles.  The  usual  well-constructed  toll  line  of  the  pres- 
ent day  is  built  close  to  the  ground,  that  is,  it  is  built  on  poles  as  short 
as  will  allow  the  use  of  the  required  number  of  cross-arms  and  at  the 
same  time  give  the  required  clearness  under  the  wires.  A  great  many 
toll  lines  are  built  of  25-foot  poles,  with  the  use  of  longer  poles  as 
required  by  the  contour  of  the  country  and  the  necessity  at  crossings. 
Such  a  line  would  be  called  a  25-foot  line.  Where  more  wires  must 
be  carried,  30-  or  35-foot  poles  are  employed,  the  line  being  graded 
with  taller  poles  as  required.  The  following  discussion  will  apply 
equally  well  to  a  25-,  30-,  or  35-  foot  pole  line. 

The  diameter  of  a  pole  enters  as  a  factor  not  only  in  its  strength 
but  in  its  lasting  quality.  For  the  highest  grade  of  construction 
nothing  smaller  than  7-inch  tops  should  be  used,  but  on  less  impor- 
tant toll  lines  6-inch  and  even  5-inch  tops  are  sometimes  used. 

Route.  The  general  route  of  the  toll  line  preferably  follows  the 
highways,  but  frequently  the  route  may  be  made  much  shorter  or 
difficult  construction  may  be  avoided  by  locating  on  private  property. 
Wherever  this  is  done  permanent  rights  of  way  should  be  obtained 
from  the  property  owners  for  all  poles,  guys,  and  braces,  and  other 
fittings,  including  the  wires  that  are  on  the  private  property.  All 
rights  of  way  so  obtained  should  be  in  writing  and  should  include 
permanent  tree-trimming  privileges. 

Locating  Poles.  In  laying  out  the  line,  a  stake  should  be  driven 
firmly  in  the  ground  to  locate  each  pole,  guy  stub,  or  anchor.  It  is  a 
good  plan  to  number  these  stakes  in  order  that  full  data  as  to  the 


POLES  AND  POLE  FITTINGS  757 

kind  of  stub  or  anchor  may  be  kept  in  the  field  book  of  the  survey. 
It  is  not  the  usual  practice  to  employ  surveying  instruments  in  laying 
out  the  line,  the  survey  being  made  by  sighting  between  stakes.  On 
the  other  hand,  in  laying  out  long  lines,  a  transit  may  sometimes  be 
used  to  advantage,  although  there  is  danger  of  wasting  time  with  it 
if  the  man  using  it  attempts  to  do  too  fine  work. 

The  line  should  be  laid  out  as  straight  as  possible  and  where 
long  curves  occur  they  should,  as  far  as  possible,  be  reduced  to  straight 
sections  joining  each  other  at  corners.  Except  for  the  very  heaviest 
type  of  construction,  forty  poles  to  the  mile  is  a  good  average,  and 
this  means  that  the  poles  on  straight  sections  will  be  about  132  feet 
apart.  On  curves  and  corners  the  distance  between  poles  should  be 
shortened.  Wherever  the  angle  between  any  two  spans  is  30  degrees 
or  over,  the  distance  between  the  poles  of  those  spans  should  be  re- 
duced to  about  75  feet.  For  smaller  angles  the  distance  between 
poles  may  be  proportionately  greater.  Wherever  possible  right- 
angle  turns  should  be  made  on  two  poles,  that  is,  the  line  wires  should 
make  two  45-degree  bends  instead  of  one  90-degree  bend.  The 
span  adjacent  to  such  corner  in  each  case  should  be  reduced  to  about 
75  feet.  It  is  also  well  at  the  terminal  of  the  line  to  reduce  the  last 
span  to  75  feet. 

Frequently,  owing  to  the  contour  of  the  ground,  longer  spans 
must  be  employed.  Sometimes  as  the  line  approaches  a  ravine  the 
choice  must  be  made  between  running  the  line  down  into  the  ravine 
or  spanning  it  with  a  single  span.  If  the  depression  may  be  cleared 
with  a  span  of  about  200  feet,  this  is  to  be  preferred  to  the  use  of 
very  high  poles  in  the  bottom  of  a  depression  or  to  very  abrupt  changes 
in  the  level  of  the  line  that  would  occur  by  setting  poles  of  ordinary 
height  in  the  bottom  of  the  depression.  Spans  of  very  much  greater 
length  than  200  feet  may  be  employed  where  absolutely  necessary, 
but  such  spans  should  always  be  made  the  subject  of  special  study. 
Wherever  the  pole  line  changes  from  one  side  of  the  road  to  the  other, 
the  crossing  should  be  made  at  an  angle  of  about  45  degrees. 

On  railroad  rights-of-way  no  poles  should  be  set  less  than  a  dis- 
tance of  12  feet  from  the  outer  edge  of  the  nearest  rail,  and  in  any 
event  the  minimum  distance  must  always  be  subject  to  the  terms  of  the 
agreement  under  which  the  right-of-way  is  secured.  In  passing 
through  towns  or  cities  the  poles  should  be  located  as  generally  as 


758  TELEPHONY 

possible  at  corners  of  intersecting  streets  or  alleys,  so  as  to  facilitate 
the  employment  of  side  guys,  if  necessary,  and  also  to  facilitate  the 
branching  off  of  wires  to  other  pole  lines  if  the  necessity  for  such 
exists. 

Grading.  The  length  of  the  poles  is,  as  stated,  determined  by 
the  character  of  the  line  being  built,  the  shortest  poles  being  deter- 
mined by  the  number  of  cross-arms  that  are  to  be  carried.  Longer 
poles  are  used  where,  on  account  of  the  profile  of  the  country,  it  is 
necessary  to  do  so  in  order  to  avoid  abrupt  changes  in  the  level  of 
the  wires. 

The  number  of  poles,  longer  than  the  standard  of  the  line,  will 
depend  on  the  character  of  the  country  through  which  the  line  passes. 
In  general  it  may  be  stated  that  an  effort  should  be  made  to  accom- 
plish the  required  grading  by  the  use  of  as  few  poles  as  possible  that 
are  over  5  feet  longer  than  the  standard  pole  of  the  line.  The  use  of 
many  long  poles  is  not  only  expensive  but  such  poles  are  not  so  strong 
or  durable. 

For  sharp  depressions  that  are  too  wide  for  a  single  span  and  that 
would  ordinarily  require  the  use  of  extra  high  poles,  the  poles  should 
be  placed  close  enough  together  to  make  the  change  in  level  on  any 
pole  as  small  as  possible.  In  such  cases,  in  order  to  facilitate  the 
grading,  poles  as  short  as  20  feet  in  length  in  a  25-foot  line,  or  25 
feet  in  length  in  a  30-foot  line  may  be  used  on  the  highest  ground  ad- 
joining depressions. 

At  highway  crossings  the  poles  should  be  of  such  length  that  no 
wire  or  attachment  will  be  less  than  18  feet  above  the  crown  of  the 
highway.  Of  course,  local  ordinances  or  laws  may  require  a  greater 
height.  At  railway  crossings  the  height  of  the  wires  or  pole  attach- 
ments above  the  rail  should  not  be  less  than  28  feet. 

To  clear  obstacles,  poles  of  such  length  or  such  method  of  con- 
struction should  be  used  as  will  give  a  clearance  of  at  least  18  inches 
from  the  obstacles  when  all  of  the  arms  are  full  of  wires.  In  avoiding 
trees  or  other  obstacles,  side  arms  may  be  used  and  their  use  is  to  be 
preferred  to  the  use  of  very  high  poles.  The  side-arm  construction 
will  be  illustrated  later  in  connection  with  city  pole  line  work. 

Distributing  Poles.  In  distributing  poles  from  wagons  or  cars, 
the  heaviest  poles  should  be  placed  at  corners  or  bends  in  the  line  and 
at  the  terminals  of  long  spans.  The  straightest  and  best  looking 


POLES  AND  POLE  FITTINGS  759 

poles  should  be  employed  through  the  cities  and  towns,  and  partic- 
ularly in  front  of  good  residences. 

Equipping  Poles.  In  general,  it  is  better  to  attach  the  cross- 
arms  and  braces  to  the  poles  before  the  poles  are  set.  The  cross-arms 
are  fitted  with  the  standard  pins  and  with  the  braces  attached  at  one 
end  before  they  are  distributed.  The  pins  are  held  in  place  on  the  arms 
by  driving  a  wire  nail  through  the  arm  and  shank  of  the  pin  after 
the  pin  is  driven  home  in  the  arm.  After  the  cross-arm  is  attached 
in  position  on  the  pole  by  the  through  bolt,  it  is  squared  with  the  pole 
and  then  the  free  ends  of  the  braces  are  overlapped  on  the  pole  and 
attached  by  a  5-irich  lag  screw,  as  already  pointed  out,  thus  maintaining 
the  square  position  of  the  arm  on  the  pole. 

In  countries  where  lightning  storms  are  common,  it  is  a  good 
plan  to  equip  about  every  tenth  pole  with  a  lightning  rod,  which  may 
be  made  of  No.  10  B.  W.  G.  galvanized  iron  wire.  This  wire  may 
be  wrapped  two  or  three  times  around  the  extreme  butt  end  of  the  pole 
before  the  pole  is  set,  and  extended  up  the  pole,  being  attached  every 
two  feet  by  a  1-^-inch  galvanized-iron  wire  staple. 

Setting  Poles.  In  setting  the  poles  it  is  important  that  all  of 
the  holes  should  be  sufficiently  large  to  allow  the  butt  of  the  pole  to 
enter  without  scraping  in  so  much  dirt  from  the  side  of  the  hole  as  to 
partially  fill  it  up.  Sufficient  space  should  be  left  all  around  for  ade- 
quate tamping.  Also,  in  order  to  prevent  the  dirt  from  being  filled 
in  faster  than  it  can  be  properly  tamped,  it  is  well  to  employ  two 
tampers  for  each  shoveler.  The  soil  should  be  piled  up  above  the 
surface  and  packed  around  the  pole  approximately  12  inches  above 
the  surface  of  the  ground. 

On  straight  lines  the  poles  should  always  be  set  so  that  the  cross- 
arms  will  be  at  right  angles  to  the  direction  of  the  line,  and  the  arms 
on  adjacent  poles  should  face  in  opposite  directions.  The  reason 
for  this  is  to  prevent  the  strain  on  the  cross-arm  on  all  the  poles  being 
away  from  the  pole  rather  than  against  it,  in  case  such  a  condition 
should  arise  as  to  cause  a  heavy  pull  of  all  the  wires  in  one  direction. 
For  the  same  reason  the  arms  of  the  last  few  poles  at  the  end  of  a 
straight  lead  should  be  placed  on  the  side  of  the  pole  toward  the  end 
of  the  lead  so  as  to  make  them  all  pull  against  their  respective  poles. 

Guying  and  Bracing.  It  is  not  sufficient  to  rely  only  on  the 
strength  of  the  poles  or  on  the  firmness  of  their  setting  in  the  ground  to 


760 


TELEPHONY 


maintain  the  rigidity  of  the  line,  particularly  when  the  line  is  subjected 
to  the  stress  of  violent  storms.  In  order  to  give  the  line  greater  stability, 
therefore,  guys  or  braces  are  used.  Guys  may  be  defined  as  tension 
members  in  the  form  of  wires  or  ropes  extending  from  a  point  near 
the  upper  end  of  the  pole  to  some  stationary  object,  such  as  an  anchor, 
tree,  or  another  pole.  Braces  may  be  defined  as  compression  members, 
usually  of  wood,  extending  at  an  angle  from  a  point  high  up  on  the 
pole  to  a  solid  foundation  in  the  ground.  Guys  act  to  resist  the  forces 
which  tend  to  pull  the  pole  out  of  its  proper  alignment  by  the  tension 
of  the  guy  wire  or  rope.  Braces  act  to  resist  such  forces  by  the  com- 
pression in  the  brace  member. 

Guys  may  be  classed  as  side  guys  when  they  are  placed  at  right 
angles  to  the  direction  of  the  line  to  prevent  the  line  from  going  over 
sidewise;  as  head  guys  when  they  are  placed  in  the  direction  of  the 


Pig.  520.     Loi  Anchors 

line  to  prevent  the  line  from  going  down  endwise;  and  as  corner  guys 
when  they  serve  to  resist  the  pull  of  the  line  wires  on  corner  poles,  due 
to  the  bend  in  the  direction  of  the  line. 

For  bare-wire  line  construction  guys  may  be  of  No.  6  B.  W.  G. 
solid  wire  for  lighter  construction,  and  of  stranded  steel  for  heavier 
construction.  For  ordinary  25-  and  30-foot  pole  lines,  ^-inch  gal- 
vanized strand  is  an  excellent  material,  since  it  possesses  the  ade- 
quate strength  and  is  more  easily  handled  than  the  larger  sizes  of 
solid  wire.  Where  necessary,  two  or  more  strands  of  this  may  be 
used  in  order  to  resist  excessive  pulls. 


POLES  AND  POLE  FITTINGS 


761 


The  subject  of  anchors  is  an  important  one.  For  heavy  work 
the  practice  is  to  bury  anchor  logs  deep  in  the  ground,  wrought-iron 
anchor  rods  extending  from  these  logs  to  a  point  above  the  surface  of 
the  ground  in  the  direction  in  which  the  guy  wire  will  run.  In  Fig. 


Fig.  521.     Plank  Anchor 


Fig.  522.     Matthews'  Anchor 


520  are  shown  the  details  of  a  number  of  anchors  made  in  this  way. 
The  anchor  log  itself  is  usually  made  of  a  section  cut  from  a  pole. 


Fig.  523.     Setting  Matthews'  Anchor 


762 


TELEPHONY 


Railway  ties  also  make  good  anchor  logs.  For  lighter  construction 
an  anchor  may  be  made  of  2-inch  planks  nailed  together,  as  shown  in 
Fig.  521. 

Except  for  very  heavy  construction  some  of  the  many  forms  of 
patent  anchors  may  be  used  with  good  results  and  economy.    A 


Fig.   524.     Solid  Iron  Wire  Guy 

familiar  type  of  these  is  the  Matthews'  anchor,  which  Is  of  such  form 
as  to  bore  itself  into  the  ground  when  turned.  Such  an  anchor  is 
shown  in  Fig.  522,  and  the  method  of  setting  it  in  Fig.  523.  Other 
forms  of  patented  anchors  require  a  hole  to  be  drilled  by  an  earth 
auger,  after  which  the  anchor  is  put  in  place  and  the  tension  which  is 
put  upon  it  sets  the  body  of  the  anchor  crosswise  of  the  hole  in  such  a 
way  as  to  resist  its  being  pulled  out.  Still  another  type,  known  as  the 
D.  and  T.  anchor  (drive  and  twist)  is  put  into  the  ground  by  driving 
it  with  a  sledge,  and  it  is  then  set  or  expanded  by  twisting  on  the  rod. 


Fig.  525.     Stranded  Wire  Guy 


The  efficacy  of  any  of  these  patented  forms  of  anchors  depends 
largely  on  the  character  of  the  soil  in  which  they  are  used.  They 
are  not  effective  in  sand,  and  indeed  it  is  hard  to  get  an  anchor  that  is. 

The  method  of  attaching  solid  iron  guy  wire  to  a  pole  is  shown 
in  Fig.  524.  There  are  two  distinct  methods  of  attaching  a  stranded 
guy  wire  to  the  pole.  One  is  to  pass  the  guy  strand  twice  around  the 
pole  and  then  fan  out  the  separate  strands  and  wrap  each  about  the 


POLES  AND  POLE  FITTINGS 


763 


main  body  of  the  guy  strand,  as  shown  in  Fig.  525.  Another  is  to 
employ  guy  clamps,  as  shown  in  Fig.  526,  and  this  is  the  plan  in  gen- 
eral to  be  preferred. 


I 


I 


Fig.   526.     Stranded  Wire  Guy 

Attaching  the  guy  wire  or  rope  to  the  eye  of  the  anchor  rod  is 
usually  done  with  the  aid  of  a  thimble,  the  form  of  which  is  shown  in 
Fig.  527.  This  may  be  done  either  with 
or  without  guy  clamps,  Fig.  528  showing 
such  a  connection  made  with  the  use  of 
guy  clamps. 

One  method  of  attaching  a  guy  to  a 
tree  is  shown  in  Fig.  529.  In  rocky 
country  it  is  often  convenient  to  anchor 
a  guy  in  rock  and  the  manner  of  doing 
this  is  made  clear  in  Fig.  530. 

Sometimes,  as  where  a  guy  must 
necessarily  cross  a  road  or  sidewalk,  in- 

tc   •  i  u  u        a     J   J 

sufficient  clearance  would  be  anorded  un- 

der the  guy  wire  if  the  guy  were  run  directly  to  an  anchor.     In  such 

cases  guy    stubs    are  used.    These  are   in   effect  poles   set  in   the 


\f\ 


527-     Guy  Thimble 


Fig.  528.     Attaching  Guy  Wire  to  Anchor 


ground,  and  to  the  top  of  these  the  guy  wire  is  fastened.     The  guy 
stub  may  be  made  sufficiently  rigid   to   need   no  guying  itself  but 


764 


TELEPHONY 


nevertheless  it  is  preferable  to  guy  it  to  an  anchor  exactly  as  if 
it  were  a  pole.  Such  a  construction  is  shown  in  Fig.  531.  If  it  is 
not  feasible  to  anchor  the  guy  stub  because  of  lack  of  space  in  which 
to  place  the  anchor  or  guy  thereto,  the  guy  stub  may  be  made  extra 
heavy  and  set  very  deep  in  the 
ground,  the  setting  being  rein- 
forced by  concrete  or  by  heavy 
planks  placed  sidewise  across  the 
hole.  The  reinforcement  at  the 
top  of  the  hole  should,  of  course, 
be  toward  the  pole  to  which  the 
guy  runs  and  those  at  the  bot- 
tom of  the  hole  on  the  side  of 
the  stub  opposite  the  pole. 

In  locating  guy  anchors  the  horizontal  distance  from  the  butt  of 
the  pole  to  the  anchor  should  be  as  great  as  possible  up  to  a  distance 
equal  to  the  length  of  the  pole. 

The  length  of  the  anchor  rod  is  usually  about  8  feet,  and 
in  any  event,  sufficient  to  allow  the  eye  of  the  anchor  rod  to 

project    6    or    8    inches 


Fig.  529.     Attaching  Guy  to  Tree 


above  the  ground.  Prac- 
tice differs  as  to  the 
galvanizing  of  anchor 
rods,  some  advocating 
that  it  be  done  and 
others  claiming  that  it  is 
useless. 

On  a  line  containing 
one  cross-arm  only,  the 
guys  should  be  attached 
to  the  pole  at  a  point  just 
below  the  arm,  but  with 
two  or  more  cross-arms, 
the  guy  should  be  at- 
tached about  midway  between  the  bottom  and  the  top  arms. 

At  the  end  of  pole  leads  the  last  pole  should  be  guyed  to  an  anchor 
beyond  the  last  pole  and  in  the  direction  of  the  last  span,  and  head 
guys  should  be  run  from  a  point  near  the  top  of  each  of  the  last 


POLES  AND  POLE  FITTINGS  765 

few  poles  to  a  point  near  the  base  of  the  next  pole  toward  the  end 
of  the  line. 

Pole  braces,  or  push  braces,  as  they  are  called,  may  be  used  in 
cases  where  a  guy  is  objectionable  or  impossible.  Where  used,  the 
pole  to  be  braced  should  be  set  deeper  in  the  ground  than  usual. 
The  butt  of  the  brace  should  be  set  about  3£  feet  in  the  ground  and 
should  be  supported  on  a  heavy  plank  or  flat  rock  laid  in  the  bottom 
of  the  hole.  The  upper  end  of  the  brace  is  beveled  at  an  angle  to  fit 
snugly  against  the  pole;  but  in  no  case  should  the  pole  itself  be  cut 
away.  The  brace  is  attached  to  the  pole  by  a  standard  through 


Fig.   531.     Guy  Stub 

bolt  of  the  type  used  in  attaching  cross-arms.  Before  drawing  up 
the  bolt  the  end  of  the  brace  and  the  part  of  the  pole  against  which 
it  is  to  rest  should  be  given  a  coat  of  carbolineum  avenarius.  A 
standard  method  of  pole  bracing,  as  employed  by  some  of  the  Bell 
companies,  is  shown  in  Fig.  532.  In  this  the  butt  of  the  brace  is 
bolted  to  an  anchor  log  instead  of  resting  directly  against  it,  and  the 
brace  is  thus  enabled  to  resist  pulling  as  well  as  pushing  stresses. 
When  so  made  they  are  called  "pull-and-push"  braces. 

Tree  Trimming.  The  question  of  tree  trimming  is  a  troublesome 
one.  The  rights  of  property  owners  have  to  be  considered  and  too 
much  cannot  be  said  against  the  way  in  which  these  rights  have  been 
ignored  and  against  the  ruthless  destruction  of  shade  trees  that  often 
has  been  practiced  by  the  employes  of  telephone  companies.  On  the 


766 


TELEPHONY 


other  hand,  a  certain  amount  of  tree  trimming  is  necessary  and  it 
should  be  done  in  the  way  least  objectionable.  All  trees  close  to  the 
line  should  be  trimmed  so  as  to  clear  the  wires  by  a  distance  of  about 
2  feet  in  all  directions.  Dead  trees,  which  would  injure  the  line  by 
falling,  should  be  cut  down. 

Stringing  Wire.     The  insulator  used  for  bare  wire  lines  is  nearly 
always  of  glass.     The  standard  line  insulator,  shown  in  Fig.  533, 


Fig.  532.     Pole  Brace 

is  provided  with  an  internal  screw  thread  to  fit  the  thread  on  the 
wooden  pins.  The  groove  is  for  the  tie  wire  by  means  of  which  the 
line  wire  is  attached  to  the  insulator.  A  factor  in  the  design  of 
insulators  is  the  path  for  surface  leakage  from  the  wire  to  the  pin  and 
cross-arm.  In  dry  weather  the  pins  and  cross-arms  are  themselves  fairly 
good  insulators,  but  in  wet  weather  they  become  better  conductors. 
The  moisture  which  collects  on  the  insulator  also  forms  a  path  for 


POLES  AND  POLE  FITTINGS 


767 


leakage  and  the  "petticoat"  or  downwardly  hanging  flange  on  the 
glass  is  to  protect  the  pin  and  the  inner  surface  of  the  glass  from 
moisture  as  far  as  possible  and  to  afford  a  long  path  over  the  surface 
of  the  glass  from  the  wire  to  the  pin. 

Transpositions: — In  making  certain  forms  of  transpositions  in 
the  line  wires  and  also  at  test  points,  it  is  required  to  dead-end  two 
wires  on  the  same  insulator.  Insulators  with  two  grooves  are  used 
for  this  purpose  and  are  called  transposition  insulators.  Fig.  534 
shows  one  form  of  these,  two  grooves  being  formed  in  the  same  glass. 
A  better  form  is  that  of  Fig.  535  in  which  the  two  grooves  are  in  sep- 


Fig.  533.     Standard  Insulator 


Fig.  534.    Transposition  Insulator 


arate  glasses.  On  account  of  the  petticoat  of  the  upper  glass,  better 
insulation  is  maintained  between  the  two  wires,  and  another  advan- 
tage in  this  form  is  that  the  opposite  stresses  of  the  two  wires  are 
taken  by  the  pin  rather  than  by  the  structure  of  the  glass,  and  as  a 
result  there  is  less  breakage. 

Tying: — The  methods  of  tying  the  wire  to  the  insulator  differ 
for  iron  and  copper  wire.  In  neither  case  does  the  line  wire  pass 
around  the  insulator;  rather  it  runs  alongside  of  it  and  is  held  in  the 
groove  of  the  insulator  by  the  tie  wire  which  passes  around  the  insu- 
lator. The  form  of  tie  largely  employed  for  iron  wire  is  shown  in 
Fig.  536;  that  for  copper  wire  in  Fig.  537.  It  is  to  be  noted  that 
the  copper-wire  tie  is  now  being  largely  employed  for  iron  wire  as 


768 


TELEPHONY 


well  as  for  copper.  In  each  case  the  tie  wire  is  of  the  same  gauge 
and  metal  as  the  line  wire  which  it  ties,  but  in  each  case  it  should 
be  soft  annealed  instead  of  hard  drawn.  For  standard  insulators 
the  copper  tie  wire  should  be  about  19  inches  long  and  the  iron  tie 

wire  about  12  inches  long. 

Joints : — The  standard  method  of 
joining  iron  wire,  known  as  the 
Western  Union  joint,  is  shown  in 
Fig.  538.  In  this  the  ends  of  the 
two  wires  to  be  joined  are  laid  side 
by  side,  in  a  pair  of  special  pliers. 
The  wires  are  then  twisted  by 
means  of  the  pliers  to  make  five 
complete  turns,  forming  what  is 
known  as  the  neck  of  the  splice. 
After  this  operation  the  splice  is 
completed  by  wrapping  each  end 
tightly  around  the  straight  section 
of  the  other  wire  four  or  five  turns. 
Tests  of  various  splices  show  that 
the  end  turns  have  very  little  virtue 
in  them,  most  of  the  holding  power 

Fig.   535.     Transposition  Insulator  .  •        i  i 

being  due  to  the  turns  in  the  neck, 

and  that  a  joint  with  five  properly  made  turns  in  the  neck  will  be  as 
strong  as  the  wire  it  is  made  of,  and  will  yield  but  slightly  at  first 
or  until  it  is  set,  after  which  there  is  practically  no  yield  up  to  the 
breaking  point. 

Copper  wire  is  usually  joined  by  means  of  the  Mclntyre  sleeve, 
which  consists  essentially  in  two  parallel  tubes  of  copper  about  4 
inches  long  secured  together  throughout  their  length,  the  internal 
diameter  of  the  tubes  being  such  as  to  just  accommodate  the  size  of 
wires  to  be  spliced.  To  make  the  joint  by  means  of  this  connector, 
the  two  wires  are  run  through  the  parallel  tubes  in  opposite  directions 
and  then  each  end  of  the  sleeve  is  grasped  in  a  special  clamp  and  the 
sleeve  twisted  through  three  complete  turns.  The  Mclntyre  sleeves  be- 
fore making  a  connection  and  a  completed  joint  are  shown  in  Fig.  539. 

Sag: — In  stringing  line  wires  it  is  desirable  that  all  wires  in  a 
span  have  a  uniform  sag.  Obviously,  the  amount  of  sag  will  depend 


POLES  AND  POLE  FITTINGS 


769 


Fig.  538.     Western  Union  Joint 


Fig.  539.     Mclntyre  Joint 


770 


TELEPHONY 


TABLE    XXVI 
Sag  at  Time  of  Erecting 


TEMP. 

DEGREES 
F. 

10 

LENGTH  OF  SPAN 

75' 

100' 

115' 

130' 

150' 

200' 

H 

3 

Sag  in 
3* 

Inches 
4* 

6 

10* 

30 

2 

3 

4 

5£ 

7 

12 

60 

2* 

*i 

5} 

7 

9 

15^ 

80 

3 

5* 

7 

8| 

H| 

19 

100 

4£ 

7   • 

9 

11 

14 

22.1 

on  the  length  of  span  and,  for  a  given  degree  of  initial  tightness,  on 
the  temperature.  Since  the  wires  are  shorter  in  winter  than  in  sum- 
mer, wires  that  are  pulled  too  tight  in  the  summer  time  may  become 
so  tight  as  to  break  during  the  winter.  Table  XXVI  shows  good 
practice  with  respect  to  the  sag  to  be  allowed  for  different  lengths 
of  span  erected  at  different  temperatures. 


Fig.  540.     Dead-Ending  with  Sleeve  Joint 

Dead-Ending : — At  the  ends  of  lines,  at  test  points,  and  sometimes 
at  transposition  points  it  is  necessary  to  dead-end  the  line  wire. 
The  method  of  dead-ending  copper  line  wire  is  indicated  in  Fig.  540. 
In  this  a  Mclntyre  sleeve  of  one-half  the  usual  length  is  employed. 
The  iron  wire  dead  end  is  shown  in  Fig.  541,  and  is  made  without  a 
sleeve,  the  wire  being  given  two  complete  wraps  around  the  insulator 
and  then  twisted  around  itself,  as  shown. 


POLES  AND  POLE  FITTINGS  771 

Test  Points: — To  facilitate  testing  on  through  lines,  it  is  com- 
mon to  establish  test  points  at  which  the  line  wire  is  cut  and  dead- 
ended  and  connected  by  some  form  of  connecting  clamp,  which  the 


Fig.  541.     Dead-Ending  Without  Sleeve  Joint 

tester  may  readily  open  in  making  his  tests.  A  form  of  this  construc- 
tion is  shown  in  Fig.  542. 

Scheme  of  Transposition.  The  necessity  for  transposing  upon 
circuit  wires  has  already  been  dealt  with.  The  scheme  of  transpo- 
sition differs  lor  the  various  cross-arms  as  well  as  for  the  different 
pairs  on  any  one  cross-arm.  The  reason  for  this  is  to  provide  for 


Fig.  542.     Test  Connection 


inductive  neutrality  between  the  wires  above  and  below  each  other 
as  well  as  those  alongside  of  each  other. 

The  poles  on  which  transpositions  occur  are  known  as  transpo- 
sition poles,  and  they  are  located  at  about  1,300-foot  intervals.     The 


772 


TELEPHONY 


length  of  a  transposition  section  for  the  present  standard  scheme 
employed  by  the  Bell  companies  is  8  miles  and  includes  32  trans- 
position poles.  The  scheme  for  one  such  section  is  shown  in 
Fig.  543,  this  being  repeated  in  each  section. 


\J-4O 


Fig.  543.     Transposition  Scheme 


There  are  two  general  methods  of  making  transpositions,  one 
of  which  requires  the  dead-ending  of  both  wires  at  the  transposition 
pole  and  the  crossing  over  of  their  free  ends.  With  this  method 

double-groove     insulators     are 
employed  at    the  transposition 
point,    each  of   the   right-hand 
wires  being    dead-ended    in    a 
groove  on  each  of  the  insulators 
and  the  two  left-hand  wires  in 
the   other  grooves    of    the    in- 
sulators.    This  method  is  shown  in  Fig.  544,  the  dead-ending  and 
splicing  of  the  free  ends  being  done  by  means  of  Mclntyre  sleeves. 

The  other  method  of  transposing,  known  as  the  single-pin  or 
running  transposition,  may  be  done  in  two  ways,  as  shown  in  Figs. 
545  and  546.  Of  the  two  methods,  the  one  shown  in  Fig.  545  is  to 


Fig.  544.     Two-Pin  Transposition 


POLES  AND  POLE  FITTINGS 


773 


be  preferred,  since  it  does  not  involve  cutting  the  wires;  however,  it 
can  only  be  made  when  the  wires  are  being  run  out.     When  it  is  neces- 


Fig.  545.     Running  Transposition 


sary  to  make  the  transposition  after  the  wire  has  been  strung,  the 
method  shown  in  Fig.  546  is  employed. 

In  the  running  transposition  the  wires  on  the  transposition  arm 


Fig.  546.     Running  Transposition 


and  the  arms  adjacent  to  it  on  each  side  should  pull  on  the  pins  and 
not  on  the  tie  wires.  On  straight  sections  of  the  line,  the  outside  pin 
is  used  for  transpositions  and  the  inside  pin  is  left  idle. 


774 


TELEPHONY 


Rural  Lines.  For  the  connection  of  rural  subscribers  in  the 
outlying  districts  of  cities  or  towns  such  a  high  grade  of  construction 
as  is  employed  on  through  toll  lines  is  not  warranted.  The  number 
of  wires  carried  on  such  lines  is  small  and  the  service  is  relatively 
unimportant.  By  this  it  is  not  meant  that  there  is  any  justification 
for  the  miserably  constructed  lines  often  found  along  country  roads 
and  aptly  termed  "bean-pole  lines."  It  is  perfectly  feasible  to 
build  lines  for  such  rural  service  of  shorter 
and  smaller  poles  than  those  required  for  more 
important  lines,  and  to  space  the  poles  farther 
apart,  thus  securing  a  cheaper  yet  adequate 


Fig.  547.     Insulator 
Bracket 


For  such  work  the  Bell  operating  com- 
panies in  certain  sections  have  adopted  as  a 
standard  what  they  term  a  22-foot  line.  Other 
companies  employ  20-foot  poles.  A  fair  sized 
pole  for  this  work  is  a  20-  or  22-foot  pole  with 
a  5-inch  top.  A  fair  spacing  is  thirty  to  a  mile. 
Where  the  line  is  to  carry  but  two  or  three 
wires,  as  is  often  the  case,  there  is  no  need  of 
cross-arms  at  all,  the  wires  being  supported 
from  brackets  nailed  directly  to  the  pole.  The  details  of  such  a  bracket 
are  shown  in  Fig.  547.  For  a  two-wire  line,  the  location  of  brackets 
on  the  pole,  on  straight  sections,  and  on  curves  is  shown  in  Fig  548, 
the  view  at  the  left  showing  the  arrangement  on  straight  sections  and 
that  at  the  right,  on  curves.  The  reason  for  placing  the  brackets 
both  on  the  same  side  of  the  pole  on  curves  is  so  that  both  wires  may 
pull  towards  the  pole.  If  more  wires  are  to  be  carried,  a  6-  or  8-pin 
cross-arm,  or,  if  the  poles  are  heavy  enough,  a  10-pin  cross-arm  may 
be  employed. 

The  wire  on  such  a  line,  if  short,  may  well  be  of  iron,  although 
there  are  certain  localities  having  much  coal  gas  in  the  atmosphere 
where  it  has  been  proven  that  iron  wire  will  last  only  a  few  years. 
In  other  places  iron  wire  may  easily  have  a  life  of  from  ten  to  fifteen 
years,  and  it  is  often  to  be  expected  that  before  this  time  shall  have 
elapsed,  the  cheap  pole  line  will  have  been  replaced  by  one  of  more 
substantial  construction,  owing  to  the  extended  requirements  of  the 
growing  community. 


POLES  AND  POLE  FITTINGS 


775 


City  Exchange  Lines.  The  requirements  for  modern  city  aerial 
lines  differ  from  those  of  cross-country  toll  lines,  in  that  they  are 
generally  required  to  carry  fewer  or  no  bare  wires  and  to  carry  cables. 
Frequently,  and  this  practice  is  growing,  city  lines  carry  no  bare 
wires  at  all  and  no  cross-arms  unless  cross-arms  are  needed  for  sup- 
porting the  cables. 

Poles.  All  that  has  been  said  regarding  the  preparation  of  the 
pole  in  the  first  portion  of  this  chapter  will  apply  to  poles  for  use 
in  city  lines.  In  addition  to  this  it  is  considered  good  practice  to 
paint  all  poles  with  one,  or  better  two,  coats  of  good  oil  paint,  an 
excellent  paint  for  this  purpose  being  known  as  Acheson  graphite. 
A  few  words  may  be  said  as  to  the  color  of  city  poles.  It  is 


Fig.  548.     Location  of  Brackets 

usual  to  paint  them  white,  or  the  upper  portion  of  them  white  and  the 
portion  6  feet  above  the  ground,  dark  green  or  black.  Such  prac- 
tice makes  the  poles  as  conspicuous  as  possible,  and  while  this  may 
be  gratifying  to  the  construction  man  or  to  the  exchange  owners, 
it  is  seldom  so  to  the  nearby  property  owners.  It  is  believed  that  a 
better  practice  is  to  paint  them  a  dull  black  or  very  dark  green  all  over. 
This  renders  them  inconspicuous  and  takes  away  the  startling  appear- 
ance of  newness  which  often  serves  to  fan  the  flame  of  dissatisfaction 
of  the  citizen. 


776  TELEPHONY 

TABLE  XXVII 
PoIe=Step  Data 


25  foot  poles 7  iron  steps 

30  foot  poles 10  iron  steps 

35  foot  poles 13  iron  steps 

40  foot  poles 16  iron  steps 

45  foot  poles 19  iron  steps 

50  foot  poles 22  iron  steps 

55  foot  poles 25  iron  steps 

60  foot  poles 28  iron  steps 

65  foot  poles 31  iron  steps 


Stepping: — As  has  been  said,  city  poles  should  be  stepped  as 
they  have  to  be  climbed  more  frequently  than  those  in  cross-country 
runs  and  the  marks  of  the  climbing  spurs  of  the  linemen  are  detri- 
mental, not  only  to  the  appearance  of  the  pole,  but  to  its  life,  and 
this  is  particularly  true  where  the  pole  is  painted.  The  method  of 
spacing  the  steps  is  shown  in  Fig.  519. 

For  guidance  in  boring  the  holes  for  the  iron  steps,  and  for  mak- 
ing the  estimates  as  to  the  number  of  steps  required  Table  XXVII 
is  given.  This  table  is  frequently  convenient  in  another  way.  In 
poles  that  have  already  been  set  it  is  often  difficult  to  judge  their 
height,  and  it  may  be  necessary  to  ascertain  this  either  in  planning 
other  construction  that  is  to  go  over  or  under  a  given  lead  or  in  mak- 
ing appraisals.  A  knowledge  of  the  number  of  iron  steps  for  given 
heights  of  poles  will,  therefore,  enable  one  to  arrive  quite  accurately 
at  the  height  of  the  pole  by  merely  counting  the  steps. 

Locating: — In  setting  poles  along  streets,  the  general  location 
will  be  within  the  curb  line.  Where  no  curbing  exists  at  the  time  of 
setting,  or  in  case  the  present  curb  line  is  likely  to  be  changed,  its 
ultimate  location  should  be  obtained  from  the  city  engineer  or  proper 
city  authority. 

In  setting  poles  within  the  curb,  it  is  desirable  to  maintain  a  sep- 
aration of  at  least  6  inches  between  the  nearest  point  of  the  pole  and 
the  curb  to  avoid  throwing  the  curb  out  of  alignment  by  movement 
of  the  pole. 

In  setting  poles  on  streets  where  their  is  an  existing  pole  line 
of  another  company,  it  is,  of  course,  preferable  to  take  the  opposite 
side  of  the  street;  but  where  the  two  must  be  placed  on  the  same  side 


POLES  AND  POLE  FITTINGS 


777 


of  the  street,  the  two  sets  of  poles  should  be  set  in  the  same  line  so  as 
to  preserve  as  far  as  possible  a  sightly  appearance  for  both  leads. 
In  alleys  the  poles  should  in  general  be  set  outside  of  and  as  close  as 
possible  to  the  abutting  private  property.  When  placed  on  private 
property  the  poles  should  be  set  to  conform  with  the  requirements 
stated  in  the  permit  from  the  owner. 

Crossings : — When  a  pole  line  for  cables  must  cross  from  one  side 
of  a  street  to  the  other,  the  crossing  is  preferably  made  at  a  street  or 
alley  intersection  and  parallel  with  the  intersecting  street  or  alley, 
as  shown  in  Fig.  549.  The  reason  for  this  is  that  the  intersecting 
street  or  alley  affords  room  for  guys,  and  if  an  intersecting  line  is  to 
be  constructed  on  this  cross-street  the  two  poles  on  which  the  crossing 
is  made  will  be  available  for  that  line. 

The  poles  at  steam  railroad  crossings  should,  if  possible,  be  so 
located  that  the  span  over  the  crossing  will  not  exceed  100  feet  in 


Gur  tote 


<  i  SVY  rote: 


Fig.  549.     Changing  Sides  of  Street 

length,  and  in  all  cases  it  must  conform  to  the  municipal  regula- 
tions or  agreements  with  the  railroad  company.  The  poles  at  such 
crossings  should  be  no  closer  than  7  feet  from  the  nearest  rail,  and, 
if  possible,  the  crossing  should  be  at  right  angles  to  the  tracks. 

Grading: — On  comparatively  level  ground  in  cities,  the  poles 
should  be  graded  so  as  to  avoid  changes  of  more  than  5  feet  in  the 
level  of  the  wires  and  cables  on  adjacent  poles.  In  general,  in  city 
work  it  is  good  practice  to  make  poles  as  low  as  possible.  This  is 
directly  contrary  to  the  plan  that  has  been  followed  in  many  eastern 
cities  where  lines  of  60-  and  70-foot  poles  are  an  eye-sore  and  a  men- 


-    778 


TELEPHONY 


ace  to  the  public.     It  is,  of  course,  necessary  to  make  all  poles  of 
such  height  as  to  give  the  cables  and  wires  which  they  support  the 


clearance  required  by  city 
should  clear  all  obstacles, 
wires,  by  at  least  4  feet  if  pos 
should  be  always  allowed  to 

Distributing :— The  same 
tribution  of  poles  should  be 
forth  for  cross-country  work, 
eration  that  the  most  sightly 
streets  and  the  worst  looking 
consideration  for  the  per 
owners  is  not  only  morally 
proven  to  be  the 
policies.  } /'''-':  / 

Mechanical      ^^l^'^flH'.rKi|nfJHrjpjrH         Protection :— Where 
there  is  danger  of       Fig.  550     Butt  Protector  or        poles  being  injured 

&  Hub  Guard  J 

by    hubs    of    wag  ons,  butt  protectors, 

consisting  of  half  cylinders  of  heavy  sheet  iron,  should  be  attached 
to  the  pole  at  such  a  height  as  to  receive  the  impact  from  the 
hubs  of  passing  wagons.  Such  a  butt  protector  on  a  pole  is  shown 


ordinances.  Cables  and  wires 
and  electric  light  and  power 
sible,  and  enough  separation 
prevent  swinging  contacts, 
considerations  as  to  the  dis- 
given  in  city  work  as  set 
with  the  additional  consid- 
poles  should  be  placed  on  the 
ones  in  the  alleys.  A  keen 
sonal  feeling  of  property 
right,  but  it  has  been  amply 
best  of  commercial 


in  Fig.  550.  Where  poles 
them  likely  to  be  used  as 
particularly  in  village 
should  be  installed  to 
ing  them.  A  pole  so  pro 
For  the  protection  of 
injury  which  might  occur 
wires  and  for  the  protec 
it  is  customary  in  cities  to 
over  the  guy  wire  for  a 
above  the  ground.  Some 
ing  an  iron  pipe  over  the 
in  place,  but  the  more 
a  cheap  wooden  box 
this  having  the  ad 
readily  applied  after 
A  guy  so  protected  is 


Fig.  551.     Cribbing 
Guards 


are  so  erected  as  to  make 

hitching  posts  for  horses, 

streets,    cribbing    guards 

prevent  the  horses  gnaw- 

tected  is  shown  in  Fig.  551. 

persons  and  teams  against 

from    running    into    guy 

tion  of  the  guy  wire  itself, 

place  a  protecting  guard 

distance   of   about  8  feet 

times  this  is  done  by  slid- 

guy  wire  before  it  is  tied 

usual   method    is   to   put 

around  the  guy  wire, 

vantage      of      being 

the   guy  is  in   place. 

shown   in    Fig.  552. 


POLES  AND  POLE  FITTINGS 


779 


Where  guys  pass  close  to  electric  light  or  trolley  wires,  they 
should  be  encased  in  a  rubber  hose  for  a  distance  of  about  4  feet  on 
each  side  of  the  point  of  crossing.  The  hose  should  be  lashed  in 
place  by  marline  bound  tightly  to  the  wire  at  each  ~  end  of 
the  hose.  Where  it  is  possible  to  put  the  protecting  0  hose 
on  before  erecting  the  wire,  it  is  preferable  to  ff  merely 
slide  the  hose  over  the  wire,  but  where,  as  is  a  usual,  the 
protector  is  put  up  after  the  erection  of  the  jjj$j9  guy  wire,  the 
hose  may  be  split  and  slipped  over  the  guy  //fa/  wire,  and  then 
lashed  in  place  with  marline,  the  lash  / jj/  mgs  extending 
throughout  the  entire  length  of  the  hose  /$Jj/  and  occurring 
about  every  4  inches. 

Alley  Arms: — Where  bare  wires 
the  poles   in  city  construction,    the 
and  equipping  the  cross-arm  and 
the  wires,  as   already  described, 
in   city   work,    particularly   in 


are  to  be  carried  on 
method    of    mounting 
of  stringing   and   tying 
is  employed.    Frequently, 
alleys,   the   pole   line  must 


necessarily     be     located     so 
abutting    the    alley    as    to 
the  standard  mounting  of 
the   so-called  alley  arm 
merely  in  an  arm  of 


close   to    walls    of   buildings 
leave  insufficient  room  to   use 
cross-arms.      For    this    purpose 
or    side    arm    is    used   consisting 
standard    length  and   pin    spacing, 


attached  to  the  pole 
alley,  and  secured 
so    equipped     is 
Messengers. 
wires     on    a 
sufficient  me 


at  one  end  so  as  to  project  out  over  the 
by  a  suitable  angle-iron  brace.    A  pole 
shown  in  Fig.  553. 

Usually  in  city  work  the  bulk  of  the  line 
pole  is  carried  in  cables.  As  cables  have  not 
chanical  strength  to  support  themselves  when 
hung  between  poles,  it  is  neces- 
sary to  provide  supporting  strands 
consisting  of  steel  wire  or  steel 
wire  rope  supported  by  the  poles. 
To  this  "messenger  wire"  the 
cable  is  fastened  at  short  intervals,  usually  of  about  18  inches,  by 
some  form  of  "cable  hanger." 

Sizes  and  Grades : — The  sizes  of  stranded  messenger  wire  range 
from  £  inch  to  £  inch  in  diameter  and,  for  supporting  smaller  cables, 
a  solid  steel  wire  No.  4  or  No.  6  B.  W.  G.  is  frequently  used.  Such 


Fig.  552.     Protected  Guy 


780 


TELEPHONY 


TABLE  XXVIII 
Size  and  Breaking  Weight  of  Messengers 


STRANDED 

SOLID 

SIZE 

i" 

7  // 
T* 

1" 

iV' 

\" 

.225" 

.192" 

Weight  per  1000  feet 

520 

400 

300 

220 

130 

134 

98 

Bessemer  Steel 

9800 

7600 

5700 

4200 

2500 

Siemens-Martin  Steel 

11000 

9000 

6800 

4860 

3056 

3500 

2500 

High  Strength  Steel 

18000 

15000 

11500 

8100 

5100 

5900 

4300 

Plow  Steel 

27000 

22500 

17250 

12100 

7600 

8000 

6000 

messenger  wire,  whether  stranded  or  solid,  is  made  in  different 
grades,  which  vary  greatly  as  to  breaking  strength.  The  principal 
standard  market  grades  are  Bessemer,  Siemens-Martin,  "high 
strength"  and  "plow  steel,"  these  increasing  in  strength  in  the  order 
mentioned.  The  weights  per  thousand  feet  and  the  breaking  strengths 
of  each  of  these  grades 
in  different  sizes,  both  -9- 
stranded  and  solid,  are 

given  in  Table  XXVIII.     889  |  99        98      911149      9 

Bessemer  steel  strand 
is    the  cheapest,  and  al- 
though frequently  employed,  it  is  finding 
less  favor  as  a  messenger  strand  on  ac- 
count of  its   liability  to  flaws.      It  has 
a    tensile    strength    of    about    60,000 
pounds  per  square  inch. 

Siemens-Martin  steel  is  an  open-hearth  process 
steel,  and  has  a  tensile  strength  of  90,000  pounds 
per  square  inch,  and  sometimes  considerably  more. 
It  is  very  uniform  and  the  likelihood  of  flaws  is 
remote.      This    is    a    thoroughly  satisfactory   steel   for    messenger 
wires  in  all  cases,  except  where  the  very  greatest  strength  is  neces- 
sary. 

The  other  grades,  known  as  the  high  strength  and  plow  steel, 
have  tensile  strengths  of  approximately  150,000  and  220,000  pounds 
per  square  inch,  respectively. 


Fig.  553.  Side  or 
Alley  Arms 


POLES  AND  POLE  FITTINGS 


781 


Good  practice  in  respect  to  the  sizes  and  the  grades  of  messenger 
wire  for  all  ordinary  spans  may  be  outlined  as  follows,  the  grade  of 
messenger  in  each  case  being  Siemens-Martin. 

For  10  to  30  pair,  22-gauge  cables  inclusive,  or  their  equivalents  in 
weight,  No.  4  B.  W.  G.  solid  wire. 

For  40  to  100  pair,  22-gauge  cables  inclusive,  or  their  equivalents  in 
weight,  f-inch  steel  strand. 

For  larger  cables  up  to  200  pair,  22-gauge,  or  their  equivalents  in  weight, 
iVinch  steel  strand. 

For  all  cables  heavier  than  200  pair,  22-gauge,  £-inch  steel  strand. 

This  practice  provides  for  a  very  large  factor  of  safety,  the  strands 
being  of  ample  strength  to  provide  for  changes  in  temperature,  wind 
and  sleet  storms,  and  for  other  contingencies. 

Methods  of  Attaching : — The  messenger  is  attached  to  each  pole, 
except  the  poles  on  which  it  is  dead-ended,  by  means  of  messenger 
supports  of  various  forms.  Most  pole  lines  in  city  work  carry  but 
a  single  cable  and  the  messenger  support  in  such  cases  is  usually  of 
malleable  or  wrought  iron  secured  directly  to  the  pole  by  lag  or 
through  bolts.  Standard  forms  of  mes- 
senger supports  for  this  class  of  work  are 
shown  in  Figs.  554  and  555. 

Under  ordinary  circumstances  on 
new  pole  lines  the  messenger  may  be 
attached  12  inches  from  the  top  of  the 
pole,  except  in  cases  where  the  poles  are 
of  extra  height,  so  as  to  provide  for  the 
other  fixtures,  such  as  distributing  ter- 
minals, in  which  case  the  messenger  may 
be  mounted  as  far  below  the  top  as  is 
desired.  The  messenger  wire  should  be 
graded  as  far  as  possible  to  avoid  too  abrupt  changes  in  level. 
It  should  always  be  attached  to  the  pole  at  such  a  height  as  to 
afford  a  clearance  of  not  less  than  20  feet  over  the  crown  of  road- 
ways when  the  cable  is  in  place,  and  a  greater  distance  may  be  re- 
quired by  city  ordinances.  Where  messengers  intersect  each  other, 
as  in  the  case  of  pole  lines  crossing  at  right  angles,  care  should  be  taken 
in  placing  the  messengers  on  the  four  corner  poles  so  that  they  will 
cross  on  the  same  level. 


Fig.  554.     Messenger  Support 


782 


TELEPHONY 


Where  a  messenger  is  erected  on  a  pole  line  carrying  open 
wires  or  cross-arms,  it  should  not  be  less  than  18  inches  below  the 
lowest  arm  unless  the  clearance  over  the  roadway  demands  that  it 
be  placed  higher  up.  This  clearance  below  the  arm  is  desirable 
so  as  to  afford  room  for  placing  terminals  and  distributing  brackets 
for  service  wires  extending  to  subscriber's  premises.  Where  it  is 
impossible  to  attach  the  messenger  this  far  below  existing  cross-arms, 
it  must,  of  coujrse,  be  placed  where  available  space 
exists. 

In  crossing  steam  railroad  tracks,  the  messen- 
ger should  be  so  placed  that  the  cable,  when 
suspended,  shall  clear  the  top  of  the  rail  by  not 
less  than  28  feet. 

Sag  and  Strains: — There  is  much  difference 
in  opinion  and  practice  as  to  the  amount  of  sag 
that  should  be  allowed  in  cable  supporting 
messengers.  Some  construction  men  believe  in 
having  the  messenger  wire  initially  as  tight  as 
possible.  This  has  been  referred  to  as  "fiddle 
string"  construction.  There  seems  to 
be  no  good  reason  for  it.  It  subjects 
the  entire  pole-line  structure  to  an  un- 
necessary initial  strain,  and  is  partic- 
ularly severe  on  guys  and  anchors. 
About  the  only  thing  that  can  be  said 
in  its  favor  is  that  it  presents,  as  long 
as  it  remains  in  place,  a  trim,  ship-shape  appearance.  The 
extreme  in  the  other  direction  is  to  hang  the  messenger  loosely  be- 
tween poles  with  a  very  large  sag,  so  that  the  cable  is,  as  it  were,  fes- 
tooned along  the  pole  line.  The  resulting  construction  is  not  so 
pleasing  in  appearance  and  it  is  subject  to  the  criticism  that  it  per- 
mits a  considerable  swinging  of  the  span  by  the  wind,  which  may 
eventually  injure  the  cable  sheath.  Whether  there  is  any  real  cause 
for  this  criticism  is  to  be  doubted,  as  this  loose  construction  method 
is  extensively  practiced  in  some  large  cities  and  no  injury  seems  to 
result.  Its  advantage  is  greatest  in  short  runs,  consisting  of  but  a 
few  poles  each,  where  conductors  of  an  underground  cable  are  led  up 
to  aerial  cables  for  distribution.  Where  the  aerial  cables  are  light, 


Fig.  555.     Messenger  Support 


POLES  AND  POLE  FITTINGS 


783 


TABLE    XXIX 

Minimum  Sag  in  Regular  Spans 


SPAN  IN  FEET 

MAIN  LINES 

DISTRIBUTING  LINES 

50 

4i" 

3*" 

60 
70 

6  " 

8  " 

4r 

80 

11" 

9  " 

90 
100 
110 

m" 

ni" 

i'  14" 

1'  5  " 

120 

2'0  " 

1'  8  " 

130 

2'3i" 

2'  0  " 

140 

2'9  " 

2'  3  " 

150 
160 

3'14" 
3'6  " 

2'  6^" 
3'  0  " 

the  very  loose  suspension  makes  it  possible  to  do  away  largely  with 
all  guying,  the  poles  being  made,  in  all  cases,  sufficiently  heavy  and 
well  set  to  be  self-supporting. 

Under  ordinary  circumstances  where  terminal  guys  are  used, 
Table  XXIX  represents  an  intermediate  practice  in  the  matter  of 
allowable  sag,  it  being  understood  that  the  sag  mentioned  is  that 
after  the  cable  has  been  put  up.  These  sags  are  worked  out  to  give 
uniform  tension  in  all  spans. 

For  short  pole  leads  carrying  light  cables,  No.  4  or  No.  6  B.  W.  G. 
wire  may  be  used.  Table  XXX  of  sags  has  been  worked  out 
for  various  spans  and  for  10,  20,  and  35  pair  cables,  with  the  idea  of 
in  no  case  subjecting  the  supporting  strand  to  more  than  700  pounds 
stress,  which,  under  normal  conditions,  would  not  place  a  stress  of 
more  than  1,000  pounds  on  the  terminal  guys. 

Where  this  practice  is  followed  the  No.  6  Siemens-Martin  wire 
is  of  sufficient  strength  for  both  messenger  and  guying,  since  this 
wire  has  a  breaking  strength  of  about  2,500  pounds.  It  will  be  un- 
derstood that  unless  the  terminal  guy  on  such  a  lead  is  in  the  same 
direction  as  the  suspension  strand,  both  vertically  and  horizontally, 
the  stress  on  the  terminal  guy  is  always  greater  than  that  on  the  sus- 
pension strand  or  wire.  This  stress  on  the  guy  increases  as  the  angle 
between  the  guy  and  the  pole  becomes  smaller,  and  Table  XXXI 
gives  a  factor  by  which  the  strain  on  suspension  strands  should  be 


784 


TELEPHONY 


TABLE  XXX 
Sags  for  Various  Spans 


LENGTH  OF  SPAN 
IN  FEET 

SAG  IN  FEET  FOR 

10  PR.  CABLE 

20  PR.  CABLE 

35  PR.  CABLE 

50 

.35 

.45 

.6 

60 

.51 

.65 

.85 

70 

.7 

.87 

1.15 

80 

.91 

1.15 

1.5 

90 

1.1 

1.41 

1.9 

100 

1.43 

1.8 

2.3 

110 

1.75 

2.15 

2.8 

120 

2.06 

2.6 

3.3 

130 

2.5 

3. 

3.9 

140 

2.8 

3.5 

4.5 

150 

3.2 

4. 

5.2 

160 

3.66 

4.6 

6. 

170 

4. 

5.15 

6.75 

180 

4.6 

5.8 

7.5 

190 

5.15 

6.4 

8.4 

200 

5.7 

7.15 

9.3 

multiplied  in  order  to  give  the  strain  on  the  guy. 

It  goes  without  saying  that  where  the  poles  are  self-supporting 
the  stresses  on  them  should  be  reduced  to  a  minimum  and  the  cables 

TABLE  XXXI 
Factor  for  Strain  on  Suspension  Strand 


WHEN  DISTANCE  FROM  ANCHOR  GUY  AT  GROUND  EQUALS 

FACTOR 

The  height  of  guy  on  pole  above  ground  

1   4 

Two-thirds  the  height  of  guy  above  ground  

1  8 

One-third  the  height  of  guy  above  ground  

3  2 

should  be  supported  as  low  down  on  them  as  possible.  The  proper 
sag  will  depend  upon  the  size  of  the  poles,  the  height  of  the  cable 
above  the  ground,  and  the  character  of  the  setting  of  the  pole. 
Table  XXXII  will  be  found  useful  in  determining  the  strain  that 
may  be  expected,  due  to  any  given  sag  on  a  given  size  of  cable. 

To  find  the  sag  for  any  other  span  such  as  will  give  the  same 
tension  in  the  wire,  divide  the  new  span  length  by  100,  square  the 
result,  and  multiply  by  the  sag  given  in  Table  XXXII. 


POLES  AND  POLE  FITTINGS 


785 


TABLE  XXXII 
Strain  at  Center  of  100  Foot  Span 

Using  No.  6  B.  W.  G.  Steel  Wire 


SAO 

10  PR.  CABLE 
.8  LB.  PER  FT. 

20  PR.  CABLE 
1  LB.  PER  FT. 

35  PR.  CABLE 
1.3  LB.  PER  FT. 

60  PR.  CABLE 
1.7  LB.  PER  FT. 

2.0  feet 

500  pounds 

630  pounds 

800  pounds 

1070  pounds 

2.5  feet 

400  pounds 

525  pounds 

650  pounds 

845  pounds 

3.0  feet 

340  pounds 

420  pounds 

560  pounds 

720  pounds 

3.5  feet 

280  pounds 

360  pounds 

470  pounds 

600  pounds 

4.0  feet 

250  pounds 

320  pounds 

400  pounds 

550  pounds 

4.5  feet 

220  pounds 

280  pounds 

360  pounds 

470  pounds 

5.0  feet 

200  pounds 

250  pounds 

325  pounds 

425  pounds 

5.5  feet 

180  pounds 

225  pounds 

300  pounds 

390  pounds 

6.0  feet 

165  pounds 

210  pounds 

270  pounds 

350  pounds 

Example.  What  sag  should  be  given  a  150-foot  span  to  give 
the  same  tension  that  a  2-foot  sag  gives  to  a  100-foot  span  in 
Table  XXXII? 


150 

Too 


=  1.5 


(1.5)2  =  2.25 
The  desired  sag  2  X  2.25  =  4.5  feet.    Answer. 

The  weight  of  cable  given  above  includes  that  of  the  supporting 
wire  and  the  marline  hangers. 

Pole  Shims: — In  pole  lines  carrying  cables,  good  practice  de- 
mands that,  in  dead-ending  messenger  wires  on  poles  and  in  attaching 
all  guy  wires  to  poles,  guy  shims  be  used.  These  shims  consist  merely 
of  short  rectangular  strips  of  iron  about  £  inch  thick,  1  inch  wide, 
and  6  inches  long,  with  a  nail  hole  at  each  end  for  attaching  them  to 
the  pole.  The  guy  shims  are  placed  around  the  pole  at  such  a  point 
that  the  attaching  messenger  or  guy  wire  will  engage  them  at  their 
middle.  In  the  case  of  guy  wires  where  there  is  an  abrupt  angular 
pull  downward  from  the  pole,  5-inch  lag  screws  may  be  placed  on 
both  sides  of  the  pole  to  prevent  the  guy  from  slipping  down. 

In  some  cases  it  is  desirable  to  insulate  the  guy  strand  from  the 
messenger  strand  for  safety  and  for  the  prevention  of  electrolysis  of 
the  guy  wire  and  anchor,  in  which  case  the  guy  strand  should  be 


786 


TELEPHONY 


attached  to  a  different  set  of  shims  from  that  used  in  dead-ending 
the  messenger.  Where  no  such  necessity  exists,  the  guy  wire  may 
be  attached  to  the  same  shims  as  the  messenger.  The  two  views  in 
Fig.  556  will  make  this  clear. 


Fig    556.    Guy  Attachment  at  Terminal  Poles 

Splices: — In  new  work,  where  it  is  necessary  to  join  ends  of 
messenger  wire,  it  is  better  to  do  so  by  dead-ending  the  two  lengths 
on  a  pole  rather  than  by  making  a  joint  at  an  intermediate  point  be- 


Pig.  557.     Wrapped  Splice 


tween  two  poles.  Where  stranded  messenger  wire  must  be  spliced 
in  the  span,  it  may  be  done  by  making  a  wrapped  splice,  as  indicated 
in  Fig.  557.  This  method  has  the  advantage  of  presenting  a  com- 


POLES  AND  POLE  FITTINGS 


787 


paratively  smooth  surface  on  the  messenger  wire  in  the  erection  of 
the  cable,  but  it  should  not  be  practiced  unless  the  proper  skill  is 
available  for  making  a  good  job  of  it.  The  usual  way  of  making 
splices  is  by  means  of  regular  guy  clamps,  the  two  wires  being  merely 
laid  side  by  side  and  clamped  together. 

Cable.  Erection: — In  erecting  aerial  cable,  the  reel  containing 
the  cable  is  placed  about  50  feet  beyond  one  of  the  poles  of  the  lead 
upon  which  the  messenger  strand  has  already  been  erected.  A  run- 
ning-up  wire,  usually  of  the  same  material  as  the  messenger  strand, 
is  fastened  to  the  pole  and  to  a  stake  in  the  ground  near  the  reel. 
This  running-up  wire  serves  to  support  and  guide  the  cable  while  it 
is  being  pulled  up  to  and  along  the  strand.  The  general  scheme  of 
erecting  is  shown  in  Fig.  558.  In  setting  up,  the  reel  should  be  so 
placed  that  the  cable  will  always  unroll  from  its  top  and  be  as  nearly 


Pig.  558.     Erecting  Aerial  Cables 

in  line  with  the  running-up  wire  as  possible.  It  is  not  generally  desir- 
able to  pull  up  more  than  1,000  feet  of  cable  in  single  lengths,  and 
that  only  in  small  sizes. 

After  the  reel  is  set  up  on  suitable  jacks  so  as  to  allow  it  to  turn 
freely,  the  pulling  rope  is  fastened  to  the  end  of  the  cable.  In  fasten- 
ing this  rope  a  wrapping  of  wire  or  marline  may  be  used,  particular 
care  being  taken  not  to  subject  the  end  of  the  cable  to  such  treat- 
ment as  will  break  through  the  lead  sheath.  The  cable  rope  should -be 
provided  with  a  swivel  hook  or  ring  so  as  to  avoid  twisting  the  cable. 
The  cable  may  be  pulled  up  by  hand-power,  horse-power,  or  by  an 
engine-driven  winch.  The  winch,  or  capstan,  or  whatever  device 


788  TELEPHONY 

is  used,  is  placed  at  the  distant  end  of  the  run  and  securely  braced. 
The  end  of  the  cable  pulling  rope  is  then  carried  to  the  capstan  drum 
and  wrapped  about  it  and  the  cable  is  slowly  and  evenly  pulled  into 
place. 

Various  methods  are  in  vogue  for  supporting  the  cable  while  it  is 
being  pulled  up.  Some  of  these  involve  the  placing  of  temporary 
trolley  wheels  on  the  messenger  from  which  the  cable  is  supported, 
these  wheels  running  along  the  messenger  wire  as  the  cable  progresses. 
In  other  cases  the  supporting  trolley  wheels  are  mounted  on  each  pole 
adjacent  to  the  messenger  and  the  cable  rolled  over  these  as  it  pro- 
gresses. Still  another  way  is  to  employ  no  trolleys  or  temporary  sup- 
ports at  all,  but  to  apply  the  cable  hangers  to  the  cables  and  to  the 
running-up  wire  as  the  cable  runs  off  the  reel,  the  hangers  running 
along  the  messenger  strand  as  the  cable  is  moved.  Unless  provision 
is  made,  by  means  of  special  attachments  at  each  pole,  by  which  the 
cable  hangers  may  ride  past  the  messenger  supports,  it  is  necessary 
to  station  a  man  at  each  pole  to  lift  each  hanger  hook  off  the  mes- 
senger wire  as  it  passes  each  pole. 


Fig.  559.    Marline  Cable  Hanger 

Hangers: — No  matter  what  method  is  used  the  cable  should  be 
slowly  and  evenly  pulled  into  place,  and  after  it  is  all  up  care  should 
be  taken  that  all  hangers  are  in  place.  The  distance  between  hangers 
on  a  cable  should  be  uniform.  For  200  pair,  22-gauge  cable  or  its 
equivalent  in  weight,  the  hangers  should  be  about  15  inches  apart. 
For  smaller  cables  this  distance  may  be  increased,  but  in  no  case 
should  it  be  over  24  inches. 

Numerous  forms  of  cable  hangers  exist,  but  the  one  that  today 
seems  best  to  meet  the  requirements  of  practice  in  practically  all 
sections  of  the  country  is  the  marline  hanger.  This  consists  merely 
of  an  S-shaped  hook  of  galvanized  steel  wire  to  which  is  attached 


POLES  AND  POLE  FITTINGS 


789 


a  loop  of  marline.  Its  construction  is  shown  in  Fig.  559,  and  the 
method  of  supporting  a  cable  by  it,  in  Fig.  560.  In  placing  the 
hangers  on  the  messenger,  the  points  of  the  hooks  should  be  faced 
toward  the  pole. 

The  length  of  the  loop  in  marline  hangers  varies  for  different 
sizes  of  cables.     Table  XXXIII  illustrates  good  practice. 

Splices: — A  subsequent  chapter  will  be  devoted  to  the  subject 
of  splicing  cables.  It  may  be 
said  at  this  point,  however,  that 
where  it  is  necessary  to  join  two 
cables  so  that  one  will  form  a 
continuation  of  the  other,  the 
wires  in  them  are  individually 
spliced  together,  taking  extreme 
pains  to  maintain  the  insulation 
and  particularly  to  keep  the 
core  dry.  After  this  a  lead 
sleeve  is  placed  over  the  joined 
wires  and  secured  to  each  end 
of  the  cable  by  a  plumber's 
wiped  joint,  so  as  to  maintain  the  continuity  of  the  enclosing  sheath 
and  keep  out  all  moisture.  This  is  called  a  straight  splice.  It  is  also 
frequently  necessary  to  tap  a  cable,  that  is,  to  lead  out  from  it  certain 
conductors  which  are  to  be  made  available  for  connection  at  an  inter- 
mediate point  on  its  length.  This  requires  a  tap  splice.  Again  it 
may  be  necessary  to  join  the  conductors  of  one  large  cable  to  those 
of  two  or  more  smaller  cables,  this  practice  being  followed  where  the 
larger  group  of  wires  is  to  be  divided  so  as  to  enable  them  to  follow 
different  routes.  When  a  cable  thus  branches  out  into  smaller  ones, 


Fig.  560.     Marline  Hanger 
Supporting  Cable 


TABLE    XXXIII 
Loop  in  Marline  Hangers 


Less  than     25  pair 

22  gauge  cable 

9  inch  loop 

25  pair 

22  gauge  cable 

11  inch  loop 

50  pair 

22  gauge  cable 

14  inch  loop 

100  pair 

22  gauge  cable 

16  inch  loop 

200  pair 

22  gauge  cable 

19  inch  loop 

790 


TELEPHONY 


the  resulting  splice  is  a  Y '-splice.     If  the  large  cable  connects  with 
two  smaller  ones,  it  is  a  two-way  splice;  and  so  on 

Supporting    Splices: — In    aerial    work    the    supporting    of    the 
splices  should  be  given  particular  attention.     While  the  covering  of 


Fig.   561.     Cable  Splice  Support 

a  well-made  splice  is  just  as  capable  of  keeping  out  moisture  as  is  the 
ordinary  sheath  of  the  cable,  it  should  always  be  looked  upon  with 
suspicion,  due  to  possible  defects  in  workmanship.  Therefore,  great 
care  should  be  taken  to  subject  the  cable  to  no  unusual  stresses  at 
splices,  particularly  such  stresses  as  would  result  in  a  bending  of  the 


Fig.   562.     Y-Splice  Support 


cable  at  the  splice.  An  excellent  way  of  supporting  a  splice  is  to 
provide  a  wrapping  of  marline  around  it,  as  shown  in  Fig.  561. 
If  the  cable  is  a  heavy  one,  such  a  marline  wrapping  may  be  pro- 
vided at  each  end  of  the  splice,  omitting  the  one  in  the  middle. 


POLES  AND  POLE  FITTINGS 


791 


The  same  practice  may  be  followed  in  the  supporting  of  tap 
and  Y-splices.  Sometimes  it  becomes  necessary  to  make  a  Y-splice 
at  a  point  where  two  messenger  wires  cross  and  where  no  pole  is  at 
the  intersection.  In  such  cases  the  messenger  wires,  which  cross 
at  the  same  level,  should  be  rigidly  secured  to  each  other  by  a  mes- 
senger clamp,  and  the  splice  and  the  cables  leading  from  it  supported 
by  marline  in  addition  to  the  usual  hangers,  as  shown  in  Fig.  562. 
It  will  be  noticed  that  the  splice  is  made  some  distance  back  from 
the  intersection  on  the  main  lead  and  that  the  two  smaller  cables 
leading  from  it  are  lashed  to  the  main  cable  by  marline,  so  as  to  make 
it  impossible  for  any  side  stress  on  the  cables  which  follow  the  inter- 
secting messenger  wire,  to  come  on  the  covering  of  the  splice  itself. 


Fig.  563.     Aerial  Cable  Turn 

In  turning  corners  with  aerial  cable,  even  where  no  splice  oc- 
curs, particular  care  should  be  taken  to  avoid  kinking  the  cable  and 
also  to  avoid  its  chafing  against  the  pole.  If  the  turn  occurs  on  a 
corner  pole,  the  cable  should  pass  on  the  inside  rather  than  on  the 
outside  of  the  pole,  and  should  be  lashed  to  the  messenger  wire  by  a 
wrapping  of  marline  in  the  same  manner  as  used  in  supporting  the 
cable  splice,  illustrated  in  Fig.  561.  If  the  turn  is  made  on  an  inter- 
mediate point  between  poles,  it  may  be  done  as  shown  in  Fig.  563. 
In  this  case  plenty  of  slack  should  be  left  in  the  cable  at  the  point  of 
turning  to  allow  for  any  drawing  up  that  may  occur,  due  to  changes 
in  temperature  or  to  any  influence  that  might  cause  the  cable  to  creep. 


792 


TELEPHONY 


In  cases  where  the  cable  is  subject  to  possible  chafing  from  trees, 
poles,  buildings,  or  other  objects,  it  may  be  protected  by  wooden 
cleats  lashed  to  it  by  marline  or  wire  as  shown  in  Fig.  564. 

Terminals.  The  subject  of  terminals  for  aerial  construction 
is  one  about  which  much  has  been  written  and  said,  and  until  recently 
no  standard  practice  has  resulted.  This  subject  particularly  is  one 
which  has  required  time  to  afford  the  necessary  experience  for  de- 
termining what  was  desirable  and  what  was  not,  and  the  art  is  so 
young  that  it  is  only  recently  that  engineers  have  approached 
anything  like  agreement. 

There  are  two  purposes  of  the  cable  terminal  for  aerial  con- 
struction: First,  to  provide  access  to  the  wires  in  the  cable  for 
connecting  un cabled  wires,  such  as  those  .leading  to  the  subscriber's 
premises  or  to  bare  wire  leads.  Second,  to  afford 
means  for  inserting  protective  devices  in  the  line  con- 
ductors at  points  between  a  section  that  is  exposed 
to  electrical  hazards  and  one  that  is  not. 

In  any  event  the  cable  terminal  must  possess  the 
following  requisites: 

It  must  afford  ready  means,  such  as  binding  posts  or 
terminal  clips,  for  attaching  the  wires  that  are  to  form  the 
continuations  of  the  cable  conductors. 

It  must  afford  desired  protection  against  the  entrance  of 
moisture  to  the  core  of  the  cable  or  cables  which  terminate  in  it. 

It  must  afford  high  insulation  between  all  of  the  terminal 
posts  or  clips  and  the  wires  leading  to  them,  so  that  the  insu- 
lation of  the  cable  conductors  as  a  whole  may  be  maintained. 

It  must  afford  ready  access  to  the  terminal  posts  and  con- 
necting wires  and  to  the  protectors,  if  such  exist. 

It  should  be  as  compact  as  is  consistent  with  the  require- 
ments for  insulation  and  good  mechanical  construction. 

It  must  be  capable  of  protecting,  in  a  reasonable  degree, 
against  the  entrance  of  dust,  moisture,  and  insects. 

It  should  be  as  sightly  as  it  is  possible  for  such  things 

Fig.  564.  Cable       to  be- 
Protector 

ror  tap  terminals  at  intermediate  points  on  aerial 

cables,  the  usual  requirement  is  for  a  relatively  small  number  of 
conductors  to  be  made  available.  This  number  ranges  from  ten 
to  fifty  pairs.  At  such  points  it  is  usual  to  continue  the  line 
wires  by  means  of  rubber-insulated  twisted  pairs  which  extend 
from  the  terminal  pole  to  the  premises  of  the  subscriber.  Such 


POLES  AND  POLE  FITTINGS 


793 


terminals  are  protected  or  unprotected  according  to  the  hazard  of 
the  continuing  wires. 

Unprotected: — An  excellent  form  of  unprotected  terminal  for 
such  work,  manufactured  by  the  Western  Electric  Company  and 
largely  employed  by  the  Bell  operating  companies,  is  shown  in  Fig. 
565.  This  consists  of  a  cast-iro»  box  forming  a  chamber,  from  the 


Fig.   565.     Western  Electric  Terminal 

lower  portion  of  which  there  extends  a  brass  tube  of  sufficient  diam- 
eter to  just  admit  the  lead  sheath  of  the  tap  cable.  The  front  of  this 
chamber  is  closed  by  a  porcelain  slab  into  which  the  lock-nut  termi- 
nal binding  posts  are  secured.  To  the  rear  end  of  the  binding  post 
studs,  the  individual  cable  wires  are  soldered,  and  the  whole  chamber 
is  then  filled  with  a  hot  insulating  compound  so  as  to  hermetically 
seal  the  end  of  the  cable  and  prevent  the  entrance  of  moisture  to 
the  insulated  wires  leading  to  the  binding  posts.  By  this  means  the 


794 


TELEPHONY 


terminals  of  the  tap  cable  are  continued  to  the  lock-nut  binding 
posts  on  the  front  of   the  porcelain  slab,  with   no   liability  of   the 

entrance  of  moisture  to  the  cable. 
The  whole  device  is  compact,  and 
owing  to  the  high  insulation  resistance 
of  porcelain  and  of  the  insulating  com- 
pound used  in  filling  the  chamber,  the 
very  high  degree  of  insulation  required 
is  maintained.  As  furnished  by  the 
Western  Electric  Company,  this  termi- 
nal has  a  length  of  lead-covered  cable 
attached  and  its  conductors  properly 
connected,  the  joint  between  the  brass 
sleeve  and  the  sheath  of  the  cable  being 
made  by  a  plumber's  wiped  joint.  The 
box  is  provided  with  a  hinged  iron 
cover  which  guards  against  the  en- 
trance of  moisture,  dust,  and  insects. 

Another  excellent  form  of  cable  ter- 
minal for  this  class  of  work,  manu- 
factured by  Frank  B.  Cook,  is  shown  in  Fig.  566.  In  this  the  binding 
posts  are  mounted  on  two  porcelain  blocks.  Two  of  these  blocks 
are  adapted  to  form  a  chamber  between  them  for  the  insulating 


Fig.  566.   Cook  Unprotected  Terminal 


567. 


Cook  Protected  Terminal 

compound.     This  terminal  may  be  procured  either  with  or  without 
a  length  of  tap  cable  attached,  according  to  whether  the  user  desires 


POLES  AND  POLE  FITTINGS 


795 


Fig. 


to  do  this  work  for    himself  or  not.     In  this   form    of  terminal  a 
cover,  consisting  of  a  cylindrical  galvanized  iron  can,  is  provided, 

which,  when  raised,  exposes  all  of  the 
binding  posts,  and  when  lowered,  affords 
them  adequate  protection. 

Protected:  —  Where  the  wires  leading 
from  a  cable  terminal  are  bare  and  of 
considerable  length,  or  where  they  pass 
through  territory  exposed  to  electrical 
hazards,  the  cable  terminal  should  be 
provided  with  protectors.  This  point 
has  already  been  dealt  with  in  Chapter 
XIX.  Cook  protected  terminals,  provid- 
ing fuse  and  carbon  arresters  for  each 
wire,  are  shown  in  Fig.  567. 

The   type   of  terminal   shown    at   the 

Terminal  BOX  left  in  Fig-  56?  ™  only  partially  equipped 
with  protectors  in  order  to  show  more 
clearly  the  method  of  mount- 
ing them  in  units.  The  one 
at  the  right  is  completely 
equipped.  This  general  type 
of  terminal  is  easily  adaptable 
for  use  at  the  junction  point 
between  aerial  and  under- 
ground cable. 

Another  form  of  protected 
terminal  —  commonly  used 
where  aerial  cable  leads  and 
bare  wire  leads  of  considera- 
ble sizes  join  —  consists  in  fuse 
and  air-gap  protectors  mount- 
ed in  wooden  boxes.  A  view 
of  such  a  wooden  terminal  box 
is  shown  in  Fig.  568.  For  use 
in  such  boxes,  strips  of  com- 
bined air-gap  and  fuse  pro- 

Pig  569     Wooden  Terminal  Box 


796 


TELEPHONY 


strips  being  added  in  units  of  ten,  twenty,  or  more,  as  required. 

This  wooden  terminal  box  has  the  advantage  of  being  able  to 
accommodate  a  large  number  of  line  conductors,  and  for  that  reason 
is  the  most  common  form  of  protected  terminal  box  for  use  between 
the  junction  of  aerial  and  underground  cables.  The  details  of  wiring 
within  such  a  box  when  used  for  joining  aerial  to  underground  cable 
wires  through  protectors,  is  shown  in  Fig.  569. 

These  wooden  terminals,  whether  used  at  the  outer  terminal  of  an 


Fig.  570.     Tap  Cable  Terminal 

aerial  lead  or  for  joining  underground  and  aerial  cables,  provide  no 
means  for  sealing  the  cables  which  terminate  in  them.  For  this  rea- 
son, it  is  necessary  to  "pot-head"  the  cables;  that  is,  they  are  termi- 
nated in  a  special  form  of  splice  which  serves  to  connect  all  of  the 
cable  conductors  with  uncabled  rubber-insulated  conductors,  and  at 
the  same  time  completely  to  seal  the  end  of  the  cable  against  the  en- 
trance of  moisture.  The  matter  of  pot-heads  will  be  considered  in  the 
chapter  dealing  with  cable  splicing. 


POLES  AND  POLE  FITTINGS 


797 


Position: — The  relation  as  to  position   between  the  cable  ter- 
minal and  the  other  features  of  aerial  construction  merits  attention. 


Fig.  571.     Aerial  Cable  Terminal 


Where  terminals  are  used  for  the  purpose  of  distributing  the  cable 
conductors  to  the  drop  wires  which  extend  to  the  adjacent  subscrib- 


Fig.  572.     Joining  of  Aerial  and  Underground  Cables 


er's  premises,  it  is  desirable  that  the  terminals  shall  be  mounted  as 
high  up  on  the  pole  as  possible,  and  at  any  rate  at  a  point  from 


798 


TELEPHONY 


Fig.  573.     Pole  Seat 


which  there  will  be  the  most  direct  and  clearest  run  to  the  neigh- 
boring houses  or  adjacent  poles. 

Fig.  570  shows  a  terminal  of  a  tap  cable  and  the  method  of  sup- 
porting the  splice  and  the  tap  cable.  The 
tap  cable  is  lashed  to  the  main  cable  beyond 
the  point  where  it  emerges  from  the  splice 
and  is  then  looped  down  and  up  vertically 
along  the  pole,  being  held  to  the  pole  by 
straps  of  sheet  iron. 

In  Fig.  571  is  shown  the  terminal  at 
the  outer  end  of  an  aerial  cable,  this  show- 
ing also  the  dead-ending  of  the  messenger 
wire  and  guying  of  the  terminal  pole.  It  also 
shows  how  the  cable  where  it  bends  around 
the  pole  adjacent  to  the  guy  shims  and  guy  and  messenger  strands 
may  be  served  with  a  wrapping  of  marline  to  guard  against  abrasion. 
The  details  of  the  most  common  method  of  inserting  protection 
between  aerial  and  underground  cables  is  shown  in 
Fig.  572.  The  cable  leading  up  from  the  under- 
ground conduits  is  shown  at  the  bottom  of  this 
figure,  and  the  aerial  cable  is  shown  extending  from 
a  messenger  wire  down  the  pole  and  then  up  to 
the  terminal  box.  This  figure  also  shows  details 
of  the  mounting  for  the  pot-heads,  in  which  both 
the  aerial  and  the  underground  cables  terminate. 
The  rubber-covered  wires  leading  out  of  the  pot- 
heads  pass  upwardly  within  the  box  alongside  of 
the  terminal  strips  of  the  protectors  and  are  there 
soldered  into  place  so  as  to  form  permanent  con- 
nections. 

Pole   Seats: — Where   50  or  more   pairs  of  wires 
terminate  in  an  aerial  terminal,  it  is  convenient  to 
Fig.  574.  Pole     equip  the  pole  with  some  form  of  seat  or  balconv 

Balcony  ^  f  .      . 

tor  convenience  ot  the  cablemen.  1ms  is  particu- 
larly true  where  protected  terminals  are  used.  This  may  be  a  simple 
seat  for  the  workmen,  as  shown  in  Fig.  573,  or  for  larger  and  more 
important  terminals  it  may  assume  the  form  of  a  "balcony,"  as 
shown  in  Fig.  574. 


CHAPTER   XLV 
UNDERGROUND  CONSTRUCTION 

Reasons  for  Placing  Wires  Underground.  The  demands  of 
the  public  and  the  welfare  of  telephone  companies  alike  require 
the  placing  of  wires  underground  in  the  most  densely  built-up 
portions  of  cities.  The  view  given  in  Fig.  575  is  in  itself  a  sufficient 
argument  for  the  placing  of  wires  underground  in  such  places.  Usually 


Pig.  575.     An  Argument  for  Underground  Wires 

the  "enlightened  self-interest"  of  the  operating  company  dictates 
a  greater  area  of  underground  construction  than  that  required  by 
the  public  through  city  ordinances. 

Interests  of  the  Public.  From  the  standpoint  of  the  public,  over- 
head wires  are  objectionable  for  several  reasons:  They  are  un- 
sightly; they  are  dangerous  to  workmen  and  passersby;  they  interfere 


800  TELEPHONY 

with  the  work  of  firemen ;  and  when  down  in  the  streets  they  obstruct 
traffic. 

Interests  of  the  Operating  Company.  The  wisely  managed 
telephone  company  will  choose  to  place  its  wires  underground  in 
congested  districts  for  still  other  reasons,  of  which  the  principal  one 
is  that  of  economy.  Where  a  large  number  of  wires  are  involved 
it  is  cheaper  under  average  city  conditions  to  place  them  under- 
ground than  overhead;  but  the  main  feature  of  economy  is  from  the 
standpoint  of  maintenance.  Well-constructed  telephone  conduits  are 
almost  as  durable  as  the  foundations  of  buildings  and  require  com- 
paratively small  expense  for  up-keep.  Wires  contained  in  such  con- 
duits are  subject  to  a  minimum  amount  of  electrical  hazard,  and  are 
less  exposed  to  the  elements  and  to  the  acts  of  careless  or  malicious 
persons.  The  depreciation  of  underground  conduits  is  almost  nil,  and 
that  of  underground  cables  very  much  less  than  of  those  overhead. 

Buried  Cable.  The  very  early  practice  in  underground  cable 
work  involved  the  placing  of  the  cable  in  a  trench  and  filling  in  the 
earth  on  top  of  it.  The  main  objection  to  this  was  that  the  cable 
was  not  accessible  for  repairs  or  changes.  A  cable  once  buried  in 
this  way  was  often  virtually  lost  if  it  became  faulty,  since  the  cost  of 
digging  it  up  was  more  j;han  its  salvage  value.  This  method  pro- 
vided no  facilities  for  growth  and  necessitated  the  frequent  digging 
up  of  the  streets.  Another  objection  was  that  the  cable  was  un- 
protected from  the  tools  of  workmen  engaged  in  making  subsequent 
excavations. 

Underground  Conduit.  The  method  that  is  now  universally  prac- 
ticed involves  the  building  of  an  underground  conduit  into  which  cables 
may  be  drawn  and  from  which  they  may  be  withdrawn  as  occasion 
requires.  Such  conduit,  once  wisely  planned  and  properly  constructed, 
affords  the  required  mechanical  protection  and  provides  not  only  for 
the  present  demands  but  for  those  reaching  far  into  the  future. 

The  conduit,  irrespective  of  cables,  consists  of  one  or  more  per- 
manently installed  ducts  or  openings  extended  between  manholes 
through  which  access  is  afforded  to  the  ducts.  These  ducts  or  open- 
ings are  made  of  sufficient  internal  diameter  to  accommodate  the 
largest  cable  that  it  seems  probable  ever  will  be  used.  The  number  of 
ducts  installed  at  the  time  of  building  the  conduit  is  made  equal 
to  the  largest  number  of  cables  that  it  seems  probable  will  be  required 


UNDERGROUND  CONSTRUCTION  .     801 

akmg  that  route  within  a  given  period  of  years.  Such  a  degree  of 
flexibility  is  provided  by  this  plan  that  only  those  cables  which  are 
required  by  the  present  needs  of  the  sy.tem  are  installed  at  the  out- 
set, others  being  added  from  time  to  time  to  meet  the  growth  or 
changed  requirements. 

Duct  Material.  The  materials  available  for  conduit  ducts  are 
vitrified  tile,  creosoted  wood  pipe,  iron  pipe,  paper  or  wood  pulp  pipe 
impregnated,  cement  lined  pipe,  concrete  pipe,  and  sometimes  the 
ducts  are  formed  in  the  concrete  structure  itself  as  it  is  being  laid. 
Of  these  materials  the  most  used  are  vitrified  tile,  impregnated  fiber 
pipe,  iron  pipe,  and  creosoted  wood  pipe.  Clay  tile  and  paper  or 
fiber  pipe  are  perhaps  the  most  used  in  main  conduit  runs  in  city 
streets  or  alleys,  where  many  ducts  are  required.  Clay  tile  has  the 
advantage  of  being  known  to  be  practically  everlasting  under  the 
action  of  the  elements.  It  has  the  disadvantage  of  being  heavy  and, 
therefore,  somewhat  expensive  on  account  of  transportation  charges 
in  those  localities  that  are  far  removed  from  the  source  of  supply. 
It  is  fragile  and,  therefore,  subject  to  a  considerable  breakage  loss  in 
handling. 

Fiber  pipe  has  the  advantage  of  being  light  and  easily  handled 
and  transported,  and,  therefore,  in  some  localities  cheaper  than  tile. 
Its  breakage  loss  is  less  than  that  of  tile.  No  one  knows  how  long  it 
will  last,  but  when  properly  made  and  laid  it  has  proven  its  ability  to 
stand,  without  appreciable  deterioration,  throughout  long  periods  of 
time.  Both  clay  tile  and  fiber  pipe  have  so  little  ability  to  withstand 
shock — as  from  the  pick  axes  of  workmen — as  to  make  it  usually 
necessary  to  provide  for  them  an  envelope  of  concrete  which  wholly 
or  partially  encloses  them. 

Iron  pipe  has  the  advantage  of  needing  no  protecting  envelope, 
as  a  rule,  but  has  the  disadvantage  of  being  very  expensive  and  of 
gradually  deteriorating  under  the  action  of  certain  soils.  It  is,  how- 
ever, largely  used  in  main  conduit  work  for  cases  requiring  special 
treatment,  and  for  the  subsidiary  or  branch  runs  from  main  con- 
duits where  but  one  or  two  ducts  are  required. 

Creosoted  wood  or  "pump  log"  conduit  is  more  expensive  in 
first  cost  than  clay  or  fiber,  but  less  expensive  than  iron  and  it  has 
such  resisting  power  against  ordinary  shocks  and  strains  as  to  make 
it  feasible  to  lay  it  directly  in  the  ground  without  an  enclosing  envelope. 


802 


TELEPHONY 


Well-impregnated  wood  is  known  to  have  a  life  of  many  years  in  all 
ordinary  soils,  and  the  consensus  of  opinion  is  that  creosoted  wooden 
pipe  has  a  long  enough  life  to  make  it  satisfactory  in  that  respect  as 
a  duct  material. 

Vitrified-Clay  Conduit.     Taking  under  consideration,  first,  the 
construction  of  conduits  employing  vitrified  clay,  it  may  be  said  that 


Fig.  576.     Single-  and  Multiple-Duct  Clay  Tile 

tile  of  this  material  is  obtainable  in  a  variety  of  forms,  which  may 
be  classified  as  single-duct  and  multiple-duct.  Illustrations  of  these 
forms,  furnished  by  McRoy  Clay  Works,  are  given  in  Fig.  576. 

Single-Duct  Tile: — In  laying,  these  are  piled  together  side  by 
side  and  tier  on  tier  in  the  trench  in  order  to  make  up  the  desired 
number  of  ducts  and  the  proper  cross-section.  The  ends  are  butted 
together,  the  joints  being  staggered  and  the  whole  mass  set  in  concrete 
so  as  to  form  a  practically  continuous  structure. 

In  order  to  maintain  the  proper  alignment  between  the  abutting 
ends  of  these  single-duct  tiles,  it  is  customary  during  the  process  of 


Fig.  577.     Dowel  Pin 


laying  them  to  employ  a  mandrel,  about  3  inches  in  diameter  and 
30  inches  long.  This  is  laid  in  the  duct  and  pulled  along  through 
it  by  workmen  as  each  additional  section  is  laid.  Otherwise  the 


UNDERGROUND  CONSTRUCTION 


803 


laying  of  this  single-duct  tile  is  about  the  same  as  that  of  ordinary 
brick. 

Multiple-Duct  Tile: — Another  form  of  tile  that  is  largely  used 
is  the  so-called  multiple-duct,  several  forms  of  which  are  shown  in 
Fig.  576.  In  laying  this,  after  providing  a  proper  foundation  usu- 
ally of  concrete,  the  tiles  are  placed  together  so  as  to  form  the  proper 
cross-sectional  arrangement  of  ducts.  The  butt  joints  are  held  in 


Fig.  578.     Conduit  of  Single-Duct  Tile 

alignment  by  dowel  pins  of  the  form  shown  in  Fig.  577.  This  pin  is 
of  iron  3  inches  long,  of  such  diameter  as  to  loosely  fit  the  dowel  pin 
holes  in  the  duct  ends,  and  having  a  washer  formed  at  its  center  so 
as  to  divide  it  equally  between  the  abutting  sections  of  conduit.  In 
order  to  prevent  dirt  or  concrete  from  entering  at  the  joints  in  the 


Fig.  579.     Conduit  of  Multiple-Duct  Tile 


ducts,  each  abutting  joint  is  wrapped  with  a  layer  of  muslin  »r  bur- 
lap cut  into  strips  about  8  inches  wide  and  long  enough  to  surround 
the  conduit  and  lap  over  several  inches.  This  strip  is  saturated  with 
water  to  make  it  cling  to  the  conduit,  after  which  it  is  plastered  with 


804 


TELEPHONY 


cement  mortar.  In  Fig.  578  a  good  idea  is  given  of  the  formation 
of  a  conduit  composed  of  single-duct  vitrified  clay,  and  in  Fig.  579  of 
one  built  of  multiple-duct. 

Use  of  Concrete  with  Clay  Tile: — Until  comparatively  recently 
it  has  been  customary  to  completely  surround  the  line  of  conduit- 
top,  bottom,  and  sides — with  a  concrete 
envelope  about  3  inches  thick,  as  shown 
in  cross-section  in  Fig.  580.  During 
recent  years  there  has  been  a  growing 
tendency  to  curtail  the  expense  in  con- 
crete work  and  several  arrangements  have 
been  tried  and  each  has  its  advocates. 
One  of  these,  shown  in  Fig.  581,  is  to  lay 
about  a  4-inch  foundation  of  concrete  in 
the  bottom  of  the  trench  and  then  build 
up  the  tile  on  this.  This  is  covered  with 
a  layer  of  heavy  creosoted  plank  and  the 
earth  is  then  tamped  in  on  sides  and 
top. 

Another  method  consists  in  laying  the 
foundation  as  just  described,  then  building  up  the  line  of  ducts  and 
tamping  in  the  earth  on  the  sides  only,  and  finally  covering  this  with 
another  layer  of  concrete  about  3  or  4  inches  thick  so  as  to  give 

protection  at  the  top  against  the  pick  axes 
of  workmen.  Both  the  top  and  bottom 
layers  of  concrete  serve  also  to  provide 
the  necessary  mechanical  rigidity.  This 
construction  is  shown  in  Fig.  579  and  in- 
|  f |  1  1 1  dicated  in  cross-section  in  Fig.  582. 

s=*°~=3>  <fc=±o!=r~o  Still  another   method    is   to   place   no 

concrete  on  the  bottom  of  the  trench,  this 
being  made  to  grade  and  tamped  to  a 
proper  degree  of  hardness  and  smooth- 
ness. On  this  the  tiles  are  laid  without 
other  foundation  and  earth  filled  in  at 
the  sides  to  a  point  a  few  inches  below  the  level  of  the  upper 
tiles.  Concrete  is  then  thrown  into  the  remaining  space  on  the 
sides  and  top  to  a  depth  of  about  3  inches  above  the  upper  tile,  as 


Fig.  580.  Conduit  Cross-Section— 
Complete  Concrete  Envelope 


Fig.  581.  Conduit  Cross-Section 

— Concrete  Bottom  and 

Plank  Top 


UNDERGROUND  CONSTRUCTION 


805 


DC 


OP 

48 


QQ 


a 

DPI 


Fig.  582.     Conduit  Section — 
Concrete  Top  and  Bottom 


shown  in  Fig.  583.  This  forms  an  inverted  channel  of  concrete 
which  gives  a  considerable  sidewise  rigidity  as  well  as  the  necessary 
protection  on  top. 

Undoubtedly  the  best  way,  where  one  wishes  to  be  perfectly 
safe,  is  to  make  the  concrete  envelope  entirely  surround  the  vitrified 
tile — top,  bottom,  and  sides.    This  makes 
a  more  rigid  structure  and  one  less  liable 
to  injury  in  the  event  of  settling  of  the 
earth  from  any  cause. 

Fiber-Pipe  Conduit.  The  bitumi- 
nized  fiber  pipe  employed  for  duct  ma- 
terial is  sometimes  made  of  paper,  being 
wound  on  a  mandrel  of  proper  size  to 
give  the  requisite  internal  diameter  and 
thoroughly  impregnated  with  asphaltum 
or  bituminous  compound.  It  is  necessary 
to  exercise  care  in  laying  this  so-called 
paper  pipe  in  hot  weather  that  it  is  not 
squeezed  out  of  its  cylindrical  form  under 
the  pressure  imposed  by  tamping  the  concrete  into  place. 

Another  form  of  fibrous  conduit  is  made  by  wrapping  very  thin 
layers  of  wet  wood  pulp  or  fiber  on  a  forming  mandrel,  under  great 
pressure,  until  the  desired  thickness  of  wall  is  obtained.     This  con- 
duit, unlike  the  paper  pipe,  has   its  individual 
fibers  felted  and  formed  into  a  solid  homogene- 
ous wall,  which  is  then  taken  off  the  mandrel, 
thoroughly  dried,  and   placed  in   a   vat  of  im- 
pregnating compound. 

This  compound  permeates  the  entire  struc- 
ture and  acts  as  a  preservative,  and  also  ren- 
ders the  wall  impervious  to  moisture.  The 
black  hard  material  which  results,  somewhat  resembles  hard  rubber 
and  is  of  such  texture  as  to  permit  its  ends  being  dressed  in  a  lathe  to 
form  the  desired  mortise  or  tendon  joints,  by  means  of  which  the 
alignment  is  secured  when  the  ducts  are  laid  in  the  trench.  This 
material  has  been  used  in  large  quantities  by  the  writers  and  has  not 
been  found  subject  to  the- fault  of  distortion  under  pressure,  mentioned 
above.  Two  lengths  of  this  fiber  conduit  are  shown  in  Fig.  584. 


Fig.  583.   Conduit  Sec- 
tion— Concrete  Top 


806  TELEPHONY 

The  length  of  the  standard  duct  is  5  feet,  its  internal  diameter,  in 
the  sizes  most  used  for  telephone  work,  3  or  3^  inches,  and  the  thick- 
ness of  wall  ^  inch. 

Recently  a  type  of  duct  formed  of  the  same  material,  with  the 
length  and  internal  diameter  the  same,  but  the   thickness   of  wall 


Fig.   584.     Fiber  Pipe 

being  ^  inch,  has  been  produced.  This  is  too  thin  to  permit  of  the 
socket  joint  formation,  so  the  joints  are  made  by  merely  butt-ending 
the  two  lengths  and  slipping  over  the  joint  a  sleeve  of  the  same  ma- 
terial about  4  inches  long.  A  good  idea  of  this  duct  and  of  the 
joining  sleeve  may  be  had  from  Fig.  585. 

Laying  Fiber  Ducts: — Fiber-pipe  conduit  when  used  in  main 
conduit  runs  must  be  set  in  concrete,  as  the  strength  of  the  wall  is  not 
sufficient  to  make  it  safe  to  do  otherwise.  In  laying  it,  after  the 
trench  has  been  properly  opened  and  graded,  a  foundation  layer  of 
concrete  about  3  inches  deep  is  laid  and  tamped,  the  upper  surface 
being  well  graded.  Upon  this  foundation  the  first  layer  of  duct  is 
laid,  the  horizontal  separation  between  the  outer  walls  of  the  ducts 
being  about  1  inch.  This  separation  is  readily  provided  by  driving 
stakes  in  the  ground  or  by  employing  so-called  combs.  These 
combs  consist  merely  of  wooden  strips  slightly  less  in  length  than 


Fig.  585.     Fiber  Pipe  and  Joining  Sleeve 

the  width  of  the  trench  and  having  short  strips  secured  at  right  angles 
to  them;  the  distance  between  these  strips  being  just  sufficient  to 
include  the  external  diameter  of  the  duct,  and  the  width  of  the  strips 
being  equal  to  the  required  horizontal  separation  between  the  ducts 
when  laid.  After  the  first  layer  of  ducts  is  in  place  and  temporarily 
held  by  the  stakes  or  combs,  concrete  is  slushed  in  and  tamped  lightly. 
If  there  is  to  be  more  than  one  layer  of  ducts,  the  depth  of  the  con- 


UNDERGROUND  CONSTRUCTION 


807 


crete  thus  applied  is  made  sufficient  to  cover  the  first  layer  about  1 
inch  deep.  The  surface  of  this  concrete  layer  is  then  leveled  and 
tamped  so  as  to  form  the  foundation  of  the  second  layer,  after  which 
the  same  operation  is  repeated  until  the  required  number  of  ducts  are 


Fig.   586.     Laying  Fiber  Duct 

laid.  After  the  top  layer  of  ducts  has  been  laid,  a  layer  of  concrete  is 
thrown  in  and  tamped  to  a  depth  of  about  3  inches  above  the  top  of 
the  ducts.  In  tamping  the  concrete  in  place,  care  must  be  taken  to 
prevent  ramming  stones  through  the  walls  of  the  ducts,  particularly 
where  the  |-inch  pipe  is  used. 


808 


TELEPHONY 


The  process  of  laying  runs  of  fiber  conduit  is  shown  in  Figs. 
586  and  587.  It  is  not  usually  necessary  to  line  the  sides  of  the  tren,ch 
with  planks,  as  shown  in  Fig.  586;  but  this  is  sometimes  done  where 


Fig.   587.     Laying  Fiber  Duct 


ie  character  of  the  soil  is  such  as  to  make  it  difficult  to  excavate 
so  as  to  leave  proper  vertical  side  walls. 

Wooden  Conduit.      Creosoted-wood  or  pump-log  conduit  is  usu- 
ally laid  without  any  concrete  whatever,  the  bottom  of  the  trench 


Fig.  588.     Creosoted  Wood  Conduit 


being  properly  graded  and  tamped  and  the  ducts  laid  directly  on 
the  bottom  of  the  trench  without  other  foundation.  Sometimes  a  foun- 
dation of  2-inch  creosoted  planks  is  used  and  the  ducts  laid  on  this ; 


UNDERGROUND  CONSTRUCTION  809 

and  on  top  of  the  ducts  a  single  layer  of  2-inch  creosoted  plank  is 
placed  to  afford  protection  in  case  of  subsequent  excavation.  In 
treacherous  soils  it  is  always  well  to  provide  the  foundation  layer 
of  2-inch  creosoted  plank  at  the  bottom  of  the  trench.  The  usual 
construction,  without  the  bottom  layer,  is  shown  in  Fig.  588. 

Iron  Conduit.  Iron  pipe  is  not  usually  employed  for  main-line 
conduit  work  except  in  places  of  especially  difficult  construction 
Where  it  is  used  in  main  conduits,  it  is  completely  encased  in  con- 
crete. A  large  amount  of  the  general  conduit  system,  employed 
by  the  telephone  and  signaling  companies  in  New  York  City,  is 
of  3^V-inch  iron  pipe  laid  in  concrete.  This  is  used  in  the 
downtown  districts  where  almost  unparalleled  congestion  occurs, 
and  where  economy  of  space  and  great  rigidity  of  construction  are 
essential  features.  Where  used  as  subsidiary  or  lateral  ducts  branch- 
ing off  from  main  conduit  runs,  it  is  customary  to  lay  the  iron  pipe 
directly  in  the  ground  without  any  protecting  covering  whatever. 
This  is  the  main  use  of  iron  pipe  in  underground  conduit  work. 

Conduit  Cross=Sections.  The  cross-sectional  arrangement  of 
the  ducts  in  the  main  lines  of  conduits  deserves  some  attention.  It 
is  common  practice  to  pay  little  regard  to  this,  but  it  is  our  belief  that, 
except  in  conduits  having  a  very  large  number  of  ducts,  arid  except 
where  it  is  difficult  to  make  the  trench  sufficiently  deep,  it  is  far  more 
convenient  to  so  arrange  the  cross-section  that  the  conduit  will  be 
no.  more  than  four  ducts  wide  and  as  many  high  as  the  total  number 
requires.  The  reason  for  this  is  that  this  arrangement  permits  a 
more  systematic  disposal  of  the  cables  in  manholes  or  vaults  than 
would  be  the  case  if  the  width  of  the  conduit  was  greater  than  four 
ducts. 

Fig.  589  shows  conduit  cross-sections  made  from  standard 
multiple-duct  tile  varying  in  number  of  ducts  from  two  to  twenty- 
four.  In  no  case  within  this  range  need  the  conduit  be  more  than 
four  ducts  wide,  except  where  unusual  conditions  as  to  digging 
or  obstructions  are  met.  Fig.  590  shows  economical  arrangements 
as  to  cross-section  of  fiber  pipe  conduits  of  various  sizes. 

General  Features  of  Conduit  Work.  Having  outlined  the  various 
kinds  of  duct  material  and  the  methods  ordinarily  employed  in  lay- 
ing them,  we  will  discuss  briefly  the  more  general  features  of  conduit 
construction,  including  that  of  manholes  and  laterals. 


DO 
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Fig.  589.     Cross-Sections  of  Tile  Duct 


UNDERGROUND  CONSTRUCTION 


811 


Main  Line  and  Lateral  Conduit.  As  has  been  stated,  manholes 
are  necessary  at  frequent  intervals  along  a  line  of  conduit  to  provide 
means  of  access  to  the  cables.  This  access  is  necessary,  first,  for 


Pig.   590.     Cross-Section  of  Fiber  Conduit 


drawing  the  cables  in  or  out  as  required;  and  second,  for  leading  off 
branch  cables  through  lateral  pipes  to  poles  or  other  aerial  struc- 
tures or  to  the  basements  of  buildings.  The  main  conduit  line, 
therefore,  consists  of  one  or  more  ducts  leading  between  adjacent 
manholes,  and  the  subsidiary  conduit  consists  in  lateral  ducts  or 


812  TELEPHONY 

pipes  leading  from  the  manholes  or  sometimes  from  intermediate 
portions  of  the  conduit  either  to  poles  or  buildings  for  affording  con- 


Fig.  591.     Section  of  Brick  Manhole 

nection  either  with  aerial  or  interior  wires  leading  to  the  subscriber's 
premises. 

Manholes.  These  consist  merely  of  holes  in  the  ground,  lined 
on  top,  bottom,  and  sides  with  brick  or  concrete.  They  have  a 
suitable  opening  at  the  top  for  the  access  of  workmen  and  have  in 
their  side  walls  openings  into  the  various  ducts. 

The  choice  between  concrete  and  brick  as  the  material  of  which 
to  construct  manholes  depends  upon  the  following  considerations. 


Fig.   592.     Concrete  Manhole 


Ordinarily  there  is  not  a  great  difference  as  to  cost,  but  this  will  always 
depend  in  part  on  the  market  and  labor  conditions  in  the  locality  in 


UNDERGROUND  CONSTRUCTION 


813 


Fig.    593. 


question.  A  factor  of  considerable  importance  that  favors  the  em- 
ployment of  concrete  is,  that  skilled  bricklayers  are  not  required,  and 
this  makes  it  possible  to  employ 
the  same  class  of  labor  for  con- 
structing manholes  as  is  used  on 
the  conduit  itself.  For  this  rea- 
son, if  for  no  other,  the  concrete 
manhole  is  usually  to  be  pre- 
ferred in  places  where  the  under- 
ground conditions  are  such  as  to 
permit  the  construction  of  the 
manholes  in  standard  sizes  and  shapes.  In  the  down-town  districts 
of  large  cities,  however,  this  is  usually  not  possible,  since  the  pres- 
ence of  pipes  and  other  under- 
ground properties  often  demands 
the  building  of  manholes  of  ir- 
regular shapes  and  sizes.  Brick- 
work lends  itself  much  more 
readily  to  the  requirements  of 
such  conditions  than  does  con- 
crete. A  good  idea  of  the  interior 
of  a  "brick  manhole  is  given  in 
Fig.  591,  which  shows  a  cross- 
section  taken  through  a  com- 
pleted manhole.  Fig.  592  shows 
a  concrete  manhole  before  the  top  has  been  placed. 

Shape  of  Manholes : — The  shape  of  a  manhole  will  always  de- 
pend to  some  extent  on  the  num- 
ber and  the  directions  of  ap- 
proach of  the  various  conduit 
runs  leading  to  it.  It  will  de- 
pend also  upon  the  character  of 
the  obstructions  found  in  exca- 
vating for  it.  Where  the  man- 
hole occurs  between  two  adjacent 
runs  of  conduit  in  the  same  line, 
and  where  there  is  no  intersecting  run,  the  general  shape  shown 
in  plan  in  Fig.  593  is  a  good  one.  Where  the  manhole  occurs  at  the 


Fig.  594.     Plan  of  Manhole 


Fig.  595.     Plan  of  Manhole 


814 


TELEPHONY 


junction  between  a  main  line  and  an  intersecting  line  this  shape  is 
necessarily  slightly  altered,  as  shown  in  plan  in  Fig.  594.     These 


Fig.  596.     Section  of  Concrete  Manhole 


shapes  lend  themselves  readily  to  the  employment  of  simple  wooden 
forms  around  which  the  concrete  is  poured,  in  the  case  of  concrete 
construction,  and  are  also  equally  desirable  from  the  standpoint 
of  simplicity  where  brick  is  the  material  employed. 


Fig.  597.     Section  of  Concrete  Manhole 


Some  prefer,  in  the  case  of  a  manhole  occurring  in  a  straight 
run,  to  curve  the  sides,  giving  a  resulting  plan  as  shown  in  Fig.  595. 
The  details  of  construction  of  the  non-intersected  and  the  inter- 
sected types  of  manholes  are  shown  in  Figs.  596  and  597. 


UNDERGROUND  CONSTRUCTION 


815 


TABLE  XXXIV 

Inside  Dimensions  of  Manholes 


Straight  Runs  Not  Intersected 

No.  or  DUCTS 

LENGTH 

WIDTH 

HEIGHT 

1   to     6 
7  to   15 
16  to  24 

4'6" 
6' 

8' 

3'6" 

4' 
5' 

4'6" 
5' 
5'6" 

Intersected  Runs 

No.  OF  DUCTS 

LENGTH 

WIDTH 

HEIGHT 

1   to     6 
7  to   15 
16   to  24 

4'6" 
6' 

8' 

4'6" 
5' 
5' 

4'6" 
5' 
5'6" 

Sizes  of  Manholes: — The  size  of  manhole  will,  of  course,  vary 
with  the  number  of  ducts  entering  it.  No  definite  rules  may  be  laid 
down,  but  the  sizes  as  given  in  Table  XXXIV  are  suggested  for 
different  sizes  of  intersected  and  non-intersected  runs. 

Location  of  Manholes: — In  laying  out  a  conduit  system  the 
location  of  the  manholes  is  an  important  factor.  It  is  not  -  always 
possible  to  locate  a  manhole  exactly  where  originally  planned,  because 
upon  excavation  it  may  be  found  that  pipes  or  other  obstructions 
that  cannot  be  moved  will  necessitate  a  change.  In  general,  however, 
manholes  should  be  located  at  street  or  alley  intersections,  since  this 
allows  a  single  manhole  to  serve  for  intersecting  runs,  and  also  places 
them  at  the  most  convenient  points  from  which  to  distribute  lateral 
pipes.  This  rule  is  by  no  means  universal,  however,  and  frequently 
manholes  other  than  at  street  intersections  are  necessary  or  desirable. 

The  distance  between  manholes  should,  of  course,  not  be  so 
great  as  to  make  it  impossible  to  draw  single  lengths  of  cable  into  the 
intervening  ducts.  However,  the  req  rements  for  distribution  in 
cities  laid  out  in  the  ordinary  way  usually  demand  the  spacing  of 
manholes  at  distances  considerably  shorter  than  the  limiting  lengths 
set  by  the  possibilities  of  cable  drawing.  In  general,  it  may  be 
stated  that  manholes  are  placed  at  distances  apart  of  about  400 
or  500  feet  or  less.  It  has  been  found  possible,  however,  to  draw 
with  success  and  safety  lengths  of  cable  considerably  in  excess  of  this 


816  TELEPHONY 

distance,  and  we  have  without  difficulty  or  danger,  in  special  cases, 
drawn  in  pieces  of  400-pair  cable,  800  feet  in  length. 

Construction  of  Conduit.  Test  Holes.  In  laying  out  the  main 
conduit,  after  its  general  method  of  construction  and  the  streets  and 
alleys  on  which  it  is  to  be  located  have  been  determined,  it  is  fre- 
quently necessary  to  dig  test  holes  at  street  intersections  to  disclose 
the  nature  of  the  existing  underground  obstacles.  These  test  holes 
are  usually  in  the  form  of  narrow  trenches  across  the  streets.  If  the 
engineer  is  fortunate  enough  to  find  city  records  of  underground 
structures  that  are  to  be  relied  upon,  or,  if  the  city  is  so  small  or  new 
that  chances  for  such  obstacles  existing  are  small,  the  digging  of  test 
holes  may  be  dispensed  with.  As  a  rule,  conduits  should  not  be  lo- 
cated in  the  center  of  the  street  on  account  of  the  possibility  of  inter- 
ference with  subsequent  sewer  or  electric  railway  work,  and  not  too 
close  to  the  curb  line  on  account  of  the  greater  exposure  to  surface 
drainage. 

Trenching.  In  opening  the  trench  the  paving  or  surface  material 
of  the  roadway  is  to  be  placed  on  one  side  so  as  to  be  available  for 
re-surfacing  when  the  work  is  completed.  Sometimes  the  trench  is 
excavated  in  parkways,  between  the  sidewalk  and  the  curb,  or  across 
lawns  on  private  right-of-way,  in  which  cases  care  must  be  taken  to 
cut  and  preserve  the  sod  and  to  replace  it  upon  the  completion  of 
the  work.  Occasionally  it  is  required  to  spread  a  cloth  on  the 
grass  to  receive  the  excavated  dirt,  so  that  as  little  of  the  grass  may 
be  injured  as  possible. 

In  general,  the  trench  for  the  main  conduit  is  to  be  excavated 
to  such  a  distance  as  will  leave  not  less  than  24  inches  from  the  top  of 
the  protecting  envelope  to  the  ultimate  grade  of  the  street.  The 
width  of  the  trench  will,  of  course,  vary  according  to  the  width  of 
the  conduit  run.  When  the  character  of  the  soil  will  permit,  the 
sides  of  the  trench  should  be  cut  clean  and  vertical  from  the  bottom 
of  the  trench  to  a  level  that  will  correspond  with  the  top  of  the  conduit 
when  laid.  From  this  point  up  to  the  surface  of  the  ground  it  is  fre- 
quently convenient,  in  some  kinds  of  soil,  to  allow  a  considerable 
slope.  In  some  soils  it  is  necessary  to  provide  shoring  to  prevent  the 
sides  of  the  trench  from  caving  in. 

Where  blasting  is  required,  as  in  rock  soils,  it  should  always 
be  done  by  a  person  competent  in  the  use  of  explosives.  In  such 


UNDERGROUND  CONSTRUCTION  817 

work  the  blast  is  covered  with  heavy  logs  and  chains  to  prevent  in- 
jury to  life  and  property,  and  only  moderate  charges  of  explosives 
should  be  used.  It  is  a  wise  precaution  to  study  the  requirements  of 
local  authorities  before  doing  any  blasting.  Where  gas,  water,  or 
other  pipes  or  underground  property  of  any  nature  is  met,  the  work, 
if  possible,  is  to  be  conducted  without  interfering  with  them,  care 
being  taken  to  properly  protect  and  support  by  temporary  braces 
or  chains  any  such  properties  which  it  may  be  necessary  to  undermine. 

Grading.  It  is  desirable  to  open  an  entire  section  of  trench 
between  adjacent  manholes  before  laying  the  ducts  so  as  to  show  the 
nature  and  location  of  all  obstructions.  The  final  grade  is  then  de- 
termined and  the  bottom  of  the  trench  graded  and  tamped,  if  necessary. 
The  grading  is  to  be  so  done  as  to  avoid  sumps  or  traps  in  which 
water  may  accumulate.  This  means  that  the  grade  should  be  a  de- 
scending one  from  one  manhole  to  the  other,  or  from  a  high  inter- 
mediate point  to  both  manholes.  A  grade  of  at  least  6  inches  for 
each  100  feet  will  do,  but  more  is  desirable. 

Where  possible,  the  various  runs  of  ducts  should  enter  a  given 
manhole  at  about  the  same  level,  those  entering  one  side  of  a  man- 
hole being  preferably  located  directly  opposite  those  in  the  other 
side.  This  is  a  convenience  in  rodding  ducts,  as  will  be  subsequently 
described,  permitting  the  rods  to  be  pushed  from  one  section  to  the 
other  without  uncoupling  them. 

Curves  in  Conduit  Line.  The  trench  should,  if  possible,  follow 
a  straight  line  between  manholes.  Often,  however,  this  is  impossible. 
Wherever  curves  are  unavoidable,  they  should  be  of  as  great  radius 
and  as  small  deflection  as  possible.  If  the  section  of  the  conduit  is 
not  more  than  100  feet  in  length,  a  60-degree  curve  may  be  permitted, 
but  it  should  have  a  radius  of  at  least  25  feet.  In  sections  between 
100  and  200  feet  in  length  a  60-degree  bend  is  allowable,  but  its 
radius  should  be  at  least  50  feet.  For  sections  longer  than  200  feet 
the  curvature  should  be  less  than  60  degrees,  and  the  radius  should 
be  proportionately  longer.  If  a  double  curvature  is  necessary  it  is 
desirable  that  neither  of  them  should  be  more  than  15  degrees  nor 
have  a  radius  less  than  50  feet.  It  goes  without  saying  that  the  longer 
the  run  the  more  undesirable  is  any  curvature. 

Lateral  Runs.  The  lateral  pipes  by  which  a  cable  is  led  from 
the  main  conduit  line  to  a  pole  or  building  for  distribution  purposes 


818 


TELEPHONY 


are  usually  of  iron,  unprotected  by  concrete,  although  sometimes 
fiber  pipe  or  creosoted  wood  may  be  used  to  advantage. 

Frequently,  when  the  lateral 
pipe  is  to  reach  a  point  on  the 
street  intermediate  between  man- 
holes, it  is  convenient  to  lay  it  for 
a  certain  distance  in  the  same 
trench  with  the  main  conduit. 
Where  this  is  done  it  is  prefer- 
ably enclosed  in  the  concrete  en- 
velope with  the  main  conduit,  and 
may  be  of  the  same  conduit  ma- 
terial as  the  main  conduit,  or  of 
the  material  of  which  the  lateral 
pipe  branching  off  from  the  main 
conduit  is  made.  As  the  cables 
in  lateral  pipes  are  usualy  smaller 


Wffiy* 

%  $?&•£& 

%: 

m*-. 


Fig.  598.     Lateral  Riser  up  Building  Wall 

than  those  in  the  main  conduits, 

it  is  frequently  permissible  to  use  smaller  diameters  of  ducts  for 
lateral  pipes  than  for  main  conduit  ducts.  2-inch 
iron  pipe  suffices  in  most  cases,  but  where  the  cable 
to  be  accommodated  is  large,  it  is  well  to  make  the 
lateral  duct  of  the  same  size  of  pipe  as  the  main- 
line duct.  Except  where  large  cables  are  to  be 
installed  in  lateral  pipes,  it  is  permissible  to  make 
right-angle  bends  in  such  pipes  on  a  30-inch  radius. 

Connections  between   the    laterals  laid   in   the 
main  trench  and  the  continuation  of  the  lateral  laid 
in  a  separate  trench  are  made  by  means  of  bends 
in  the  duct   material.      Iron  pipe  may  readily  be 
bent,    and    fiber    conduit    and  _^______— -— — , 

some  other  types  of  duct  ma- 
terial provide  specially  bent 
lengths  of  duct  for  this  and 
similar  purposes. 

Ordinarily  it  is  not  neces- 
sary to  dig  the  trench  for  lateral  P1*-  599-    Lateral  Riser  up  Pole 
pipe  as  deep  as  that  required  for  a  main  trench — a  trench  18  inches 


UNDERGROUND  CONSTRUCTION 


819 


deep  and  as  narrow  as  a  man  can  dig  with  a  spade  being  sufficient.  If 
iron  pipe  is  laid,  all  protection  to  it  may  ordinarily  be  omitted ;  but  if 
fiber  or  wood  pipe  is  used,  it  is  a  good  plan,  outside  of  the  property 
lines,  to  fill  in  the  space  between  the  duct  and  the  side  of  the  trench 
and  over  the  duct  to  a  depth  of  about  3  inches  with  concrete  lightly 
tamped  in  place.  Inside  of  the  property  lines  on  private  property 
this  protection  may  be  omitted. 

Where  possible  the  lateral  should  slope  toward  the  manhole. 
This  is  particularly  important  in  northern  localities,  where  much 
trouble  has  been  experienced  due  to  the  freezing  of  water  in  lateral 
iron  pipes,  resulting  in  a  consequent  compression  of  the  cable,  which 
causes  the  short-circuiting  of  its  conductors. 

Lateral  Risers.  Where  the  lateral  cable  is  to  extend  up  a  pole 
or  up  the  outside  wall  of  a  building,  it  should  make  connection  with 
a  wrought-iron  pipe  bend  not  having  less  than  30  inches  radius,  and 
a  straight  length  of  wrought-iron  pipe  should  continue  up  the  pole, 
preferably  to  a  distance  of  8  feet  above  the  ground.  This  lateral 
riser,  as  it  is  called,  should  be 
strapped  to  the  pole  or  to  the 
wall  of  the  building  by  stand- 
ard pipe  hooks  or  straps.  The 
method  of  ending  a  lateral  at  a 
building  is  shown  in  Fig.  598, 
and  of  extending  it  up  a  pole  in 
Fig.  599. 

Cable  Supports.  Some  form 
of  support  is  necessary  for  the 
cables  leading  around  the  walls 
of  the  manhole.  A  cable  sup- 
port manufactured  by  the  Stand- 
ard Underground  Cable  Com- 
pany is  shown  in  Fig.  600.  This 

is  of  sectional  construction  and  may  be  extended  as  desired.  It 
is  frequently  found  convenient  to  provide  cable  supports  by  insert- 
ing into  the  wall  of  the  vault  during  construction  short  lengths 
of  f-inch  pipe  at  the  proper  heights.  The  ends  of  these  are 
set  flush  with  the  inner  surface  of  the  wall.  When  cables  are  installed 
a  f-inch  wrought-iron  rod  12  or  14  inches  long  is  inserted  into  this 


Fig.  600.     Cable  Support 


820 


TELEPHONY 


pipe,  on  which  is  placed  a  wooden  shoe  for  supporting  the  cable. 
The  details  of  this  support  are  shown  in  Fig.  601.  This  has  the  ad- 
vantage of  not  requiring  the  installation  of  the  support  until  the  cable 
is  installed,  and  of  permitting  its  installation  without  any  drilling  of 
the  walls  of  the  manhole. 

Concrete.  The  matter  of  the  proper  kind  of  concrete  for  use  in 
conduit  construction  is  one  concerning  which  there  is  much  difference 
in  opinion  and  practice.  Some  engineers  employ  the  same  strong 

mixture  of  sand,  stone,  or  gravel 
and  cement  that  would  be  em- 
ployed where  the  greatest  strength 
is  required.  The  later  tendency, 
however,  has  been  to  employ  a 
much  cheaper  mixture,  realizing 
that  under  no  ordinary  circum- 


stances will  the  concrete  be  sub- 
jected to  very  great  strains  of 
any  nature.  Any  good  building 
cement  will  do.  The  ordinary 
concrete  is  made  of  a  mixture  of 
sand,  broken  stone  or  gravel,  and 

cement  in  various  proportions.  Frequently  "run-of-crusher"  stone  is 
procurable,  which  contains  about  the  right  quantity  of  small  pieces 
to  pack  well,  and  if  this  is  found  upon  experiment  to  make  a  good 
concrete  when  mixed  with  the  proper  portion  of  cement,  the  use  of 
sand  may  be  dispensed  with.  Frequently,  also,  gravel  as  it  comes 
from  the  bank  or  bar  has  about  the  right  proportion  of  sand,  in  which 
case,  after  experimenting,  this  may  be  used  with  cement  to  form  the 
concrete. 

The  general  subject  of  concrete  construction  is  too  large  a  one 
to  treat  of  here,  and  much  good  literature  is  available  on  it.  It  may 
be  repeated,  however,  that  the  demands  of  underground  conduit 
structures  are  not  such  as  to  require  an  exceedingly  strong  or  ex- 
pensive mixture.  For  the  concrete  envelope  surrounding  the  ducts, 
a  mixture  as  weak  as  1  part  of  cement  and  4  parts  of  sand  and  8  parts 
of  stone  or  gravel,  known  as  a  1-4-8  mixture,  is  considered  by  some 
sufficient.  1-3-5  or  1-3-6  mixtures  may  be  considered  standard  prac- 
tice. For  manhole  construction  a  1-3-5  mixture  is  good.  Frequently 


Fig.  601.     Cable  Support 


UNDERGROUND  CONSTRUCTION  821 

much  money  may  be  saved  by  a  careful  study  on  the  part  of  the 
engineer  of  the  cost  of  the  materials  available  at  the  point  where  the 
construction  is  to  be  done  and  by  a  few  simple  experiments  to  de- 
termine the  mixture  that  will  give  the  best  results  with  these  mate- 
rials. Concrete  should  be  used  within  one  hour  of  the  time  it  is 
gauged  in  mixing. 

Mortar.  Cement  mortar  consists  of  a  mixture  of  cement  and 
sand  or  screenings.  A  mixture  of  1  part  of  cement  to  3  parts  of  sand 
or  screenings  makes  a  good  strong  mortar.  Mortar  should  be  used 
within  thirty  minutes  of  the  time  it  is  gauged  in  mixing. 

Safeguards.  It  is  necessary  always  to  guard  the  excavations 
wherever  made,  for  the  protection  of  the  public.  This  is  done  by 
means  of  fences  or  barriers  carried  as  a  part  of  the  construction 
equipment.  The  location  of  all  excavations,  fences,  barriers,  or  of 
material  piled  in  the  street,  should  at  all  times  between  sunset  and 
sunrise  be  indicated  by  a  sufficient  number  of  red  lanterns. 

All  construction  work  should  be  done  with  due  regard  to  the 
rights  of  the  public,  temporary  bridges  being  provided  where  neces- 
sary to  facilitate  traffic  across  trenches.  No  excavations  should  be 
made  or  material  piled  where  they  will  interfere  with  the  access  of 
the  public  to  fire-alarm  boxes  or  of  firemen  to  water  hydrants. 

Installing  Underground  Cables.  The  drawing  of  cables  into 
underground  ducts  is  a  very  simple  matter,  but  owing  to  the  fragile 
nature  of  sheaths  and  to  the  necessity  for  keeping  their  cores  always 
absolutely  dry,  it  must  be  done  with  great  care. 

Rodding.  It  is  first  necessary  to  have  some  sort  of  a  pulling 
connection  through  the  duct  into  which  the  cable  is  to  be  drawn. 
Sometimes  a  "fish  wire,"  consisting  of  No.  9  galvanized  steel  wire, 
is  drawn  into  the  duct  as  it  is  being  laid.  Where  this  is  done,  the 
pulling  rope  or  cable  that  is  to  be  used  in  actually  drawing  the  cable 
into  the  duct  may  be  attached  to  one  end  of  the  fish  wire  and  drawn 
by  it  into  the  duct.  Where  there  is  no  fish  wire,  the  process  of 
rodding  is  necessary.  This  consists  ordinarily  in  pushing  short 
wooden  rods  into  the  duct  from  one  manhole  to  another,  these 
rods  being  provided  with  connecting  sockets  by  means  of  which 
they  may  be  joined  together  end  to  end.  As  each  rod  is  pushed 
into  the  duct  another  is  joined  to  it  and  pushed  in,  until  the  first  rod 
emerges  from  the  duct  at  the  distant  manhole,  the  chain  of  rods  is 


822 


TELEPHONY 


then  used  in  place  of  a  fish  wire  to  pull  the  pulling  rope  or  cable 
into  the  duct. 

The  process  of  rodding  in  this  way  has  the  disadvantage  of 
being  slow.  Where  the  interior  of  the  ducts  is  smooth  and  where 
the  length  is  not  prohibitive,  it  is  feasible  to  use  a  heavy,  stiff  steel 
wire  instead  of  the  rods,  this  being  pushed  through  from  one  man- 
hole to  the  other.  A  No.  6  steel  wire  is  desirable  for  this  purpose. 
Before  actually  pulling  in  the  cable  a  mandrel  should  be  pulled 
through  the  ducts  to  make  sure  that  the  passage  is  clear. 

Drawing  In.  The  power  used  for  drawing  in  the  cable  may  be 
that  of  man,  horse,  or  engine.  For  small  installations  and  light 


Fig.  602.     Cable  Pulling  Apparatus 

cables  a  man-  or  horse-driven  capstan  is  frequently  employed.  Some- 
times the  direct  pull  of  a  horse  or  team  or  of  an  automobile  is  employed. 
Where  much  and  heavy  work  is  to  be  done  an  engine-driven  capstan 
is  much  to  be  preferred.  It  is  more  powerful,  more  easily  controlled, 
and  is  capable  of  exerting  a  steadier  pull  and  of  drawing  with  greater 
rapidity  than  any  of  the  other  devices.  For  such  work  the  capstan 
may  be  driven  by  an  electric  motor,  connected  by  flexible  insulated 
leads  to  the  nearest  trolley  wire  or  other  source  of  power  current 
where  the  distribution  of  such  is  sufficiently  universal  to  make  it 
always  available  within  a  reasonable  distance. 


UNDERGROUND  CONSTRUCTION 


823 


A  better  way  is  to  employ  a  gasoline  engine,  since  then  no  con- 
nections have  to  be  provided  for  electric  wires.  Such  a  cable  pulling 
outfit  is  shown  in  Fig.  602.  This  particular  equipment  has  a  6- 
horse-power  engine  connected  by  belt  and  gear  to  the  cable  pulling 
drum.  The  reel  from 
which  the  cable  is  being 
pulled  may  be  seen  in 
the  distance. 

For  handling  the 
reel  from  which  the  ca- 
ble is  to  be  pulled,  the 
simplest  and  most  con- 
venient device  consists 
of  a  pair  of  large  wagon 
wheels,  greater  in  diam- 
eter than  that  of  the 
largest  reel  head.  A 
heavy  steel  shaft  passes  Fjg  603  Reel  Supported  on  Wheels 

through  these  as  an  axle, 

also  passing  through  the  hole  in  the  center  of  the  reel.  This  device 
may  be  used  as  a  cart  in  transporting  the  reel  through  the  streets 
and  as  a  stand  to  support  the  reel  while  the  cable  is  being  drawn 
off  into  the  duct.  Such  an  arrangement,  set  up  for  drawing  in,  is 
shown  in  Fig.  603. 

In  setting  up  for  pulling,  the  reel  should  be  in  line  with  the  duct 
and  ahead  of  the  vault  openings  rather  than  back  of  it,  the  reel  turn- 
ing in  such  direction  that  the  cable  will  feed  from  its  top.  Usually 
a  steel  rope  is  employed  for  pulling.  The  end  of  the  pulling  rope 


Fig.  604.     Grip  for  Drawing  In  Cables 


is  attached  to  the  cable  either  by  a1  lashing  of  wire  or  by  cable 
grips  designed  to  clamp  the  end  of  the  cable  securely  without 
subjecting  the  sheath  to  injury.  One  form  of  such  a  cable  grip  is 
shown  in  Fig.  604,  this  being  a  device  which  increases  its  grip  on 
the  cable  as  the  lengthwise  pull  of  the  rope  increases. 


824 


TELEPHONY 


Skids  and  sheaves  should  be  set  up  in  the  manhole  so  as  to  guide 
the  cable  into  the  mouth  of  the  duct  from  a  direction  as  nearly  in  a 
line  with  the  duct  as  possible.  Likewise,  sheaves  and  skids  should 
be  provided  in  the  manhole  toward  which  the  pulling  is  being  done, 
so  that  the  rope  will  not  chafe  against  the  side  of  the  duct. 

The  general  arrangement  of  the  cable  reel  and  of  the  apparatus 
employed  in  pulling  in  cables  is  shown  in  Fig.  605. 

Lubrication.  In  pulling  in  heavy  runs  of  cable,  the  work  is 
greatly  facilitated  and  the  strain  on  the  cable  is  greatly  reduced  by 
lubricating  the  cable  as  it  passes  into  the  duct.  Grease,  soap,  graph- 
ite, and  other  forms  of  lubricant  have  been  used,  but  in  our  experience 
powdered  soapstone  is  the  best,  and  it  has  the  advantage  of  being 
cheap  and  cleanly.  It  may  be  fed  into  the  duct  opening  as  the  cable 
enters  by  a  funnel  and  scoop,  or  sprinkled  on  the  top  of  the  cable  at 
the  point  where  it  is  entering  the  duct. 

The  greatest  care  should  be  taken  to  inspect  the  cable  as  it  is 
being  pulled  in,  the  man  at  the  reel  watching  for  any  imperfections 


Fig.  605.     Pulling  In  Cable 

in  its  sheath.  Care  also  should  be  taken  that  the  reel  turns  at  uniform 
speed,  since  if  it  runs  ahead  it  is  likely  to  cause  sharp  kinks  in  the 
cable. 

After  the  cables  are  drawn  in,  their  ends  should  be  left  pro- 
jecting in  the  manholes  a  proper  distance  for  making  the  splices. 
The  forward  end  of  the  cable,  where  the  hitch  was  made  to  the  pull- 
ing rope,  if  at  all  damaged,  should  be  cut  off  and  resealed  with  molten 
solder  to  prevent  the  entrance  of  moisture. 


UNDERGROUND  CONSTRUCTION 


825 


Speed  of  Drawing  In.  John  M.  Humiston  has  recorded  some 
tests  made  on  the  power  required  to  pull  a  cable  into  a  duct  at 
varying  speeds  with  and  without  lubricant.  A  300  pair,  No.  19  gauge, 


Ffg.  606.     Arrangement  of  Cables  in  Manholes 

lead-covered  paper-insulated  cable,  weighing  7  pounds  and  7  ounces 
per  foot,  and  having  an  outside  diameter  of  2f -inches,  was  used. 
The  duct  was  of  vitrified  clay  rectangular  in  cross-section,  682  feet 
in  length.  The  cable  was  first  pulled  in  by  a  capstan  turned  by  a 
horse.  The  maximum  stress  on  the  rope  noted  was  3,500  pounds, 
this  occurring  about  50  feet  before  the  finish  was  reached,  arid  being 
due  to  the  drawing  having  stopped  and  started  again.  The  maxi- 
mum stress  during  continuous  drawing  occurred  at  the  end  of  the  pull 
and  was  3,200  pounds.  The  rate  of  pulling  in  this  experiment  was 
25  feet  per  minute.  The  same  cable  was  then  drawn  in  at  a  higher 

TABLE  XXXV 
Drawing  In  Stress  on  Cable 


WEIGHT  OF     :       LENGTH  OF 
CABLE                     SECTION 
POUNDS                      FEET 

SPEED 
FEET  PER 
MINUTE 

LUBRICANT 

STRESS 
POUNDS 

5060 

628 

25 

None 

3200 

5060 

679 

120 

None 

2800       . 

5060 

682 

120 

Soapstone 

1200 

826  TELEPHONY 

rate  of  speed  by  means  of  a  heavy  automobile  truck.  The  maximum 
stress  in  the  cable  when  being  drawn  at  the  rate  of  120  feet  per  min- 
ute, was  2,800  pounds.  Thus  practice  indicates  that  a  higher  rate 
of  travel  resulted  in  a  lower  friction,  as  theory  wrould  indicate.  The 
same  cable  was  then  pulled  into  the  duct,  using  powdered  soapstone 
as  a  lubricant.  In  this  case  the  stress  was  only  1,200  pounds,  showing 


Fig.  607.    Arrangement  of  Cables  in  Manholes 

marked  saving  in  power  by  this  very  simple  scheme  of  lubrication. 
Table  XXXV  summarizes  these  experiments. 

Long  Conduit  Sections.  Humiston  also  gives  some  information 
as  to  successful  drawing  in  of  heavy  cables  into  very  long  runs  of  con- 
duit. He  concludes  from  these  that  where  no  street  obstructions 
intervene,  it  is  entirely  feasible  to  locate  the  vaults  as  far  apart  as  800 
feet.  He  cites  one  example  where  heavy  cables  have  been  pulled  into 


UNDERGROUND  CONSTRUCTION  827 

a  section  of  conduit  882  feet  in  length  with  two  reversed  curves  or 
offsets  in  it.  These  instances  are  interesting  as  illustrating  possible 
extremes  in  this  phase  of  underground  cable  practice. 

Arrangement  of  Cables  in  Vaults.  Practically  all  of  the  splices 
in  underground  cables  occur  in  manholes.  A  well-made  splice  may 
be  spoiled  by  subsequent  misuse.  Much  depends,  therefore,  on  the 
way  the  cables  are  stowed  away  in  the  manholes.  It  has  already 
been  stated  that  where  possible  it  is  preferable  to  have  the  conduit 
runs  not  more  than  four  ducts  wide.  This  makes  possible  a  very 
systematic  arrangement  of  cables  in  the  manholes  as  suggested  in 
Fig.  606,  which  shows  the  sides  of  a  square  manhole  laid  out  flat  for 
purposes  of  illustration. 

When  the  conduit  is  not  more  than  four  ducts  wide  it  is  possible 
to  arrange  the  cables  on  each  side  of  the  vault  in  not  over  two  tiers, 
thus  making  the  cables  next  to  the  wall  more  easily  reached.  This 
way  of  arranging  the  cable  is  indicated  in  Fig.  607,  which  shows  the 
cables  leading  from  the  two  left-hand  vertical  rows  of  ducts  around 
that  side  of  the  manhole.  The  splices  are  staggered.  This  arrange- 
ment is  somewhat  idealized,  since  it  is  not  always  practicable  to  ac- 
complish it,  especially  where  old  work  has  to  be  dealt  with.  It  is 
good,  however,  at  least  to  have  an  ideal  at  which  to  aim. 

Another  feature  to  be  borne  in  mind  in  the  arrangement  of 
splices  and  cables  in  manholes  is  the  disposal  of  small  tap  cables 
leading  to  laterals.  If  the  proper  thought  is  given  before  making  the 
tap  splice,  it  is  easy  to  lead  the  small  cables  from  the  splices  to  the 
lateral  duct  around  the  wall  of  the  manhole,  and  behind  the  large 
cables,  as  shown  in  Fig.  607,  and  in  this  manner  the  smaller  cables, 
which  are  least  able  to  stand  rough  usage,  are  protected  by  the  larger 
ones. 


CHAPTER  XLVI 
CABLE  SPLICING 

Necessity  for  Dryness.  On  no  feature  of  telephone  work  is  the 
proper  condition  of  a  telephone  plant  more  dependent  than  on  the 
use  of  proper  methods  and  care  in  splicing  of  its  lead-covered  paper- 
insulated  cables.  Dry  paper,  by  virtue  of  its  low  electrostatic  capac- 
ity, its  high  insulation  resistance,  and  its  low  cost,  is  the  only  material 
that  has  been  found  suitable  for  the  insulation  of  cable  conductors 
where  such  conductors  form  considerable  portions  of  the  length  of 
telephone  lines.  It  is  subject,  however,  to  the  grave  objection  that 
even  a  very  small  amount  of  moisture  will  destroy  the  insulation 
between  the  conductors.  The  avidity  with  which  this  paper  will 
absorb  any  moisture  makes  it  necessary  to  keep  it  completely  iso- 
lated from  atmospheric  conditions,  and,  in  fact,  in  a  state  of  extreme 
dryness.  The  difficulties  in  the  way  of  doing  this  would  seem  to  the 
uninitiated  to  be  unsurmountable.  Aerial  cables  are  exposed  to  rain, 
snow,  sleet,  and  ice,  and  underground  cables  are  sometimes  com- 
pletely immersed  in  water  for  considerable  periods  of  time,  as  when 
the  manholes  and  the  conduit  system  are  flooded.  A  pin-hole  any- 
where in  the  lead  sheath  of  a  cable  will  permit  enough  moisture  to 
enter  to  make  it  unfit  for  use. 

The  cables  employed  for  both  aerial  and  underground  work 
differ  in  no  respect  except,  perhaps,  in  size;  cables  with  400,  600, 
and,  in  rare  cases,  900  pairs  of  conductors  being  employed  in  under- 
ground work,  while  those  having  more  than  300  pairs  are  seldom,  and 
more  than  400  pairs  are  never,  so  far  as  we  know,  suspended  aerially. 

The  rule  that  must  govern  the  cable  splicer,  first,  last,  and  always, 
is — Keep  the  core  of  the  cable  dry.  This  does  not  merely  mean 
that  no  noticeable  moisture  shall  be  present,  but  that  it  shall  be 
dry  in  the  sense  that  a  thing  is  dry  after  it  has  been  baked  in  an 
oven  for  a  long  time. 

Whenever  possible,  splicing  should  be  done  in  a  dry  place  in  dry 


CABLE  SPLICING  829 

weather.  If  conditions  demand  that  it  be  done  in  a  damp  place, 
as  is  frequently  necessary,  extreme  care  should  be  taken  to  guard 
against  the  entrance  of  moisture  and  to  expel  whatever  moisture 
does  enter,  as  will  be  explained. 

General  Method  of  Splicing.  In  general,  the  method  of  making 
a  splice  consists  in  stripping  back  the  lead  sheath  from  the  two  ends 
to  be  spliced  for  a  sufficient  space  to  afford  the  proper  working  length 
of  the  exposed  conductors,  then  the  individual  wires  are  joined 
together  and  covered  with  paper  sleeves.  Before  beginning  the 
splicing  of  the  conductors,  and  during  its  progress  if  necessary,  and 
always  after  its  completion,  the  exposed  conductors  are  subjected 
to  a  boiling-out  process  to  expel  all  moisture.  After  the  conductors 
are  spliced  and  boiled  out,  they  are  bunched  together  and  enclosed 
in  a  lead  sleeve  of  sufficient  diameter  not  to  crowd  the  wires  too 
much  together,  and  of  sufficient  length  to  lap  over  the  lead  sheaths 
of  the  joined  cables.  Then  the  edges  of  the  sleeve  are  beaten  down 
to  the  cable  sheath  at  each  end  and  secured  thereto  by  a  plumber's 
wiped  joint. 

Straight  Splice.  A  lead  sleeve  of  proper  size  is  first  slipped 
over  the  end  of  one  of  the  cables  to  be  joined  and  pushed  back  out  of 
the  way.  A  mark  is  then  made  on  the  sheath  of  each  cable  to  desig- 
nate the  point  at  which  the  sheath  is  to  be  cut  and  removed.  The 
distance  from  the  end  of  the  cable  at  which  this  mark  is  made  should 


—  L£ff6TH 
Fig.  60S.     Preparing  Cable  for  Splice 

be  for  all  sizes  of  cable  about  2  inches  less  than  the  length  of  the  lead 
sleeve  employed,  as  indicated  in  Fig.  608.  A  portion  of  the  cable 
sheath  about  4  inches  back  from  this  mark  is  then  scraped  bright  so 
as  to  afford  a  proper  surface  for  making  the  wiped  joint  at  the  end 
of  the  splicing  operation,  and  this  brightened  surface  is  then 
rubbed  with  tallow  to  keep  it  bright  during  the  subsequent  opera- 
tions. The  reason  for  doing  this  scraping  and  brightening  operation 
at  this  point  in  the  work  is  to  prevent  the  small  particles  of  lead, 
scraped  off,  from  getting  in  the  splice,  and  endangering  the  insulation. 


830  TELEPHONY 

Preparing  the  Conductors.  The  lead  sheath  is  then  removed  from 
the  end  of  each  cable  back  to  the  mark  previously  made.  This  is 
readily  done  by  indenting  the  cable  around  its  circumference  with 
the  edge  of  a  cable  knife,  after  which  a  slight  bending  back  and  forth 
will  break  the  lead  sheath  on  this  mark  and  permit  the  end  to  be 
drawn  off.  The  core  of  the  cables  is  now  exposed,  and  from  this 
time  on  the  greatest  care  should  be  taken  not  only  to  keep  it  dry,  but 
to  guard  against  mechanically  injuring  the  insulation.  The  core 
of  each  end  should  now  be  bound  tightly  with  narrow  strips  of  dry 
muslin,  just  at  the  end  of  the  cable  sheath,  packing  this  rnuslin  back 
under  the  sheath  for  a  slight  distance  to  prevent  the  sharp  edges  of 
the  sheath  from  injuring  the  insulation  of  the  outer  layer  of  wires. 
As  soon  as  possible  after  this  is  done  the  cable  ends  should  be  boiled 
out.  In  the  case  of  a  small  cable  this  may  be  done  by  immersing 
the  exposed  ends  of  the  core  in  a  kettle  of  hot  paraffin,  but  for  larger 
cables  this  is  not  feasible,  the  process  then  consisting  in  pouring 
hot  paraffin  over  the  exposed  portions  of  the  core,  the  drippings  being 
caught  in  a  pan  placed  directly  beneath.  This  boiling-out  process 
should  include  the  entire  length  of  the  exposed  core  and  the  muslin 
wrapping  under  the  end  of  the  sheath.  Great  care  should  be  exer- 
cised as  to  the  temperature  of  the  paraffin.  It  should  be  very  hot, 
and  yet  not  hot  enough  to  scorch  the  paper.  Overheated  paraffin  not 
only  injures  the  insulation  of  the  cable  conductors,  but  is  dangerous 
to  life,  since  it  may  take  fire.  Underheated  paraffin  will  not  expel 
the  moisture.  If  the  paraffin  is  so  hot  that  white  fumes  rise  from  it, 
it  should  be  allowed  to  cool  slightly  before  being  used. 

In  boiling  out,  one  should  begin  at  the  cable  sheath  and  work 
toward  the  core  end,  or  if  the  wires  have  been  spliced,  one  should 
begin  at  the  ends  of  the  cable  sheath  and  work  toward  the  middle  of 
the  splice.  This  prevents  a  tendency  for  the  heat  to  drive  what  mois- 
ture there  is  in  the  exposed  ends  back  into  the  core  within  the  lead 
sheath.  The  hot  paraffin  itself  should  soak  back  into  the  core  cov- 
ered by  the  sheath  for  some  distance  to  keep  moisture  out  during 
the  splicing.  After  the  ends  are  boiled  out,  the  two  cables  should 
be  placed  in  proper  alignment,  the  distance  between  the  ends  of  the 
sheaths  being  about  4  inches  less  than  the  length  of  the  lead  sleeve, 
as  indicated  in  Fig.  608.  The  conductors  are  then  bent  back  close 
to  the  sheath  out  of  the  way  and  spliced  in  the  following  manner: 


CABLE  SPLICING 


831 


Joining  the  Wires.  Starting  with  the  center  wires  of  the  cores, 
or  with  the  lower  back  sides  of  the  cores,  two  pairs,  one  from  each  ca- 
ble, are  loosely  brought  together  with  a  partial  twist,  as  shown  in 
a  of  Fig.  609.  The  bend  thus  made  in  each  pair  marks  the 
point  at  which  the  joint  is  to  be  made.  A  paper  sleeve  is  then 
slipped  over  each  wire  of  the  pair  and  pushed  back  out  of  the  way 
so  as  to  make  room  for  the  joint,  as  shown  in  b  of  Fig.  609. 


Fig.  609.     Splicing  Cable  Conductors 

The  two  colored  wires  of  the  pairs  chosen  are  then  twisted  together 
for  about  three  twists,  as  indicated  in  6,  and  the  insulation 
stripped  off  beyond  this  twist.  This  twist  is  made  at  the  points 
indicated  by  the  bends  previously  made,  as  in  a .  In  removing  the 
insulation,  care  should  be  taken  not  to  nick  the  conductors.  The 
reason  for  including  the  insulation  in  the  twist  of  the  joint  is  to  pre- 
vent its  stripping  back  and  exposing  the  wires.  The  exposed  ends 
of  the  wires  are  then  twisted  together,  and  the  method  of  doing  it  is 
indicated  in  c.  The  wires  are  bent  so  as  to  form  a  crank,  the 


832  TELEPHONY 

handle  of  the  crank  being  held  between  the  thumb  and  finger  of  the 
right  hand,  while  the  portion  of  the  wires  just  beyond  the  insulation 
is  similarly  held  with  the  left  hand.  The  twisting  is  accomplished  by 
merely  turning  this  crank.  There  is  a  knack  about  it  which  requires 
practice  to  acquire.  After  the  wires  themselves  are  twisted  together, 
they  are  cut  off  so  as  to  let  the  twisted  wire  project  about  1  inch 
from  the  end  of  the  insulation.  The  twist  is  then  bent  down  along  the 
insulated  wire,  as  shown  in  d,  and  the  paper  sleeve  slipped  over  it. 
The  completed  joint  is  shown  in  e. 

This  process  is  repeated  until  all  of  the  pairs  are  spliced.  In 
order  that  all  of  the  wire  joints  shall  not  occur  at  the  same  point  in 
the  splice,  they  are  staggered,,  thus  preventing  the  splice  attaining  too 
great  a  diameter  at  any  one  point.  By  reasonable  care  in  dis- 


Pig.  610.     Sections  of  Completed  Splice 

tributing  the  wire  joints  along  the  length  of  the  splice,  it  may  be 
kept  uniform  in  size  and  shape. 

Final  Boiling-Out.  When  all  of  the  wire  joints  have  been 
made,  the  splice  is  again  boiled  out  in  hot  paraffin  until  all  moisture 
has  been  expelled.  It  will  usually  be  found  that  some  moisture  has 
been  gathered  by  the  paper  insulation  from  the  hands  of  the  work- 
man or  from  the  air. 

Immediately  after  this  final  boiling-out  and  while  the  splice  is 
still  "piping  hot,"  it  should  be  wrapped  with  strips  of  dry  muslin  2  or 
3  inches  wide,  so  as  to  compress  the  wires  only  to  a  sufficient  extent 
to  permit  the  lead  sleeve  easily  to  be  slipped  over  them.  Unless 
there  is  a  suspicion  that  some  moisture  may  have  entered  during  this 
binding  process,  no  further  boiling-out  need  be  done. 

Enclosing  the  Splice.  The  lead  sleeve  is  then  slipped  into  place 
while  the  splice  is  hot  and  its  ends,  which  will  overlap,  are  dressed 
down  so  as  to  engage  the  cable  sheath.  After  this  a  plumber's  wiped 
joint  is  carefully  made  at  each  end  of  the  sleeve  and  the  splice  is  com- 
plete, as  shown  in  section  in  Fig.  610. 


CABLE  SPLICING 


833 


In  making  the  wiped  joints,  strips  of  gummed  paper  are  used 
to  limit  the  flow  of  the  wiping  solder  on  the  sheath.  Inspection  of 
the  completed  wiped  joints  is  an  important  feature,  and  sometimes  a 
mirror  will  aid  in  examining  the  underside  of  the  joint.  As  an  in- 
ducement to  proper  workmanship,  each  splicer  working  on  a  job 
should  be  given  a  number  and  a  steel  stamp  bearing  this  number, 


Fig.  611.     Tap  Splice 

which  number  must  be  stamped  on  each  splice  when  completed.  By 
this  method  careless  work  may  of  ten  be  traced  back  to  its  proper  source. 
Tap  Splice.  The  method  of  making  a  tap  splice  where  the 
wires  of  a  branch  cable  join  those  of  a  continuous  cable,  differs 
somewhat  from  that  just  described,  since  three  wire  ends  have  to  be 
handled  rather  than  two.  The  method  to  be  followed  in  this  will  be 
clear  from  an  inspection  of  Fig.  611.  It  will  be  seen  in  the  second 


834  TELEPHONY 

operation  that  a  short  piece  of  wire  must  be  inserted  in  order  to 
give  sufficient  length  of  wire  to  make  the  splice  with  the  branch  con- 
ductor. 

The  external  appearance  of  a  finished  tap  splice  is  shown  in  Fig. 
612.  The  tap  cable  should  always  lead  out  from  one  end  of  the  splice 
and  should  be  lashed  to  the  main  cable  with  marline  about  6  inches 


Fig.   612.     Finished  Tap  Splice 

beyond  the  end  of  the  splice  to  prevent  any  side  strain  being  exerted 
by  the  tension  of  the  tap  cable  on  the  splice  itself. 

Y-Splices.  For  Y-splices  that  are  not  in  the  nature  of  tap  splices, 
but  in  which  the  conductors  of  a  large  cable  are  spliced  into  two 
smaller  cables,  the  same  method  of  procedure  is  followed  as  in  the 
straight  splice,  so  far  as  the  joining  of  the  wires  is  concerned.  Fre- 
quently a  larger  cable  will  be  spliced  into  two  smaller  ones  where 
the  sum  of  the  conductors  in  the  smaller  ones  more  than  equals  the 
number  in  the  larger.  In  this  case  certain  of  the  conductors  in  the 
larger  cable  will  be  spliced  straight  through  to  the  respective  con- 
ductors in  the  smaller  cables,  and  certain  others  in  the  larger  cable 
will  be  joined  to  two  conductors,  one  in  each  of  the  smaller  cables. 
This  results  in  some  of  the  conductors  in  the  large  cables  being  made 
available  at  the  terminals  of  both  of  the  smaller  cables,  this  being 
one  of  the  phases  of  multiple-tap  distribution. 

Sizes  of  Lead  Sleeves.  Table  XXXVI  gives  the  sizes  of  lead 
sleeves  for  straight  and  Y-splices  in  various  sizes  of  22-gauge  cable, 
the  sleeves  in  each  case  being  of  pure  lead,  %  inch  in  thickness. 

Where  a  branch  cable  is  spliced  into  a  continuous  cable,  and 
under  certain  other  conditions  of  practice,  it  is  necessary  to  employ 
split  sleeves,  i.  e.,  sleeves  that  are  cut  through  one  side  of  their  length 
so  as  to  enable  them  to  be  opened  and  slipped  over  the  cable.  Of 
course,  when  such  a  sleeve  is  used,  the  joint  along  its  side  should  be 
carefully  closed  by  solder  so  as  to  make  it  a  continuously  closed  pipe. 
The  paper  sleeves  for  the  individual  wires  are  usually  about  3  inches 
long  and  approximately  \  inch  in  diameter.  They  should  always 
be  boiled  in  paraffin  before  using. 

Pot=Heads.    The  pot-head,  as  already  stated,  is  a  special  form 


CABLE  SPLICING 


835 


TABLE  XXXVI 

Lead  Sleeves 


STRAIGHT  SPLICES  22  GAUGE 

Y-SPLICES  22  GAUGE 

No.  PBS. 

INSIDE  DIAM. 
IN  INCHES 

LENGTH 
INCHES 

No.  PRS. 

I  NIDE  DIAM. 
IN  INCHES 

LENGTH 
INCHES 

10 

1 

16 

10 

1 

16 

15 

1 

16 

15 

1 

16 

20 

.        1 

16 

20 

H 

16 

30 

1* 

16 

30 

li 

16 

40 

2 

18 

40 

2 

18 

50 

2 

18 

50 

2* 

18 

60 

2 

18 

60 

2* 

18 

80 

2* 

18 

80 

3 

20 

100 

2i 

18 

100 

3* 

22 

200 

3 

20 

200 

4 

22 

400 

8i 

22 

400 

4* 

22 

600 

4 

26 

600 

4J 

26 

of  splice.  Instead  of  joining  two  lead-covered  paper-insulated  cables, 
it  joins  the  wires  in  one  such  cable  with  an  equal  number  of  individ- 
ual rubber-covered  wires,  its  purpose  being  to  terminate  the  paper 
cable  in  wires  that  will  not  be  injured  by  exposure  to  the  atmosphere. 
Briefly,  a  pot-head  is  made  by  opening  the  end  of  the  paper- 
insulated  cable  and  splicing  to  its  conductors  rubber-insulated  wires, 
and  then  enclosing  the  splice  in  a  chamber  filled  with  insulating 
compound,  so  that  no  moisture  can  enter  the  core  of  the  cable. 

The  method  of  making  a  pot-head  is  as  follows:  A  lead  pot- 
head  sleeve  of  proper  size  is  slipped  over  the  end  of  the  paper  cable 
and  run  back  out  of  the  way.  The  end  of  the  paper  cable  is  then 
prepared  in  the  same  manner  as  described  for  making  a  straight 
splice,  the  same  care  being  exercised  in  boiling  it  out.  The  core  is 
then  wrapped  with  muslin  at  the  point  where  it  emerges  from  the 
lead  sheath,  this  wrapping  being  tucked  in  under  the  edges  of  the 
sheath  to  prevent  injury  to  the  conductors,  as  in  the  ordinary  splice. 
Rubber-covered  twisted-pair  pot-head  wires,  usually  of  the  same 
gauge  as  the  wires  of  the  cable,  are  then  spliced  to  the  cable  wires 
in  exactly  the  same  manner  as  described  in  making  a  straight  splice, 
paper  sleeves  being  used  in  the  same  way.  To  do  this,  the  pot- 
head  wire  is  skinned  for  a  distance  of  about  l£  inches,  the  colored 
or  otherwise  distinguished  wire  of  the  rubber-covered  pair  being 
spliced  to  the  colored  wire  of  the  cable.  The  same  care  as  to  stag- 


836 


TELEPHONY 


gering  of  the  joints  is  also  necessary  to  prevent  the  splice  assuming 
unequal  diameters  along  its  lengths. 

After  the  wires  are  spliced,  they  are  neatly  bunched  and  wrapped 
with  twine  or  wicking,  this  being  drawn  only  tight  enough  to  com- 
press the  bunch  so  that  the  lead  sleeve  will  readily  slip  over  it.  In  no 
case,  however,  should  the  twine  or  wicking  extend  higher  up  on  the 
splice  than  a  point  about  3^  inches  below  the  top  of 
the  pot-head  sleeve,  after  the  latter  has  been  put  into 
place.  This  same  instruction  as  to  twine  or  wicking 
will  apply  to  paper  sleeves  and  to  any  other  material 
of  a  fibrous  nature,  none  of  which  should  be  allowed  to 
extend  further  up  than  about  3^  inches  below  the  top 
of  the  sleeve  after  it  is  in  place. 

After  the  wires  are  joined  and  bunched  in  this  man- 
ner, the  pot-head  sleeve  is  drawn  up  over  the  splice 
and  its  lower  end  beaten  or  dressed  into  form  against 
the  cable  sheath,  after  which  it  is  secured  in  this  position 
by  a  regular  wiped  joint  in  exactly  the  same  manner  as  a 
straight  splice. 

Filling.  The  method  so  far  described  results  in  a 
splice  that  is  open  at  one  end  and  from  this  end  pro- 
jects the  bunch  of  rubber-insulated  conductors.  The 
splice  is  now  secured  in  a  vertical  position,  open  end 
up,  and  is  ready  for  filling.  Before  pouring,  the  wires 
in  the  top  of  the  sleeve  should  be  loosened  as  much  as 
possible  so  as  to  allow  the  insulating  compound  to  flow 
freely  between  them.  The  entire  sleeve  is  then  warmed  thoroughly 
with  a  blow  torch,  and  the  insulating  compound,  which  has  been 
heated  so  as  to  flow  freely,  is  poured  in  it  within  -J  inch  of  the  top  of 
the  lead  sleeve.  As  the  compound  settles,  more  should  be  added  to 
keep  it  to  the  height  mentioned,  the  sleeve  being  kept  hot  to  facili- 
tate the  process  of  settling.  Sometimes  after  the  pot-head  has  cooled 
it  will  be  found  that  the  compound  has  settled  slightly,  and  in  this 
case  more  should  be  added  to  bring  the  insulating  surface  to  the  re- 
quired height.  After  the  pot-head,  Fig.  613,  has  been  filled  and 
allowed  to  cool,  the  remaining  space  in  the  top  of  the  sleeve  may 
be  filled  with  Cimmerian  asphalt,  which  has  been  heated  so  as  to 
flow  freely.  Cimmerian  asphalt  is  a  compound  which  does  not  be- 


Fig.  613. 
Section  of 
Pot-head 


CABLE  SPLICING 


837 


come  hard  and  brittle  with  age  and  is,  therefore,  used  as  an  added 
precaution  to  insure  a  perpetual  seal. 

Central-Office  Pot-Heads.  In  making  central-office  pot-heads 
or  splices  for  joining  the  conductors  of  the  incoming  paper  cables 
to  those  of  the  silk  and  cotton  cables  or  the  wool  cables,  which  lead 
to  the  distributing  frame,  the  same  general  practice  as 
to  making  the  joints  in  the  conductors  is  followed. 
Where  a  large  line  cable  is  joined  to  many  silk-  and 
cotton-insulated  lead-covered  cables,  the  resulting  splice 
or  pot-head,  illustrated  in  Fig.  614,  is  formed  as  fol- 
lows: The  pot-head  sleeve  is  first  slipped  over  the 
main  cable  and  back  out  of  the  way;  likewise,  the 
terminal  cables  are  passed  through  the  holes  in  the 
wooden  disk  shown  in  the  drawing,  and  this  is  slipped 
back  out  of  the  way.  The  lead  sleeve  is  then  re- 
moved from  each  of  the  terminal  cables  and  from 
the  main  cable,  and  splicing  is  done  in  the  same 
manner  as  in  a^straight  cable  splice.  After  the  splic- 
ing is  completed  the  lead  sheath  is  put  in  place,  the 
joint  is  wiped,  and  the  wooden  disk  is  slipped  down  on 
the  terminal  cable  so  that  its  lower  surface  will  be  flush 
with  the  ends  of  their  lead  sheaths.  If  the  cables  do 
not  completely  fill  the  holes  in  the  wooden  disk,  muslin 
should  be  crowded  in  around  the  cables.  The  top  of  section  of 
the  wooden  disk  is  now  about  1  inch  below  the  top  of 
the  pot-head  sleeve,  arid  upon  it  is  placed  a  layer  of  fine  dry  sand 
about  %  inch  deep  to  form  the  foundation  of  the  wiping  solder  to 
be  used  in  sealing  the  top  of  the  lead  sleeve.  This  wiping  solder 
is  filled  flush  with  the  top  of  the  sleeve  and  its  surface  wiped  so  as 
to  join  perfectly  and  continuously  with  the  lead  sheath  of  each  ter- 
minal cable  and  with  the  walls  of  the  pot-head  sleeve. 

Splices  should  always,  if  possible,  be  finished  the  same  day  they 
are  begun.  If  the  surroundings  be  dry  and  are  of  such  a  nature  as 
to  continue  so,  then  a  splice  may  be  left  unfinished  over  night,  but  it 
should  always  be  protected  from  the  atmosphere  by  a  rubber  blanket 
completely  enclosing  it  and  bound  tightly  against  the  cable  ends. 
Wherever  paper  cable  is  cut,  its  ends  should  be  sealed  with  solder 
before  leaving  it. 


CHAPTER   XLVII 
OFFICE  TERMINAL  CABLES 

In  a  modern  plant  the  line  side  of  the  main  distributing  frame  may 
be  considered  as  the  dividing  point  between  the  outside  cable  plant 
and  the  inside  apparatus  plant,  since  it  is  at  this  point  that  the  con- 
ductors of  the  outside  plant  may  be  said  properly  to  terminate.  The 
matters  now  to  be  considered  are:  the  method  of  leading  the  outside 
cables  into  the  central-office  building,  and  the  method  of  so  termi- 
nating the  conductors  of  these  cables  that  their  insulation  will  not  be 
impaired,  either  by  the  entrance  of  moisture  or  by  mechanical  in- 
jury to  the  insulation  of  the  wires  where  they  emerge  from  the  lead 
sheath. 

The  entrance  of  the  outside  cables  to  the  office  building  may 
be  either  aerial,  underground,  or  both.  Only  in  small  plants,  if 
they  are  modern,  will  the  entrance  be  aerial,  since  if  there  is  any 
underground  work  at  all  in  the  exchange,  it  will  probably  occur  in  the 
immediate  vicinity  of  the  central  office  where  the  cable  runs  are  al- 
ways heaviest. 

Aerial  Cable  Entrance.  Where  aerial  cables  enter  the  central 
office,  a  heavy  pole  is  set  near  the  wall  of  the  building,  and  all  of  the 
aerial  cables  are  run  to  this  pole.  From  this  pole  the  cables  are  led, 
usually  on  an  iron  rack,  to  the  wall  and  into  .the  building,  where  they 
are  connected  to  suitable  terminal  apparatus. 

In  very  small  towns,  where  only  bare-wire  lines  exist,  the  line 
wires  are  brought  to  the  office  pole  and  there  joined  to  the  various 
conductors  in  an  office  cable  which  leads  into  the  building.  For 
this  purpose  rubber-insulated  cable  is  usually  employed,  since  by 
its  use  special  treatment  of  the  cable  ends  is  avoided.  Owing  to  the 
very  short  lengths  of  cables  so  used,  the  relatively  high  electro- 
static capacity  of  the  rubber-insutated  wires  is  not  a  serious  factor. 

Underground  Cable  Entrance  for  Small  Plants.  Sometimes 
when  the  wire  plant  is  almost  wholly  aerial,  a  short  length  of  under- 


OFFICE  TERMINAL  CABLES 


839 


840 


TELEPHONY 


ground  conduit  will  be  used,  and  underground  cables  led  through  this 
to  the  central  office.  A  somewhat  idealized  arrangement  for  a  very 
small  exchange,  embodying  an  underground  entrance  for  a  wire 
plant  that  is  otherwise  all  aerial,  is  shown  in  Fig.  615. 

Underground  Entrances  for  Larger  Plants.  Either  of  two  gen- 
eral plans  may  be  followed  for  effecting  the  entrance  of  the  cables  of 
an  underground  conduit  system  into  the  office  building.  An  office 
manhole  may  be  employed  to  which  all  lines  of  conduit  lead,  and 
from  which  all  cables  pass  through  a  tunnel  or  regular  conduit  ducts 
to  the  basement  of  the  building,  Fig.  616.  Or,  the  office  manhole 


Fig.  616.     Underground  Cable  Entrance 
through  Office  Manhole. 

may  be  dispensed  with  and  the  conduits  extended  directly  to  the 
basement  of  the  building,  which,  in  this  case,  forms  in  itself  the  office 
manhole,  Fig.  617. 

Where  the  building  is  located  directly  on  the  street  and  where 
the  conditions  in  the  street  are  such  as  to  permit  the  approaching 
ducts  to  extend  directly  to  the  basement,  as  required  by  the  scheme 
shown  in  Fig.  617,  this  practice  is  simpler,  cheaper,  and  better.  Often 
the  basement  or  cellar  may  be  extended  out  under  the  sidewalk 
and  street  so  as  to  intersect  the  conduit  lines,  without  the  necessity 
of  curving  the  conduit  approaches  or  running  them  diagonally,  as 
was  done  in  the  installation  shown. 


OFFICE  TERMINAL  CABLES 


841 


By  using  the  cellar  of  the  central-office  building  as  the  office 
manhole,  a  large  amount  of  splicing  that  would  otherwise  be  done 
in  a  street  manhole  is  done  within  the  walls  of  the  building,  and 
generally  the  amount  of  splicing  is  reduced — both  advantageous 
features. 

Treatment  of  Cable  Ends.  There  are  three  principal  reasons 
why  it  is  not  good  practice  to  run  the  paper-insulated  cables  to  the 
distributing  frame:  First,  the  cable  end,  if  thus  exposed,  would  be 
likely,  during  wet  weather,  to  absorb  sufficient  moisture  to  lower  the 


Fig.  617.     Basement  Cable  Entrance. 

insulation  of  the  conductors;  second,  paper-insulated  wires  are  not 
well  adapted  to  stand  the  handling  necessary  in  the  work  of  fanning 
out  the  conductors  and  leading  them  to  their  respective  terminals;  and, 
third,  the  line  cables  entering  the  central  office  are  usually  of  large 
size,  having  400  or  600  pairs,  and  it  is  not  convenient,  as  a  rule,  to 
subdivide  such  large  units  at  the  distributing  frame.  This  latter 
fact  makes  it  desirable  to  subdivide  the  large  cables  entering  the  cen- 
tral office  before  leading  them  to  the  distributing  frame,  entirely 
aside  from  any  considerations  as  to  the  maintaining  of  the  insulation. 
Two  general  methods  of  overcoming  these  difficulties  are  prac- 
ticed. One  is  to  splice  on  to  the  paper-insulated  cables,  at  a  point 
some  distance  from  the  main  distributing  frame,  cables  of  such  char- 
acter that  moisture  will  not  be  likely  to  work  back  through  them.  Such 


842 


TELEPHONY 


cables  are  made  of  wool-insulated  wires,  lead  encased.  They  are 
well  adapted  to  stand  the  necessary  exposure  and  handling  at  the 
distributing  frame  and  they  serve  as  a  seal  for  the  paper  cables. 
The  other  method  is  to  terminate  the  paper  cables  entering  the  office 
in  pot-heads,  which  form  the  seal  for  the  paper  cables.  From  these 
pot-heads,  cables,  either  of  wool  or  of  silk  and  cotton  insulation— 
either  of  which  are  not  so  susceptible  to  moisture  and  are  better 
able  to  withstand  rough  treatment  than  paper  cables — are  led  to  the 
distributing  frame.  The  making  of  pot-heads  for  such  use  is  con- 


Fig.  618.     Cable  Splices. 

sidered  in  another  chapter;  the  general  arrangement  employed  for 
disposing  of  these  splices  and  pot-heads  and  of  the  cables  leading 
from  them  to  the  distributing  frame  will,  however,  be  considered  here. 
Wool  Cable  Ends.  In  using  wool-insulated  terminal  cables,  the 
splices  may  be  made  of  ordinary  form  in  the  office  manhole,  or  at  the 
point  where  the  cables  enter  the  central-office  building,  or  in  fact  at 
any  point  that  is  not  so  close  to  the  main  distributing  frame  as  to 
leave  a  distance  of  less  than  about  25  feet  in  the  length  of  each  wool 


OFFICE  TERMINAL  CABLES 


843 


cable.  It  is  not  considered  necessary  to  fill  the  splices  with  insu- 
lating compound,  the  length  of  wool  cable  being  relied  on  to  keep 
moisture  out  of  the  paper  cable.  The  wool  insulation  on  the  wires', 
after  they  are  exposed  by  the  removal  of  the  lead  sheath  at  the  distrib- 
uting frame,  is  well  adapted  to  withstand  the  necessary  handling, 
being  much  tougher  and  less  easily  damaged  than  paper  insulation. 
In  Fig.  618  are  shown  the  splices  between  the  400-pair  paper 
cables  entering  the  Howard  Street  office  of  the  San  Francisco  Home 
Telephone  Company.  These  splices  are  made  within  the  building 


Fig.  619.     Pot-Head  Rack 

in  the  vertical  part  of  the  cable  run  between  the  basement  and  the 
distributing  frame  room.  The  arrangement  of  the  cables  in  a  single 
row  gives  ready  access  to  all  of  the  splices. 

Pot-head  Method  of  Terminating.  Where  pot-heads  are  em- 
ployed at  the  central  office,  the  terminal  cables  that  are  spliced  on  to 
the  paper  cables  are  lead  covered,  and  usually  insulated  with  silk 
or  cotton  and  less  frequently  with  wool.  It  is  customary  to  make 
these  carry  a  relatively  small  number  of  conductors,  as  small  cables 
are  much  more  convenient  to  handle  and  may  be  led  with  greater 
neatness  and  with  a  smaller  amount  of  exposed  wire  to  their  rela- 
tive distributing  frame  terminals. 


844  TELEPHONY 

It  is  convenient  to  support  the  pot-heads  vertically  on  an  iron 
rack  in  the  basement  of  the  building,  from  which  rack  the  smaller 
cables  are  led  to  the  distributing  frame  room  above.  The  details  of 
such  an  office  pot-head  rack  are  shown  in  Fig.  619.  In  this  installa- 
tion each  400  pair  of  paper  cable  was  spliced  to  two  200  pair  of  wool 
cables.  The  vertical  terminal  strips  on  the  line  side  of  the  main 
distributing  frame  were  equipped  for  200  pairs,  and  consequently 
each  of  the  wool  cables  occupied  all  the  terminals  on  one  of  the  ver- 
tical distributing  frame  strips. 

The  pot-heads  in  this  case  are  each  supported  on  a  cast-iron 
shoe  or  bracket,  as  shown,  the  shell  of  the  pot-head  being  held  in 
vertical  alignment  on  this  shoe  by  means  of  iron  straps  bolted  to  the 
horizontal  members  of  the  frame. 

Subdivision  into  Small  Terminal  Cables.  Frequently  the  main 
line  cables  are  subdivided  into  very  much  smaller  cables  than  200 
pair.  The  terminal  strips  on  the  line  side  of  the  main  distributing 
frame  commonly  carry  blocks  of  20  or  40  pairs  of  terminals.  A  neat 
arrangement  is  to  subdivide  the  main  cable  at  the  pot-head  into  as 
many  40-pair  silk-  and  •cotton-insulated  lead-covered  cables  as  are 
necessary  to  carry  the  total  number  of  wires,  and  then  to  lead  each 
one  of  these  40-pair  cables  to  a  different  one  of  the  40-pair  ter- 
minal strips  on  the  frame.  In  this  way  the  conductors  of  the  line 
cables  are  carried  in  lead  sheaths  right  to  the  connecting  strips  on 
which  they  terminate,  rather  than  having  the  lead  stripped  off  at  a 
point  a  greater  distance  from  the  ends  of  the  conductors,  as  is 
necessary  where  larger  terminal  cables  are  employed. 

Cable  Runs.  The  method  of  conducting  the  terminal  cables  to 
the  distributing  frame  is  a  matter  which  must  always  be  carefully 
worked  out  in  view  of  the  particular  requirements  of  each  case,  and 
should  be  provided  for  in  detail  in  the  design  of  the  building.  This 
is  one  of  the  important  points  of  conference  between  the  architect 
and  the  engineer.  The  method  chosen  will  depend  on  the  relative 
locations  of  the  cable  entrance,  the  pot-head  rack,  and  the  distribut- 
ing frame.  These  all  enter  into  the  design  of  the  building.  It  is  also 
dependent  on  the  vertical  height  of  the  main  distributing  frame, 
since  the  number  of  terminals  in  a  strip  or  in  a  column  is  a  matter 
which  affects  the  size  of  the  terminal  cables.  This  also  affects, 
and  is  affected  by,  the  design  of  the  building. 


OFFICE  TERMINAL  CABLES 


845 


The  general  method  to  be  employee!  in  running  from  the  pot- 
head  rack  to  the  main  distributing  frame  depends  usually  on  whether 
the  two  racks  are  on  the  same  floor,  or  on  adjacent  floors,  or  on  floors 
separated  by  intermediate  stories. 

Where  they  are  on  the  same  floor  the  terminal  cables  may  be 
run  in  the  most  direct  manner  from  the  top  of  the  pot-head  rack  to 
the  top  of  the  distributing  frame  and  then  fed  down.  Usually,  how- 
ever, these  racks  are  not  on  the  same  floor.  Where  the  main  frame  is 
on  the  floor  immediately  above  that  of  the  pot-head  rack,  the  simplest 


Fig-  620.    Cables  to  Main  Distirbuting  Frame 

way  is  to  mount  the  main  frame  immediately  above  the  pot-head  rack 
and  run  the  cables  straight  up  to  the  main  frame,  as  shown  in  Fig. 
619.  A  good  way  of  doing  this  is  to  provide  a  slot  in  the  floor  extend- 
ing the  entire  length  of  the  main  frame,  and  care  in  disposing  the 
longitudinal  spacing  of  the  pot-heads  on  the  pot-head  rack  will  result 
in  each  main  cable  being  terminated  directly  under  the  correspond- 
ing vertical  strips  of  the  main  frame. 

Where  there  are  intervening  stories  between  the  pot-head  rack 
and  the  main  frame,  it  is  usually  not  possible  to  run  the  cable  straight 


Fig.  621.   Cable  Entrance  of  the  Grant  Avenue  Office 
of  the  San  Francisco  Home  Telephone  Company 


-  622.    Cable  Entrance  of  the  Grant  Avenue  Office 
of  the  San  Francisco  Home  Telephone  Company 


OFFICE  TERMINAL  CABLES  847 

up  on  account  of  interfering  with  the  space  on  the  intervening 
floors.  One  good  way,  which  usually  results  in  a  minimum  length  of 
terminal  cables,  is  to  mount  the  pothead  rack  parallel  with  and 
near  to  one  of  the  walls  of  the  building  and  provide  in  this  wall 
a  sufficient  number  of  ducts,  either  of  iron  pipe  or  fiber  conduit, 
leading  vertically  to  the  distributing  frame  room.  The  terminal  ca- 
bles are  led  up  from  the  pot-heads  to  a  horizontal  iron  rack  and  passed 
over  this  to  the  lower  ends  of  the  wall  ducts,  through  which  they 
pass  to  the  ceiling  of  the  floor  below  the  main  frame,  thence  through 
an  iron  rack  to  a  point  beneath  the  main  frame,  and  thence  up  through 
holes  in  the  floor  to  the  terminal  strips  which  they  are  to  feed.  From 
this  point  the  individual  cables  are  led  along  the  vertical  members 
of  the  iron  rack  of  the  distributing  frame  to  the  proper  horizontal 
member,  and  thence  horizontally  along  that  to  the  terminal  strip. 
Such  an  arrangement,  employing  wall  ducts,  is  shown  in  Fig.  620. 
Where  a  cable  shaft  is  employed  rather  than  wall  ducts,  a  good 
way  is  to  build  an  iron  rack  or  lattice  work,  extending  from  the  pot- 
head  rack  horizontally  to  the  vertical  shaft  leading  to  the  distributing 
frame  room,  the  cables  being  supported  in  this  shaft  by  strapping  them 
at  frequent  intervals  to  iron  supports. 

In  Figs.  621  and  622  are  shown  details  of  the  cable  entrance  of  the 
Grant  Avenue  office  of  the  San  Francisco  Home  Telephone  Company. 
Fig  621  shows  the  cables  passing  from  the  conduit  to  the  bottom  of 
the  cable  shaft,  and  Fig.  622  is  a  view  in  the  other  direction,  show- 
ing the  cables  turning  up  to  enter  this  shaft.  Note  that  in  support- 
ing the  cables  on  the  side  walls  no  more  than  two  vertical  tiers  are 
used. 


CHAPTER   XLVIII 
SERVICE  CONNECTIONS 

In  gas-  and  water-supply  systems  individual  pipes  are  run  from 
the  main  in  the  street  to  the  consumer's  premises,  and  these  are 
called  service  connections.  In  electric-light  and  power-distributing 
systems,  and  in  telephone  systems,  the  service  connections  consist 
in  individual  pairs  of  wires  leading  from  the  line  wires  of  the  pole  or 
conduit  route  to  the  subscriber's  premises. 

Telephony,  unlike  all  the  other  public  utility  systems  wherein 
the  commodity  distributed  is  furnished  by  means  of  wires  or  pipes  ex- 
tended from  a  central  station,  requires  that  the  line  supplying  each 
subscriber,  or  party-line  group  of  subscribers,  be  individualized  the 
entire  distance  from  the  central  station  to  the  subscriber's  premises. 
In  water,  gas  and  electric-light,  and  power  systems,  the  service  con- 
nector is  ordinarily  the  only  part  of  the  supply  line  that  is  individual 
to  the  consumer. 

Connections  from  Bare  Wire  Lines.  "Where  a  line  drops  off 
a  bare  wire  lead  to  reach  a  subscriber,  the  service  connection  may 
be  made  by  means  of  bare  or  insulated  wires,  strung  from  the  main 
pole  line  to  the  house,  where  they  terminate  on  brackets.  If  the  line 
from  which  the  connection  is  made  is  a  party  line,  and  the  station  is  not 
the  end  one  on  the  line,  the  service  wires  are  merely  tap-connected  to 
the  line  wires.  If  the  station  is  on  an  individual  line  or  the  end  one 
on  a  party  line,  the  service  wires  are  merely  a  continuation  of  the 
line  wires.  In  all  cases  the  service  wire  should  be  properly  dead- 
ended  to  resist  stress,  both  on  the  line  pole  and  at  the  house. 

Connections  from  Cable  Lines.  In  cable  construction  several 
methods  may  be  employed  for  connecting  the  cable  terminal  with 
the  subscriber's  house  wiring.  These  naturally  fall  into  three  classes: 
aerial  or  drop-wire  distribution;  wall  or  fence-wire  distribution;  and 
distribution  from  underground  terminals. 

Drop-Wire  Distribution.  A  drop  connection  or  drop  is  an  aerial 
pair  of  wires  strung  from  a  distributing  pole  near  the  subscriber's 


SERVICE  CONNECTIONS  849 

• 

premises  to  a  point  on  his  building,  from  which  they  lead  to  the  ter- 
minals of  the  house  protection  apparatus  and  interior  wiring.  Drop 
wires  may  be  bare,  but  modern  practice  has  proven  that  they  are 
better  of  insulated  wire.  In  some  installations  the  drop  connection 
consists  of  two  separate  wires,  one  bare  and  the  other  insulated,  the 
idea  being  that  even  if  they  swing  together  there  will  still  be  an  in- 
sulating protection.  Consensus  of  opinion  now,  however,  is  that 
both  wires  should  be  insulated,  and  there  has  resulted  a  form  of 
wire  known  as  drop  wire,  consisting  of  two  rubber-covered  and 
braided  wires  twisted  together.  As  has  been  pointed  out,  this 
rubber-covered  twisted-pair  drop  wire  may  be  of  iron,  copper,  or  of 
copper-clad  steel.  It  should  always  be  of  sufficient  strength  to  be 
self-supporting  in  rather  long  spans,  even  under  the  most  severe 
conditions  of  weather.  Obviously,  therefore,  the  climatic  con- 
ditions affect  the  requirements  as  to  its  strength,  and  it  may  be  said, 
therefore,  that  for  northern  climates,  where  sleet  and  wind  storms  are 
to  be  expected,  the  drop  wire,  if  of  copper,  should  not  be  smaller 
than  No.  14  B.  &  S.  gauge.  In  those  climates  where  sleet  is  not  to 
be  expected  and  wind  storms  are  not  severe,  No.  16  B.  &  S.  gauge 
copper  suffices.  Owing  to  the  low  cost  of 
iron  wire,  the  saving  in  using  smaller  sizes 
than  No.  14  is  not  enough  to  warrant  doing 
so.  No.  18  copper-clad  steel  wire  may  be 
made  with  such  a  proportion  of  steel  as  to 
have  a  strength  approximately  equal  to  a 
No.  16  hard-drawn  copper,  but  where  this 
bimetallic  wire  has  been  used  for  drop-wire 
purposes,  it  has  usually  been  of  No.  17  B. 
&  S.  gauge  High  conductivity  in  drop 
wires  is  not  an  essential  and  any  of  these  Fig.  623.  Drop-Wire 
wires  may  be  considered  as  satisfactory  in 

that  respect.     The  great  difficulty  with  iron  wire  for  drop  purposes 
is  its  liability  to  rust  at  the  terminals  and  at  exposed  portions. 

Stringing  Drop  Wires.  The  attachment  of  the  drop  wire  to 
the  distributing  pole  and  to  the  cable  terminal,  already  shown  in 
Fig.  570,  is  again  referred  to.  Usually  an  iron  bracket  is  bolted  to 
the  pole  carrying  one  or  more  porcelain  insulators.  These  porce- 
lain insulators  are  preferably  provided  with  double  grooves  to  accom- 


850 


TELEPHONY 


rnodate  the  twin  wires.  For  dead-ending  on  distributing  poles,  the 
insulators  are  sometimes  provided  with  two  pairs  of  grooves,  so  that 
each  may  accommodate  two  drops.  Such  an  insulator  is  shown  in 
Fig.  623. 

A  common  form  of  bracket,  adapted  to  hold  two  of  these  insu- 
lators, is  shown  with  the  insulators  attached  in  Fig.  624.  Some 

trouble  has  been  experienced  in  attach- 
ing the  insulators  to  the  brackets,  due 
to  the  fact  that  if  the  nut  was  tight- 
ened sufficiently  to  bind  and  prevent  its 
unscrewing,  there  was  danger  of  crack- 
ing the  insulator.  It  is  for  this  reason 
that  the  four  round  leather  washers  are 
used. 

The  method  of  tying  the  drop  wire 
to  such  insulators  deserves  attention, 
since  if  this  is  improperly  done  the  drop 
wires  are  likely  to  become  crossed,  even 
though  a  good  grade  of  material  is  used. 
These  knobs  are  used  not  only  for  at- 
taching the  wire  to  the  distributing  pole 
where  the  drop  swings  off,  but  also  to 
intermediate  poles — where  such  exist 
in  the  path  from  the  distributing  pole 
to  the  subscriber's  premises.  They  are 
also  attached  to  the  house  wall  of  the 
subscriber  at  the  point  where  the  span 
ends  and  at  other  points  on  the  wall,  in  order  to  lead  the  wires 
along  the  house  to  the  place  where  they  pass  through  the  wall. 

The  principal  point  to  be  remembered  in  making  any  of  these 
ties  to  the  insulators  is  that  the  two  separate  grooves  are  intended  to 
hold  separately  the  two  wires  of  the  pair.  The  two  wires  should  not 
cross  each  other  in  passing  around  the  grooves,  but  should  lie  as  far 
as  possible  parallel  with  each  other  in  the  grooves. 

The  method  of  dead-ending  the  drop  wire  on  the  distributing 
pole  is  shown  in  Fig.  625.  It  will  be  seen  that  the  drop  is  bent  once 
around  the  insulator  and  then  the  free  end  of  it  is  wrapped  about 
five  times  around  the  portion  that  is  to  lead  off  into  the  span.  On 


Fig.  624.     Drop- Wire  Insulators 
and  Bracket 


SERVICE  CONNECTIONS  851 

distributing  poles,  the  wire  may  thus  be  wrapped  about  itself  easily, 
because  there  is  always  a  short  end  that  is  to  lead  up  into  the  terminal 
box  or  can. 

Where  the  drop  wire  is  to  be  tied  but  not  dead-ended  on  an 
intermediate  insulator,  as  between  two  spans  that  are  in  the  same 
straight  line,  or  nearly  so,  the  drop  is  laid  in  the  insulator  groove,  but 
not  passed  around  the  insulator.  It  is  tied  in  place  by  a  tie  wire  cut 


Fig.  625.     Dead-Ending  Drop  Wire  on  Distributing  Pole 

from  the  scrap  ends  of  the  regular  twisted  pair  drop  wire.  The 
method  of  making  this  tie  is  shown  in  four  successive  steps  in  Fig.  626. 
At  the  point  on  the  subscriber's  wall  where  the  span  ends,  it  is 
necessary  to  dead-end  the  wire,  but  it  is  not  usually  feasible  to  make 
the  same  sort  of  dead-end  tie  that  is  illustrated  in  Fig.  625,  because 
of  the  fact  that  it  is  not  desirable  to  cut  the  wire  at  this  point,  as  would 
be  necessary  in  order  to  wrap  it  around  itself.  It  is  necessary, 
therefore,  to  dead-end  the  wire  without  wrapping  it  about  itself,  and 
the  method  of  doing  this  is  shown  in  Fig.  627.  In  this  case  the  wire 


852 


TELEPHONY 


is  wrapped  once  about  the  insulator  and  tied  in  place,  as  shown,  by 
a  tie  wire  of  the  same  material  which  is  also  passed  once  around  the 
insulator  and  then  given  about  eight  turns  about  the  twisted  pair. 
In  both  the  regular  tie  and  the  house  dead-end,,  each  end  of  the  tie 
wire,  after  wrapping,  should  be  inserted  between  the  two  wires  of  the 


Pig.  626.     Tie  for  Drop  Wire 

strand  and  then  cut  off.     This  prevents  the  tie  wire  from  unwrap- 
ping. 

At  the  distributing  pole,  after  dead-ending  the  drop  wire,  the  free 
end  is  looped  down  through  a  bridle  ring  screwed  into  the  pole  below 
the  terminal,  and  its  free  end  is  passed  up  into  the  terminal  can,  where 
the  wires  are  attached  to  the  proper  binding  posts.  The  bridle  ring 


SERVICE  CONNECTIONS 


853 


used  for  this  purpose  is  usually  a  3-irich  enameled  iron  ring  with  a 
screw  shank  for  fastening  it  to  the  pole.  The  looping  of  the  drop- 
wire  ends  down  through  these  rings  disposes  of  the  slack  in  the  drop- 
wire  ends  in  a  neat  manner.  This  arrangement  has  already  been 


Fig.  627.     Tie  for  Drop  Wire  at  House  End  of  Span 

referred  to  in  connection  with  Figs.  570  and  625.    Another  example 
is  shown  in  detail  in  Fig.  628. 

In  Fig.  629  are  shown  the  details  of  the  connections  of  the  drop 
wire  to  the  outside  wall  of  the  house.  Where  the  span  leading  to  the 
house  terminates,  the  house  dead-end,  shown  in  Fig.  627,  should  be 


854 


TELEPHONY 


used,  and  at  all  other  insulators  on  the  house,  the  regular  tie  of  Fig.  626 
should  be  used,  these  insulators  being  placed  not  further  than  7  feet 
apart  on  horizontal  runs,  and  not  over  15  feet  apart  on  vertical  runs. 
The  drip  loop  is  provided  just  below  the  point  where  the  wires 
enter  the  house  to  prevent  moisture  from  following  the  wire  into 
the  house.  The  details  of  passing  the  wires  through  the  wall  are 
given  in  Fig.  630. 

Splicing  Drop  Wire.     It  is  preferable  that  continuous  lengths 
of  drop  wire  be  used  from  the  distributing  pole  to  the  subscriber's 


Fig.  628.     Connecting  Drop  Wires  to  Cable  Terminals 

premises,  but  nevertheless  economy  of  material  makes  other  de- 
mands and  a  carefully  prepared  splice  need  cause  no  trouble.  The 
method  of  splicing  a  twisted-pair  drop  wire  is  shown  in  Fig.  631. 
It  is  well  not  to  use  all  of  the  scrap  ends  of  the  drop  wire  oil  any  one 
connection,  and  this  may  be  prevented  by  limiting  the  number  of 
splices  to  two  in  any  run  of  ordinary  length. 

Circle-Top  Distribution.     In  districts  where  the  buildings  are  of 
such  a  nature  as  to  warrant  the  installation  of  individual  underground 


SERVICE  CONNECTIONS 


855 


terminals,  it  has  been  the  practice  in  the  past  to  provide,  usually  on 
the  interior  of  the  blocks,  very  high  distributing  poles  on  which  the 


Fig.  629.     Attaching  Drop  Wire  to  House 

cable  terminal  was  placed  and 
from  which  the  drop  wires  radi- 
ated to  the  various  houses  within 
reach,  much  like  the  ribs  of  an 
open  umbrella.  Often  these  poles 
are  required  to  be  of  very  great 
height  and  the  drop-wire  insula- 
tors are  mounted  on  large  rings 
encircling  the  pole  and  secured 
thereto  by  iron  brackets.  This 
is  called  "circle-top  distribution," 
and  an  excellent  example  of 
it  is  shown  in  Fig.  632.  The 
expense  of  these  poles,  the  cost 
of  up-keep,  their  equipment,  and 
their  general  unsightliness,  are 
all  objectionable  features. 

Rear=Wall  or   Fence   Distri= 
bution.     The  better  method  of  interior  block  distribution,  where  it 


Fig.  630.     Drop  Wires  Entering  House 


856  TELEPHONY 

is  possible,  is  to  terminate  the  cables  on  the  walls  of  the  houses,  or 
on  back  fences,  or  on  medium  height  poles  near  the  rear  walls  or 
fences,  and  to  lead  from  these  terminals  either  paper  cable  or  small 
gauge  rubber-covered  wire  carried  on  rings,  or  both,  along  the  walls 
or  fences  to  the  points  of  entrance  to  the  various  buildings. 

Wall-Ring  Wiring.  Where  the  wall  wiring  is  done  by  means 
of  open  wire  rather  than  cables,  a  wire  very  similar  to  the  ordinary 
drop  wire,  but  of  smaller  gauge,  is  used.  It  is  usually  a  No.  18  B. 
&  S.  gauge,  rubber-covered  and  braided,  twisted  pair.  The  wire 
is  carried  along  the  walls  in  split  bridle  rings  of  the  form  shown  in 


Pig.  631.    Splicing  Drop  Wire 

Fig.  633.  The  size  of  the  ring  is  determined  by  the  number  of  pairs 
of  wires  that  will  ultimately  follow  the  routes,  good  practice  in  this 
respect  being  to  use  a  3-inch  ring  for  all  runs  that  will  ultimately  carry 
more  than  ten  pairs  and  for  the  slack  loop  ring  at  all  terminals;  a 
1^-inch  ring  for  all  runs  that  will  ultimately  carry  not  less  than  three 
and  not  more  than  ten  pairs;  and  a  f-inch  ring  on  all  runs  that  will 
carry  one,  two,  or  three  pairs.  A  f-inch  ring  is  also  well  adapted 
for  the  final  connection  of  the  single  pair  at  the  point  where  the  wires 
enter  the  building. 


SERVICE  CONNECTIONS 


857 


Where  rings  are  to  be  secured  to  wooden  walls,  they  are  screwed 
directly  into  the  wood.  Where  attached  to  brick,  concrete  or  stone 
walls,  a  hole  is  drilled  into  the  wall  and  this  may  be  plugged  with 


Fig.  632.     Circle- Top  Distribution 

wood,  into  which  the  ring  is  screwed,  or  it  may  be  fitted  with  any  one 
of  several  forms  of  standard  expansion  screw  plugs  that  automatically 
tighten  and  hold  against  the  interior  of  the  hole.  In  all  cases  the 
rings  should  be  turned  so  that  they  are  at  right  angles  to  the  direction 
of  the  run.  When  the  run  is  horizontal  the  open  sides  of  the  split 
rings  should  be  at  the  top,  and  at  all  corners  the  open  side  should 
be  at  the  outer  side  of  the  bend.  In  horizontal  runs  the  rings 
should  be  placed  not  over  4  feet  and  on  vertical  runs  not  more  than 
8  feet  apart. 

The  method  of  mounting  a  terminal  on  a  wall  is  shown  in  Fig. 
634.  This  illustration  not  only  shows  the  lead-covered  cable  leading 
up  from  the  underground  lateral  to  the  terminal  can,  but  it  also  shows 
the  method  of  leading  off  the  twisted-pair  wall  wires  through  the  rings. 


858 


TELEPHONY 


The  method  of  dead-ending  the  individual  pairs  of  wires  at  the  bridle 
ring  is  shown  in  detail  at  the  left  of  Fig.  634,  while  the  method 
of  running  the  wire  through  the  wall  of  the  house,  preferably  in  the 

upper  casement  of  the  window, 
is  shown  in  Fig.  635.  Other  de- 
tails of  wall-ring  construction 
are  shown  in  Fig.  636. 

Aerial  Conduit.  Often  it  is 
convenient  to  mount  the  cable 
terminal  on  a  pole  in  the  interior 
of  the  block  so  as  to  be  able  to 
reach  the  rear  walls  of  several 
houses.  Where  this  is  done  an 
Fig.  ess.  Bridie  Ring  aerial  conduit  is  provided  for 


Fig.  634.     Wall  Terminal  and  Wiring 


supporting   the   wires    from    the   pole   to   the  nearest  wall  of    the 
house.      This  is  readily  done  by  extending  a  messenger  wire  from 


SERVICE  CONNECTIONS 


859 


the  pole  to  the  house.  As  such  construction  is  always  relatively 
light,  a  No.  4  or  No.  6  messenger  wire  is  amply  strong.  The  mes- 
senger wire  may  be  dead-ended 
on  the  pole  in  the  ordinary  way, 
and  on  the  house  in  a  heavy 
screw-eye  bolt.  This  messenger 
wire  so  run  may  support  a  lead- 
covered  cable  leading  from  the 
pole  to  a  terminal  on  the  house; 
and  from  this  terminal  on  the 
house;  the  rubber  bridle  wires 
may  be  run,  as  already  de- 
scribed. Usually,  however,  it  is 
preferable  to  place  the  terminal 
can  on  the  pole  and  to  extend 
the  bridle  wires  from  it  along 
the  messenger  wire.  For  this 
purpose  aerial  conduit  rings  are 
used,  Fig.  637, which  are  clamped 
directly  to  the  messenger  wire  so  ~.g  63g  Wall  Wire  Enterlng  Building 
as  to  hang  below  it.  They  are 
usually  spaced  about  18  inches  apart,  and  the  bridle  wires  are  car- 


Pig.  636.     Details  of  Wall  Wiring 


ried  through  them  along  the  messenger  wire  to  the  wall  of  the  house. 
A  general  idea  of  this  construction  is  given  in  Fig.  638. 

Fence   Wiring.    The   same  general   methods   that   have   been 


860 


TELEPHONY 


outlined  for  wall  wiring  will  apply  to  fence  wiring,  except  that  on 
fences  greater  care  must  be  taken  to  protect  the  wiring.  Where  the 
terminal  is  placed  on  the  fence,  it  should  be  in  as  inconspicuous  a 

place  as  possible  and  at  the  same  time 
reasonably  accessible  to  linemen,  who 
should  be  able  to  reach  it,  if  possible, 
without  the  necessity  of  annoying  the 
occupants  of  the  property  on  which  the 
terminal  is  located.  It  should  always  be 
so  placed  that  it  will  be  least  liable  to  in- 
jury by  children  walking  or  playing  on  the 
fences.  Where  necessary,  it  may  be  suit- 
ably housed  in  a  wooden  cabinet  provided 
with  lock  and  key.  The  underside  of  the 
upper  fence  stringer  forms  the  best  place  for  the  bridle  wires,  and  if  a 
cable  is  also  run  along  the  fence  for  any  distance,  it  is  preferably  run 
underneath  the  bottom  stringer.  Details  of  this  construction  are 
shown  in  Fig.  639.  Where  it  is  not  possible  to  mount  a  fence  cable 


Pig.  637.    Aerial  Conduit  Ring 


Fig.  638.     Aerial  Ring  Conduit 

under  the  stringer  of   the  fence,  it  should  be  enclosed  in  a  simple 
wooden  moulding. 

Back  fences  form  almost  the  only  available  way  of  distributing 
wires  and  cables  in  some  congested  districts  in  large  cities  like  New 
York  and  San  Francisco,  where  back  yards  are  small  and  where,  in 
many  cases,  no  alleys  exist.  This  method  allows  a  malicious  or  mis- 
chievous person  to  tamper  with  the  cables  or  wires,  but  is  not  as 
unsightly  as  aerial  block  distribution. 


SERVICE  CONNECTIONS 


861 


Distribution  from  Underground  Terminals  in  Buildings.  Large 
office  buildings  are  practically  always  found  in  localities  fed  by  under- 
ground cables.  The  proper  way  of  making  the  service  connections 
in  such  buildings  is  to  run  an  underground  lateral  from  the  nearby 
conduit  manhole  directly  to  the  basement  of  the  building,  and  to  draw 
into  this  lateral  a  cable  having  a  sufficient  number  of  pairs  to  serve 
all  the  subscribers  in  that  building.  Some  of  the  large  office  buildings 
in  New  York  and  Chicago  have  a,s  many  as  1200  cable  pairs  thus 
entering  them  directly  from  the  underground  conduits.  In  small 
office  buildings  and  business  blocks,  the  question  as  to  whether  the 
service  connections  shall  be  made  directly  from  an  interior  terminal  or 
by  the  rear-wall  method  must  always  be  governed  by  the  size  of  the 


Fig.  639.     Fence  Wiring 

building,  by  the  ease  of  access  to  the  basement  of  the  building  for 
the  underground  cable,  by  the  provision  of  proper  runways  for  the 
wires  in  the  building,  and  by  the  character  of  the  outside  of  the 
building  as  affecting  the  feasibility  of  using  rear-wall  wiring.  Under- 
ground service  connections  are  to  be  preferred,  in  many  cases  are  the 
cheapest,  and  in  other  cases  are  the  only  practical  way. 

Where  the  distribution  is  from  an  underground  terminal  within 
the  building,  the  cable  leading  into  the  building  should  be  terminated 
at  some  point  where  it  will  not  be  liable  to  injury  and  where  it  will  be 
reasonably  accessible  to  the  workmen  of  the  telephone  company. 
The  same  sort  of  a  terminal  that  is  employed  for  aerial  cable  work 
may  be  used  for  terminating  the  cables,  unless  they  are  of  very  great 
size,  in  which  case  special  terminal  racks  are  provided. 


CHAPTER  XLIX 
SUBSCRIBERS'  STATION  WIRING 

The  simplest  case  of  subscriber's  station  wiring  is  that  of  an 
unexposed  open  wire  line,  such  as  a  private  line  in  the  open  country, 
entering  a  dry  wooden  house  to  connect  with  a  telephone.  In  this 
case  the  outdoor  line  is  terminated  on  insulators  on  the  outside  of 
the  house  at  such  a  point  as  to  make  the  wiring  to  the  telephone  with- 
in as  short  as  possible.  A  twisted  pair  of  wires  insulated  with  a  good 
grade  of  rubber  with  a  braid  over  each  wire  is  then  run  into  the 
house  through  a  hole  slanting  inwardly  upward  and  is  carried  to 
the  instrument  along  vertical  and  horizontal  lines  of  the  woodwork 
of  the  house.  It  is  attached  to  that  woodwork  by  means  of  insulating 
nails  or  staples. 

General  Conditions.  All  telephones  in  subscribers'  stations  may 
be  classified  as  exposed  or  unexposed,  depending  on  the  character  of 
the  line  outside.  Protectors  are  required  at  exposed  stations,  but 
none  are  required  at  unexposed  stations. 

The  construction  rules  of  governing  fire-insurance  bodies  pre- 
scribe that  the  wiring  of  an  exposed  line  shall  be  considered  high- 
tension  wiring  up  to  the  point  of  the  protector,  but  that  between  the 
protector  and  the  telephone  the  wiring  may  be  considered  as  without 
exposure  to  hazard  and  may  be  run  in  any  desired  way. 

The  wires  from  the  exposed  line  to  the  protector,  therefore,  if 
the  latter  is  within  the  building,  are  to  be  supported  on  insulators. 
From  the  protector  to  the  telephone,  "inside  wire"  shall  be  used. 
As  so  much  of  the  success  of  telephone  service  and  the  securing  of 
low  maintenance  costs  depend  on  the  quality  of  inside  wire,  the  fol- 
lowing condensed  specifications  for  such  wire  are  offered : 

The  wires  shall  be  of  No.  19  B.  &  S.  gauge  and  shall  be  of  soft 
copper,  well  tinned.  Each  tinned  copper  conductor  shall  be  evenly  and 
smoothly  covered  with  an  approved  rubber  insulating  compound  to  an 
outside  diameter  not  less  than  ^-inch.  The  insulated  covering  shall 


SUBSCRIBERS'  STATION  WIRING  863 

be  flexible  and  not  liable  to  deteriorate  under  ordinary  conditions  nor 
to  act  injuriously  on  the  conductor.  Each  insulated  conductor  shall  be 
covered  with  a  close  braid  of  cotton.  This  shall  either  be  polished  or  shall 
be  treated  with  a  paint  or  compound,  insoluble  in  water,  and  this  shall 
not  act  injuriously  on  the  insulating  compound.  The  braid  on  one  wire 
shall  have  a  raised  thread  or  an  approved  equivalent  marker  so  that  the 
wires  may  be  easily  distinguishable  from  each  other.  The  two  insulated 
braided  conductors  constituting  a  pair  shall  be  twisted  together.  The 
twists  shall  be  regular  and  uniform.  The  length  of  the  twists  shall 
be  not  less  than  If-inches  nor  more  than  2^  inches.  The  completed  wire 
shall  be  capable  of  withstanding  a  pressure  of  1,000  volts  alternating  cur- 
rent and  shall  have  an  insulation  resistance  of  at  least  200  megohms  per 
mile  at  a  temperature  of  60°  Fahrenheit.  The  resistance  of  each  conduc- 
tor shall  not  exceed  50  ohms  per  mile  of  completed  wire  at  60°  Fahrenheit. 

Where  a  protector  is  required,  if  the  chosen  type  mounts  inside 
the  house,  it  shall  be  located  as  close  to  the  entrance  as  possible. 
It  shall  be  mounted  on  a  wall  7  feet  from  the  floor,  shall  be  placed  so 
as  to  avoid  dampness  and  inflammable  material,  such  as  window 
shades  and  curtains,  and  never  shall  be  mounted  in  a  show  window. 

The  ground  wire  from  the  protector  shall,  if  possible,  be  run  to  a 
water  pipe;  if  this  is  not  possible,  a  gas  pipe  is  next  preferred,  con- 
nection being  best  made  between  the  meter  and  the  street.  A 
ground  rod  not  less  than  6  feet  long  driven  into  permanently  damp 
earth  is  the  third  choice.  A  good  ground  clamp  is  the  most  con- 
venient way  of  attaching  a  ground  wire  to  a  pipe,  however,  soldering 
the  wirfc  to  the  pipe  is  as  good  practice  but  less  convenient. 

Where  a  protector  is  not  required  because  the  line  is  unexposed, 
it  is  good  practice  to  install  a  connection  block  in  its  place,  as  this  is  a 
handy  way  of  joining  the  entrance  wires  to  the  inside  wires.  A 
connection  block  is  merely  a  small  slab  of  insulating  material  with 
binding  posts  on  it.  Two  connected  pairs  of  such  posts  may  be  used, 
or  a  single  pair,  each  post  taking  one  inside  and  one  outside  wire. 

The  inside  wire  of  two  insulated  and  braided  conductors  should 
be  carried  around  rooms  in  picture  mouldings  where  possible,  if 
this  does  not  require  crossing  open  plastered  walls.  To  reach  wall 
telephones,  it  is  generally  possible  to  drop  the  wire  from  the  picture 
moulding  to  the  instrument  within  a  plastered  wall  and  to  bring  it 
out  at  the  telephone  so  as  not  to  show  at  all. 

In  carrying  the  inside  wire  along  other  woodwork  than  a  picture 
moulding,  the  best  fastening  is  an  upholsterer's  tack.  This  is  a  small, 
sharp-pointed  steel  nail  with  a  large  head  of  insulating  material. 


864  TELEPHONY 

The  stem  of  the  tack  is  slipped  between  the  two  wires  of  the  pair. 
Care  should  be  taken  never  to  drive  the  tack  through  the  insulation. 
The  next  best  fastener  is  a  staple  having  a  saddle  of  insulating  ma- 
terial. For  ground  wires,  which  are  single,  staples  are  necessary. 
Neither  ground  wires  nor  twin  wires  shall  have  spirals  in  them. 
Ground  wires  carry  away  lightning  best  if  entirely  straight. 

The  more  complicated  cases  of  wiring  office  buildings,  hotels,  and 
apartment  houses  require  some  use  of  inside  wire  in  these  ways.  It 
is  not  possible,  however,  to  bring  the  outside  lines  to  the  telephones 
as  simply  as  in  the  case  of  isolated  houses.  The  preparation  for 
wiring  an  office  building  or  hotel  should  begin  at  the  time  the  archi- 
tect makes  his  first  preliminary  sketch,  and  telephone  wires  should 
be  considered  and  arranged  for  during  all  the  processes  of  planning 
and  constructing  the  building.  Owners,  architects,  building  con- 
tractors, and  wiremen  should  have  exact  knowledge  of  the  needs  of 
modern  telephone  installations.  For  this  reason,  the  following  fun- 
damental matter  is  presented. 

There  are  three  classes  of  buildings  which  require  particular 
attention  in  preparing  for  telephone  service:  Office  Buildings; 
Hotels;  and  Apartment  Houses. 

Other  buildings  which  require  consideration  are:  Flats  and 
Private  Dwellings. 

Office  Buildings.  Assume  that  office  buildings  will  require  about 
one  telephone  per  office.  While  the  number  of  telephones  so  figured 
often  is  exceeded,  the  excess  stations  are  taken  care  of  by  private 
exchanges,  having  fewer  lines  to  the  central  office  than  to  local  sub- 
stations. Since  the  location  of  telephones  cannot  be  determined 
in  advance,  it  is  necessary  that  a  very  flexible  arrangement  be  pro- 
vided and  one  that  will  permit  wires  to  be  run  to  any  part  of  every 
room.  Such  an  arrangement  in  nearly  all  cases  can  be  obtained 
best  by  the  use  of  raceway  mouldings  in  halls,  picture  mouldings  in 
rooms,  and  by  the  proper  distribution  of  hall  terminal  boxes.  These 
boxes  are  served  by  riser  cables  and  provision  is  made  for  the 
riser  cables  to  be  carried  vertically  from  the  basement  to  the  top  floor. 

Raceway  Mouldings.  Common  and  good  forms  of  raceway 
mouldings  are  shown  in  Fig.  640.  In  the  hall  a  larger  moulding  is 
required  than  in  the  individual  rooms,  the  moulding  in  each  room 
being  connected  with  the  hall  moulding  by  a  short  piece  of  f-inch 


SUBSCRIBERS'  STATION  WIRING 


865 


conduit  which  will  make  any  subsequent  boring  of  the  walls  unneces- 
sary. Mouldings  in  adjacent  offices  should  be  similarly  connected, 
in  which  case,  the  conduit  shouldbe  placed  in  the  dividing  wall  as  close 
to  the  hall  as  possible.  This  in- 
terconnection of  room  mouldings 
will  be  found  particularly  useful 
in  wiring  a  suite  of  offices  for  a 
private  exchange  with  a  switch- 
board in  some  one  room.  The 
mouldings  should  be  continuous 
along  the  entire  length  of  a  hall 


Pl_ASTER 


Fig.  640.     Race-Way  Mouldings 


and  around  the  walls  of  a  room 
and  preferably  should  be  above 
all  doors  and  window  sashes, 
or,  at  the  lowest,  on  a  level  with  their  tops. 

Ceiling  Conduits.  Where  terminal  boxes  are  not  placed  on 
both  sides  of  the  hall,  the  mouldings  on  each  side  should  be  con- 
nected by  1  i-inch  conduits,  placed  in  the  ceiling,  as  shown  in  Fig. 
641.  Where  very  long  halls  exist,  even  with  terminal  boxes  on  both 


-FLOOR  L.1NE 


CONDUIT 


CEIL.IMQ 


STANDARD  OUTLET    BOXES 
HAUL-    MOULDINGS 


Fig.  641.     Ceiling  Conduits 


sides,  connecting  conduits  l£  inches  in  diameter,  and  in  duplicate, 
should  be  placed  at  intervals  not  exceeding  100  feet.  The  total 
length  of  a  hall  should  be  considered  as  that  measured  throughout 
its  entire  length  on  all  sides  of  a  hjiilding. 

In  placing  conduit  under  these  conditions  and  elsewhere,  the 
usual  precautions  should  be  taken  to  round  the  exposed  edges  at  the 


866 


TELEPHONY 


ends  so  as  to  remove  all  burrs  which  might  injure  the  insulation  of 
the  wires  or  the  covering  of  the  cables. 

Hall  Terminal  Boxes.  It  is  distinctly  desirable  to  use  but  one 
size  of  hall  terminal  box  throughout  a  building.  This  can  be  done 
with  ease,  if  the  following  simple  rule  is  adhered  to:  Always  place 
enough  boxes  on  each  floor  so  that  not  more  than  ten  to  thirteen  offices 
will  be  served  from  one  box. 

A  good  form  of  hall  terminal  box  suitable  for  the  above  arrange- 
ment is  shown  in  Fig.  642.  The  boxes  should  be  placed  just  below 
the  hall  moulding,  the  back  of  the  box  projecting  up  behind  the  mould- 
ing to  afford  a  means  of  connection  between  the  moulding  and  the 
box.  In  placing  the  boxes  along  a  hall,  an  effort  should  be  made  to 
have  the  box  as  close  as  possible  to  the  center  of  the  group  of 
offices  which  is  to  be  served  by  the  box. 

In  order  to  connect  the  terminal  strips  in  the  hall  terminal  boxes 
with  the  riser  cable,  the  cable  shaft,  or  closet  accommodating  the  riser 


HAI_t_    MOULDING 


DOOR    FRAME 


PLASTER   l-INC 


Fig.  642.     Hall  Terminal  Box 

cable,  should  itself  be  connected  with  the  hall  terminal  box  directly 
by  means  of  the  hall  moulding,  or  by  a  special  conduit-run,  or  di- 
rectly by  placing  the  hall  terminal  box  in  the  shaft  close  to  the  hall 
moulding. 

Hall  terminal  boxes  should  be  located  with  reference  to  the  offices 
they  are  to  serve  and,  if  a  choice  exists,  the  location  of  the  riser-cable 
shafts  should  be  made  with  reference  to  the  location  of  the  hall 


SUBSCRIBERS'  STATION  WIRING 


867 


terminal  boxes  so  as   to   reduce   the   length   of  distributing  cable 
from  the  riser  shaft  to  the  hall  distributing  boxes  to  a  minimum. 

Riser-Cable  Shafts.  In  all  except  the  smallest  buildings  (four 
to  five  offices  in  the  longest  side)  at  least  two  riser-cable  shafts  should 
be  provided  for  at  diagonally  opposite  corners  of  the  building.  There 


ri_oow  LINE 


row  COVERS 

WAINSCOTING 


FRONT    E.L-EVATIOH 

Or  SHAFT    WITH 
COVERS  IN   PLACE 


SECTION  B-B 

Fig.  643.     Riser-Cable  Shaft 


should  be  as  many  shafts  as  necessary  to  keep  the  distance  from  the 
shaft  to  the  farthest  terminal  box  under  75  feet.  Enclosed  shafts  are 
preferable  to  open  ones,  elevator  ways  and  vents  being  classed  as 
open  shafts.  Cables  in  open  shafts,  even  though  covered,  are  un- 
sightly and  in  danger  of  injury. 

Separate  shafts  of  small  dimensions  are  recommended  and 
they  may  be  located  in  some  part  of  the  hall  walls  so  as  to  be  nearly 
midway  between  the  hall  terminal  boxes  they  serve.  Such  a  shaft  is 


868  TELEPHONY 

shown  in  Fig.  643,  in  this  particular  case  it  being  in  the  hall 
wall  opposite  a  partition  between  offices.  In  many  cases  such  a 
shaft  may  be  recessed  into  a  wall  of  the  janitor's  closet,  elevator  shaft, 
toilet  room,  or  other  space,  so  as  to  be  inconspicuous.  The  shaft 
should  have  an  inside  dimension  of  at  least  6  by  12  inches.  It  may 
well  be  somewhat  larger.  The  long  dimension  preferably  should  be 
parallel  with  the  hall;  however,  this  position  of  the  shaft  may  be 
reversed  if  necessary.  The  shaft  shown  is  provided  with  a  remov- 
able paneled  cover  on  each  floor,  extending  from  the  wainscoting  to 
the  ceiling,  but  cut  at  the  hall  moulding.  If  the  location  of  the 
shaft  permits,  one  of  the  hall  terminal  boxes,  Fig.  642,  may  be  placed 
in  the  shaft  directly  below  the  moulding. 

As  riser  cables  generally  are  not  self-supporting,  means  must 
be  provided  to  permit  the  cables  to  be  strapped  to  the  walls  of  the 
shaft.  This  may  be  arranged  by  sinking  flush  with  the  walls  of  the 
shaft  near  the  top  and  the  bottom  of  the  opening  on  each  floor,  2- 
inch  by  4-inch  wooden  sleepers.  Where  tile  walls  are  used,  the 
sleepers  may  be  omitted  and  toggle  bolts  used.  It  is  not  generally 
possible  to  extend  all  cable  shafts  to  the  basement,  as  the  arrange- 
ment of  the  first  floor  generally  is  different  from  the  rest  of  the  floors; 
this  condition  may  easily  be  met,  however,  by  extending  to  the  base- 
ment from  the  bottom  of  each  cable  shaft,  two  3^-inch  conduits. 

Cable  in  Elevator  Shaft.  Where  it  is  not  possible  to  construct 
separate  shafts  for  telephone  purposes,  other  shafts — such  as  elevator 
shafts,  pipe  shafts,  and  vents — may  be  used,  providing  they  are  within 
a  reasonable  distance  of  the  hall  terminal  boxes.  They  are  distinctly 
not  the  best  way,  however. 

When  an  elevator  shaft  must  be  used,  provision  should  be  made 
at  each  floor  so  that  the  distributing  cables  may  be  run  from  the  riser 
cable  to  the  hall  distributing  boxes.  This  may  be  accomplished  either 
by  running  the  hall  moulding  to  the  shaft  or  by  connecting  the  shaft 
to  the  hall  moulding  by  a  conduit  run.  Where  possible,  the  riser  cable 
should  be  enclosed,  both  for  appearance  and  for  protection.  This 
being  done  by  means  of  a  small  box  with  removable  doors,  extend- 
ing vertically  from  the  basement  to  the  top  floor.  This  box  should 
have  an  internal  cross-section  approximately  6  by  12  inches. 

Combined  Pipe-and-Wire  Shaft  Where  a  combined  pipe-and- 
wire  shaft  is  to  be  used,  an  effort  should  be  made  to  separate  the  tele- 


SUBSCRIBERS'    STATION   WIRING 


869 


phone  wire  portion  from  the  rest  by  a  substantial  partition,  preferably 
fireproof.  Telephone  cables  and  wires  may  not  be  kept  close  to 
steam  pipes.  Provision  should  be  made  so  that  access  may  be  had 
to  the  cable  at  each  floor.  The  shaft  should  be  properly  connected 


KEV. 

RISCR    CABLES    IN    SMAl_i_    VERTICAL.     SMARTS 
1  HAUL.    DISTRIBUTING     BOXES 
DISTRIBUTING    CABLE    RUNS 
WIRE.   RUNS    TO    ROOMS 


-----    CC 


Fig.  644.     Office  Building  Wiring 


with  the  hall  moulding  or  to  the  distributing  boxes.     A  separate 
door  for  each  part  of  the  shaft  is  not  necessary. 

Cable  in  Vent  Shafts.  Where  vent  shafts  exist  in  suitable  loca- 
tions, they  should  be  treated  the  same  as  elevator  shafts,  the  cable 
being  accessible  at  each  floor.  As  lead-covered  cables  are  used  in 


870  TELEPHONY 

riser  shafts,  it  is  not  necessary  that  the  vent  shaft  should  be  water- 
proof. Care  should  be  taken,  however,  that  the  connection  to  the 
hall  moulding  is  waterproof  on  the  hall  side.  Where  conduit  is 
used,  this  result  may  be  obtained  by  slanting  the  conduit  up  from  the 
outside. 

Main  Distributing  Terminals.  In  order  that  wires  within  the 
building  may  be  connected  to  the  outside  wires  coming  from  the  cen- 
tral office,  it  is  necessary  that  both  sets  of  wires  be  carried  to  some 
common  distributing  point.  The  place  at  which  this  occurs  is  either 
a  main  terminal  cabinet  or  main  terminal  room. 

The  main  terminal,  except  for  special  reasons,  always  should  be 
located  in  the  basement  and  the  location  should  be  dry  and  not  too 
close  to  steam  pipes  and  the  like,  but  convenient  to  vertical  riser- 
cable  shafts  and  to  the  point  or  points  at  which  the  service  cables  will 
enter  the  building. 

If  the  vertical  riser-cable  shafts  do  not  terminate  at  the  main 
distributing  terminal  they  should  be  connected  to  the  latter  by  3J- 
inch  conduits,  two  conduits  being  run  to  each  shaft.  Should  it  be 
evident  that  a  shaft  will  never  have  to  accommodate  a  full  cable  of 
four  hundred  pairs  of  wires,  smaller  conduits  may  be  used. 

Typical  Arrangement.  Fig.  644  shows  a  typical  office  building 
floor  plan,  laid  out  for  wiring. 

Hotels.  The  entire  telephone  traffic  of  hotels  generally  is  han- 
dled by  a  private-exchange  switchboard.  This  switchboard  must  be 
connected  with  practically  every  room  in  the  hotel  and  with  a  tele- 
phone central  office,  the  latter  by  a  few  trunk  lines.  The  location  of 
the  telephone  outlet  in  each  room  of  a  hotel  may  be  determined 
readily;  therefore,  it  is  always  possible  to  install  permanent  wiring 
at  the  outset  for  the  entire  telephone  system.  This  distinguishes 
hotel  wiring  from  office-building  wiring,  since  the  permanent  wiring 
in  the  latter  must  terminate  at  the  hall  terminal  boxes. 

In  a  degree,  the  wiring  of  a  hotel  for  telephone  purposes  is  similar 
to  wiring  it  for  electric  lights,  the  difference  being  that  for  telephone 
service  a  separate  pair  of  conductors  must  be  carried  to  each  tele- 
phone, while  in  electric-light  work,  all  lights  may  be  connected  in  one 
way  or  another  to  a  single  source  of  current. 

The  method  of  wiring  best  suited  to  the  average  hotel  is:  A 
riser  cable,  which  is  installed  at  a  convenient  point,  is  properly  con- 


SUBSCRIBERS'  STATION  WIRING  871 

nected  at  each  floor  by  smaller  cables  to  suitable  floor  distributing 
boxes.  From  the  proper  box  a  pair  of  twisted  wires  is  run  to  each 
telephone  in  each  room.  More  than  one  riser  cable  may  be  needed 
in  a  large  hotel.  As  in  office  buildings,  the  economy  of  such  a  system 
will  depend  very  largely  upon  the  care  exercised  in  choosing  the 
locations  of  the  riser-cable  shafts  and  the  floor  distributing  boxes, 
which  locations  should  be  such  that  the  length  of  conduit,  wire,  and 
cable  runs  will  be  a  minimum. 

The  first  thing  to  do  in  preparing  wiring  plans  for  a  hotel  is  to 
decide  upon  the  location  of  the  private  exchange  switchboard.  A 
good  location  generally  is  found  in  or  close  to  the  office.  This,  in 
most  cases,  brings  the  switchboard  on  the  first  floor  and  in  occasional 
cases  on  the  second  or  third  floors. 

All  house  lines,  incoming  and  outgoing  trunks  from  and  to  the 
central  office,  and  all  lines  to  the  private  exchange  switchboard 
should  end  at  the  main  distributing  terminal.  The  terminal  affords 
a  means  of  cross-connection,  so  that  any  room  line  or  trunk  line 
may  be  connected  as  desired  to  a  chosen  circuit  in  the  private 
exchange  switchboard.  It  will  be  seen  that  in  this  way  the  number 
of  wires  between  the  private  exchange  switchboard  and  the  main 
distributing  terminal  will  be  large,  and  to  reduce  cost  to  a  min- 
imum the  distance  between  the  main  distributing  terminal  and  the 
switchboard  should  be  made  as  short  as  possible. 

Arrangement  of  Apparatus.  In  hotels  of  moderate  size,  not 
exceeding  150  rooms,  it  is  good  practice  to  arrange  for  a  main  dis- 
tributing terminal  directly  at  the  rear  of,  or  close  to,  the  private  ex- 
change switchboard.  Where  this  is  not  advisable,  and  in  the  larger 
hotels,  the  main  distributing  terminal  should  be  located  in  a  separate 
room.  This  room,  in  the  larger  hotels,  will  also  be  required  to 
care  for  certain  power  apparatus  and  other  telephone  accessories  in 
addition  to  the  wiring. 

Where  the  switchboard  is  located  on  the  first  floor,  an  ideal 
location  for  the  terminal  room  is  directly  below  it  in  the  basement. 
Where  the  private  exchange  is  not  located  on  the  first  floor,  the  ter- 
minal room,  from  a  cost  viewpoint,  should  be  located  on  the  same 
floor  as  the  switchboard.  Where  a  separate  room — or  a  room  used  in 
conjunction  with  electric  fuse  cutouts,  etc. — is  to  accommodate  the 
main  distributing  terminal,  a  cabinet  is  not  necessary.  If  the  terminal 


872  TELEPHONY 

must  be  located  in  a  hall  or  other  room,  or  at  the  back  of  the  switch- 
board so  as  to  be  exposed  to  general  view,  a  cabinet  should  be  pro- 
vided which  should  be  approximately  1  foot  deep  in  the  clear.  The 
entire  front  of  the  cabinet  should  be  fitted  with  removable  doors, 
and  its  design  should  harmonize  with  its  surroundings.  Provision 
also  should  be  made  to  carry  the  incoming  wires  from  the  point 
where  they  enter  the  building  to  the  distributing  cabinet.  A  2-inch 
conduit  generally  will  suffice  for  this  purpose. 

Location  of  Outlets.  If  ordinary  wall  telephone  sets  are  to  be 
used  in  rooms,  the  telephone  outlet,  consisting  of  an  ordinary  con- 
duit outlet  box,  should  be  placed  4  feet  10  inches  above  the  floor. 
If  sets  mounted  flush  with  the  wall  are  to  be  used,  a  special  conduit 
outlet  box  is  required. 

Where  portable  telephones  are  used,  the  outlets  in  most  cases 
may  be  located  with  advantage  in  the  baseboard.  This  location  and 
the  outlet  box  required  must  be  considered  special  and  should  be 
given  special  consideration  in  each  case.  It  is  often  desirable  to 
arrange  for  telephone  service  in  dining  rooms  and  grills,  from  part  or 
all  of  the  tables  and  such  provision  requires  the  use  of  floor  or  wall 
outlet  boxes  and  special  apparatus. 

From  the  individual  outlet  or  outlets  in  each  room,  there  should 
be  run  to  the  proper  floor  distributing  box  a  twisted  pair  of  No.  19 
B.  &  S.  gauge,  braided,  rubber-covered,  tinned,  soft  copper  wire. 
Where  a  separate  circuit  is  run  to  each  outlet — the  usual  case — a 
third  or  ground  wire  is  not  necessary  unless  coin-prepayment  or  other 
special  service  is  desired;  in  the  latter  case,  a  third  wire  tap  should 
be  brought  out  at  each  outlet.  The  third  wire  taps  from  all  outlets 
should  be  spliced  to  a  common  wire  at  the  distributing  box  or  other 
convenient  point.  For  mechanical  reasons  this  common  ground 
wire  should  not  be  smaller  than  No.  12  B.  &  S.  gauge  and  should 
be  of  the  best  quality  of  braided,  rubber-covered,  tinned,  soft  copper 
wire.  At  each  floor  distributing  box,  the  wires  from  the  various  out- 
lets should  be  connected  to  the  distributing  cable,  preferably  by 
means  of  suitable  binding  posts  mounted  on  strips  of  suitable  insu- 
lating material. 

Distributing  Cables.  The  distributing  cables  from  the  riser- 
cable  shafts  should  be  lead-covered  and  should  consist  of  a  number 
of  twisted  pairs  of  No.  22  B.  &  S.  gauge,  double  silk-  and  cotton- 


SUBSCRIBERS'  STATION  WIRING 


873 


insulated,  soft,  tinned,  copper  wires.  The  size  of  the  distributing 
cable  is  determined  by  the  number  of  outlets  it  is  to  serve.  An  extra 
pair  or  so  should  be  allowed  where  possible.  This  will  naturally 
follow  where  a  15-pair  cable  serves  from  twelve  to  fourteen  rooms. 
Even  where  one  conduit  serves  more  than  one  distributing  box, 
only  one  cable  should  be  placed  in  a  conduit  where  practicable. 


'    j:1 J~'M    'ind    ilS    !w!~~E !  ~3T 
i  fl  BI      r ii      fii      r^i  la*1 


CABLC 
/Z^?/f  ZW  T/TJBUTJN6  BOX 

-WOK jftwrtraff fioofr 

CA3LE  ffVHS 


Fig.  645.     Hotel  Wiring 


This  may  be  done  in  most  cases  by  tapering  the  cable  at  each  box 
so  that  only  the  wires  actually  required  will  be  carried  ahead.  At 
the  riser-cable  shaft  or  shafts,  the  distributing  cables  should  be  per- 
manently spliced  to  the  riser  cable  or  cables,  as  the  case  may  be. 

Riser  Cables.  The  capacity  of  the  riser  cable  or  cables  should 
be  made  to  equal  the  combined  capacity  of  the  distributing  cables 
unless  there  is  an  unusually  large  number  of  dead  wires  in  the  latter. 
In  case  this  number  is  large,  the  capacity  of  the  riser  cable  or  cables 


874  TELEPHONY 

may  be  decreased  as  required  and  certain  of  the  dead  wires  in  the 
distributing  cables  on  each  of  the  floors  connected  to  the  same  con- 
ductors in  the  riser  cable  or  left  dead,  if  desirable. 

In  hotels  of  not  over  five  or  six  stories,  the  riser  cable  should 
preferably  be  of  the  same  kind  as  the  distributing  cables;  namely, 
lead-covered,  with  No.  22  B.  &  S.  gauge,  double  silk-  and  cotton- 
insulated  wires.  The  distributing  cables  then  may  be  spliced  to  the 
riser  cables  at  each  floor. 

In  the  larger  hotels,  especially  where  a  large  number  of  rooms 
are  to  be  served  on  each  floor,  the  first  cost  may  be  materially  reduced 
by  using  lead-covered  cables  with  paper-insulated  conductors  such 
as  are  used  in  underground  conduits.  Where  this  type  of  riser 
cable  is  used,  it  is  better  to  make  one  splice  to  the  distributing  cables 
at  every  fourth  or  fifth  floor,  thus  paralleling  certain  of  the  distribut- 
ing cables  for  a  floor  or  so,  rather  than  to  splice  the  distributing  cable 
to  the  riser  cable  at  each  floor.  The  reason  for  this  is  that  splices  to 
paper  cables  are  expensive  to  make  and  must  be  made  with  great 
care,  since  the  paper  insulation  of  the  conductors  will  not  stand  rough 
usage  or  exposure  to  moisture. 

To  convey  the  riser  cables  to  the  main  distributing  terminal 
where  silk  and  cotton  riser  cables  are  used,  they  are  attached  directly 
to  the  terminal  strips  on  the  main  terminal  rack,  the  exposed  insu- 
lation first  being  dipped  in  hot  beeswax.  Where  paper-insulated 
cables  are  used,  the  cable  must  be  pot-headed  before  being  at- 
tached to  the  terminals. 

Typical  Arrangement.  Fig.  645  illustrates  an  actual  example 
of  the  method  of  wiring  for  hotels. 

Apartment  Houses.  The  following  notes  refer  in  particular  to 
those  apartment  houses  which  are  of  sufficient  size,  or  of  such  a  type 
as  to  warrant  a  private  exchange.  The  requirements  of  this  class 
of  apartaaent  houses  are  similar  to  the  requirements  of  hotels. 

In  apartment  houses,  one  telephone  generally  is  required  for 
each  apartment.  This  makes  the  ratio  of  telephones  to  rooms 
considerably  less  than  in  hotels.  It  is  for  this  reason  that  the  method 
of  wiring  best  suited  for  hotels  is  not  generally  the  most  economical 
or  the  best  suited  for  apartment  houses,  except  in  cases  of  very  large 
buildings  or  where  the  arrangement  of  the  apartments  on  each  floor 
is  irregular. 


SUBSCRIBERS'  STATION  WIRING  875 

In  most  apartment  houses,  the  apartments  on  the  several  floors 
are  arranged  similarly.  Should  the  telephones  in  each  vertical  tier 
of  apartments  be  located  in  the  same  relative  position,  it  will  be 
seen  that  one  vertical  riser  conduit  running  from  the  basement  to  the 
top  floor  can  be  made  to  serve  all  telephones  in  each  tier  of  apart- 
ments, and  in  many  cases,  one  conduit  can  be  made  to  serve  more 
than  one  tier  of  apartments. 

Most  of  the  preceding,  relative  to  the  location  of  the  private 
exchange  switchboard  and  the  main  distributing  terminal  in  hotels, 
applies  equally  well  to  their  location  in  large  apartment  houses. 

Conduits  and  Outlets.  Care  should  be  taken  not  to  overlook 
the  conduit  that  may  be  required  between  the  mam  distributing  ter- 
minal and  the  private  exchange  switchboard.  The  size  of  this  con- 
duit will  depend  on  the  size  of  the  cable  it  is  to  accommodate.  Where 
possible,  the  conduit  also  should  be  run  from  the  main  distributing 
terminal  to  the  point  at  which  the  service  wires  will  enter.  A  2-inch 
conduit  generally  is  sufficient  for  this  purpose. 

To  permit  the  use  of  the  system  above  described,  it  is  essential 
that  there  be  an  outlet  in  each  apartment  of  each  vertical  tier,  in  line 
with  the  common  vertical  riser  conduit.  In  most  cases,  the  telephone 
may  be  located  at  this  outlet.  Should  another  location  be  desirable 
in  certain  apartments,  an  additional  outlet  box  should  be  installed 
at  the  desired  location  and  connected  to  the  outlet  box  referred  to 
above  by  a  suitable  run  of  conduit.  Whether  the  outlet  box  is  to 
be  of  standard  or  special  design  such  as  required  for  flush  sets,  or 
whether  it  is  to  be  located  for  a  wall  set,  or  a  portable  set,  will 
depend  upon  the  type  of  instrument  desired. 

The  size  of  each  of  the  vertical  riser  conduits,  which  is  deter- 
mined by  the  number  of  wires  it  is  to  accommodate,  may  be  tapered 
from  floor  to  floor  as  the  number  of  wires  to  be  carried  by  the  conduit 
decreases.  All  riser  conduits  should  be  carried  to  the  main  dis- 
tributing terminal. 

Main  Distributing  Terminal.  The  type  of  main  distributing 
terminal  best  suited  to  most  apartment  houses  is  one  which  mounts 
in  a  cabinet,  the  size  of  which  is  determined  by  the  number  of 
apartments  the  terminal  is  to  serve. 

The  telephone  outlet  in  each  apartment  should  be  connected 
to  the  main  distributing  terminal  by  a  twisted  pair  of  No.  19  B.  &  S. 


876 


TELEPHONY 


gauge,  rubber-covered  wires,  which  wires  may  be  fished  easily  through 
the  conduit  after  the  latter  has  been  installed.  In  some  of  the  larger 
apartments,  it  will  be  found  economical  to  splice  the  individual  wires, 
at  the  bottom  of  each  vertical  conduit,  to  a  lead-covered  cable  con- 
taining a  proper  number  of  No.  22  B.  &  S.  gauge  double  silk-  and 
cotton-insulated  wires,  thus  conveying  the  circuits  from  the  splices 


KKY: 
•  RISER  CONDUIT 

—  WIRE  RUN  OR  OUTLET 

Pig.  646.     Apartment  House  Wiring 


to  the  main  distributing  terminal  in  the  cable.  Similar  cable  should 
be  used  to  connect  the  main  distributing  terminal  to  the  private 
exchange  switchboard. 

Typical  Arrangement.  Fig.  646  illustrates  the  wiring  of  a  typ- 
ical apartment  house.  In  this  case,  a  number  of  risers  have  been 
located  so  as  to  serve  more  than  one  apartment  each. 


SUBSCRIBERS'  STATION  WIRING  877 

Flats.  Small  apartment  houses  may  be  considered  as  flats,  in 
that  both  classes  of  buildings  do  not,  as  a  rule,  require  private  ex- 
changes. It  may  be  said  further  that  intercommunication  between 
such  apartments  or  flats  generally  is  not  necessary  and  sometimes 
is  undesirable. 

Location  of  Outlets.  The  location  of  each  telephone  outlet 
having  been  determined,  there  should  be  run  to  each  outlet  from 
the  predetermined  common  distributing  point  a  pair  of  No.  19  B. 
&  S.  gauge,  braided,  rubber-covered  copper  wires. 

Distributing  Point.  The  common  distributing  point  preferably 
should  be  located  in  the  basement.  Should  the  building  be  served 
by  an  underground  system,  further  provision  for  wiring  need  not  be 
made  unless  the  basement  walls  are  finished,  in  which  case  conduits 
should  be  run  through  the  finished  portions  of  the  basement,  avoiding 
the  necessity  of  carrying  exposed  wire  or  cable  through  a  finished 
room.  Should  the  building  be  served  by  an  aerial  lead,  either  at 
the  front  or  at  the  rear  of  the  building,  two  conduits  should  be  run 
from  the  common  distributing  point  to  a  point  or  points  on  the  front 
or  on  the  rear  of  the  building,  directly  opposite  the  aerial  leads  from 
which  the  service  wires  or  cables  will  be  taken. 

In  small  buildings  of  not  over  six  or  eight  apartments,  a  dis- 
tributing cabinet  is  not  necessary.  In  larger  buildings,  a  distributing 
cabinet  should  be  arranged  for  at  the  common  distributing  point. 
It  should  have  a  removable  door,  hinged  or  otherwise,  and  should 
be  approximately  6  inches  deep. 

Special  Facilities.  When  desired,  special  facilities  for  local 
service  can  be  provided  which  will  permit  a  caller  in  the  vestibule  to 
ring  and  speak  with  the  occupant  of  any  apartment  or  with  the 
janitor.  The  occupant  of  each  apartment  also  may  call  the  jani- 
tor or  be  called  by  him.  To  provide  for  such  a  system  of  course 
will  require  some  additional  wiring  and  special  apparatus,  but  such 
apparatus  does  away  with  the  usual  tin  speaking  tube  and  push 
button. 

Private  Dwellings.  The  setting  of  the  protectors  and  instru- 
ments, and  the  running  of  the  wires  may  be  carried  out  in  private 
dwellings  in  accordance  with  the  instructions  already  given.  Whether 
or  not  concealed  wiring  is  to  be  used  will  depend  on  the  character 
of  the  dwelling  an  1  the  desires  of  the  owner. 


The  practical  result  of  the  electrolytic  hazards  described  in 
Chapter  XVIII  is,  that  if  currents  are  allowed  to  flow  away  from 
cable  sheaths  or  other  metallic  property  in  the  presence  of  moisture 
for  a  long  enough  time,  the  metals  will  be  injured.  Fig.  647  is  a 


Fig.  647.     Part  of  Cable  Sheath  Corroded  by  Electrolysis 

photograph  of  a  piece  of  cable  sheath  from  which  current  at  a  pres- 
sure of  5  volts  flowed  to  rnoist  earth  for  a  few  months.  If  it  had 
flowed  a  few  months  longer,  the  pitting  would  have  perforated  the 
sheath  and  moisture  would  have  entered  the  cable  core. 

Early  Controversy.  As  the  principal,  if  not  the  only  cause,  of 
this  hazard  is  the  return  current  from  ground  return  electric  rail- 
ways, it  will  be  instructive  briefly  to  recall  certain  phases  of  a  con- 
troversy between  telephone  and  electric-railway  interests  in  the  early 


ELECTROLYSIS  OF  UNDERGROUND  CABLES         879 

days  of  each  of  these  industries.  It  was  not  known  then  that  this 
controversy  involved  the  electrolytic  hazard,  and  it  was  fought  out 
on  other  lines;  but  the  point  of  particular  interest  is  that,  had  the  con- 
troversy been  decided  the  other  way,  electrolysis  of  underground 
cables  would  not  be  a  serious  enough  matter  to  write  about  here. 

It  has  been  stated  that  all  telephone  lines  were  single-wire  cir- 
cuits in  the  beginning  of  practical  telephone  development,  and  for 
a  number  of  years  thereafter.  It  was  the  advent  of  electric  light 
and  electric  street-car  systems,  particularly  the  latter,  which  made 
grounded  circuits  unsuitable  for  the  best  telephone  service,  the 
high-tension  circuits  while  in  operation  making  these  single-wire 
telephone  circuits  noisy,  and  the  street-railway  systems  causing 
not  only  noises  but  false  signals  as  well,  the  annunciators  of  the  sub- 
scribers' lines  being  frequently  dropped  by  stray  currents  flowing 
over  the  lines. 

As  the  street-car  systems  became  more  extensive  in  the  cities, 
and  particularly  as  their  traffic  grew  heavier,  the  difficulties  from  these 
two  troubles  became  quite  serious.  No  remedy  was  discovered  to 
be  reasonably  possible  so  far  as  changing  the  street-car  system  was 
concerned,  except  to  provide  each  car  line  with  two  trolley  wires,  and 
each  car  with  two  trolley  poles,  making  the  street-car  system  wholly 
metallic  circuit,  the  rails  not  being  used  at  all  for  the  return  of  the 
current  to  the  power  station.  Such  systems  have  been  installed 
abroad,  and  such  was  the  plan  adopted  in  Cincinnati,  Ohio.  In 
that  city,  grounded  telephone  lines  were  successfully  operated  for  a 
considerable  time  after  the  establishment  of  the  electric-traction 
system.  So  long  as  the  car  trucks  and  wheels  were  kept  insulated 
from  the  electric  circuit  of  the  motors,  the  telephone  lines  passing  or 
ending  near  the  trolley  line  were  free  from  disturbances.  If,  by  any 
defect  in  the  car,  a  cross  occurred  between  some  part  of  the  motor 
circuit  and  the  truck,  the  progress  of  that  car  through  the  streets 
could  be  traced  by  one  listening  upon  the  telephone  lines,  and  it  was 
by  means  of  this  detective  work  of  the  telephone  system  that  such 
defective  cars  were  located,  the  listening  subscribers  observing  from 
their  windows  the  numbers  of  the  noisy  cars  as  they  passed  by. 
These  cars  were  then  withdrawn  from  service  and  repaired.  The 
double  trolley  system  with  its  distinct  freedom  from  being  the  cause 
of  trouble  is  still  extensively  in  service  in  Cincinnati. 


880  TELEPHONY 

The  existence  of  a  so  thoroughly  possible  traction  system  with 
its  absence  from  disturbances  to  the  telephone  system  caused  legal 
action  to  be  instituted  in  Ohio,  wherein  it  was  contended  by  the  tele- 
phone company  that  as  the  traction  interests  possessed  a  means  of 
operating  their  cars  without  interfering  with  the  telephone  system, 
the  former  should  apply  that  method,  or  be  compelled  to  continue 
it  if  they  had  applied  it.  The  decision  of  the  lower  courts  was  fav- 
orable to  this  contention,  and  it  seemed  at  that  time  as  if  the  single- 
trolley  rail-return  traction  system  must  disappear. 

Court  Decisions.  In  the  higher  courts  this  decision  was  reversed, 
it  being  held  that  the  telephone  company  did  not  possess  such  a  right 
to  the  use  of  the  streets  and  the  earth  under  the  street,  as  to  permit 
it  to  interfere  with  the  proper  use  of  the  streets  for  transit  and  trans- 
portation. The  ruling  in  effect  was  that  the  telephone  company 
did  not  own  the  earth,  as  some  phases  of  the  attitude  of  the  original 
monopolists  have  indicated  they  were  inclined  to  believe.  From 
this  time  the  growth  of  single-trolley  traction  systems  continued 
without  further  opposition  from  the  telephone  interests,  and  the  bur- 
den of  relief  lay  upon  the  latter. 

McCluer  System.  One  of  the  measures  of  relief  was  that  devised 
by  McCluer  of  Richmond,  Virginia,  and  consisted  in  the  simple  ex- 
pedient of  providing  for  each  main  and  branch  route  of  the  tele- 
phone system,  a  copper  return  wire  to  which  the  telephones  of  that 
district  were  connected,  instead  of  being  connected  to  ground.  For 
a  given  route  of  100  lines,  for  example,  there  would  be  100  line  wires 
plus  one  common  return  wire.  The  office  of  this  was  merely  to  pro- 
vide a  return  path  to  the  central  office  quite  as  the  ground  had  pro- 
vided such  a  path  before,  and  its  success  depended  upon  its  being 
kept  clear  of  ground.  In  a  sense,  an  exchange  having  single- wire 
lines  and  a  common  return  wire  on  each  route — the  other  returns 
being  united  in  the  central  office — might  be  called  a  metallic-circuit 
system,  because  if  the  circuit  is  not  grounded  in  any  degree  it  must  neces- 
sarily be  metallic.  In  the  accepted  sense,  however,  a  system  utilizing 
a  common  return  is  only  to  be  called  metallic  circuit  by  an  excess  of 
courtesy,  because  it  partakes  in  a  large  degree  of  the  disadvantages  of 
a  grounded  system,  the  chief  disadvantage  being  the  lack  of  similarity 
in  the  two  sides  of  the  line,  one  side  being  its  actual  line  wire,  and  the 
other  the  common  return.  As  the  latter  has  many  lines  connected 


ELECTROLYSIS  OF  UNDERGROUND  CABLES         881 

to  it,  and  as  it  is  usually  of  copper  and  the  line  wire  frequently  of 
steel  or  of  iron,  the  conductivity  and  electrostatic  capacity  of  the 
two  sides  of  a  given  line  were  distinctly  unlike.  The  common 
return  wire  did,  however,  successfully  cure  the  trouble  due  to  the 
falling  of  drops  from  stray  traction  currents.  These  troubles,  as 
they  were  due  wholly  to  currents  flowing  between  separated  points  of 
the  earth,  would  no  longer  exist  when  the  use  of  the  earth  as  a  return 
had  been  abandoned. 

Metallic  Circuits  and  Cables.  Following  this  partial  cure  of 
new  difficulties,  the  existence  of  metallic-circuit  long-distance  lines 
assisted  in  introducing  the  present  general  practice  of  making  all 
subscribers'  lines  of  two  wires,  with  the  consequent  advantages, 
assuming  proper  transposition,  of  complete  balance,  complete  quiet- 
ness, and  absence  from  false  signals.  This  made  it  further  advan- 
tageous to  combine  many  lines  in  one  cable,  even  though  it  must 
be  of  considerable  length,  because  the  complete  balance  of  the  sub- 
scribers' metallic  circuit  rendered  it  free  from  cross-talk  troubles 
as  well  as  from  inductive  noises.  When  it  became  possible  to 
place  many  wires  in  one  cable,  it  was  further  possible  to  place  the 
cables  underground,  and  such  practice  became  general. 

Electrolysis  Troubles.  In  1895,  I.  H.  Farnham,  chief  engineer 
of  the  New  England  Telephone  Company,  observed  the  begin- 
ning of  an  epidemic  of  underground-cable  troubles.  In  all  of  these 
new  troubles,  the  difficulty  was  a  failure  of  the  underground  cable 
due  to  loss  of  insulation,  and  the  loss  of  insulation  was  found  to  be 
the  result  of  the  entrance  of  moisture  due  to  an  unaccountable  ap- 
pearance of  holes  in  the  lead  sheath.  With  characteristic  thor- 
oughness, Farnham  investigated  all  the  elements  accompanying  this 
new  trouble  and  conducted  a  series  of  experiments  which  led  him 
finally  to  discover  that  the  cause  was  the  eating  away  of  the  lead 
sheath  of  the  underground  cable  at  a  certain  point,  or  at  certain 
points  in  its  length,  which  destruction  of  the  sheath  was  due  to  the 
passage  of  current  from  the  sheath  to  the  earth  in  the  presence  of 
moisture,  as  has  already  been  stated. 

Causes.  The  corrosion  of  a  lead  sheath  by  currents  passing 
from  it  to  the  earth  must  not  be  confused  with  any  electrical  use  of  the 
conductors  of  the  cable,  as  it  is  entirely  independent  of  such  actions. 
The  reasons  the  lead  sheath  is  attacked  at  all  are  several:  First,  the 


882  TELEPHONY 

existence  of  a  single-trolley  traction  system,  using  the  rails  of  the 
track  as  a  return  for  current,  does  more  than  that  statement  implies. 
Whatever  may  be  the  condition  of  the  soil  in  any  city,  the  rails  laid 
upon  the  earth  will  be  assisted  in  carrying  current  by  the  earth's 
mass  itself.  This  is  true,  however  well  the  rails  may  be  supple- 
mented by  copper  return  wires  connected  to  them  at  intervals.  Sec- 
ond, the  cable  sheaths  themselves,  being  formed  of  a  considerable 
amount  of  lead,  a  fairly  good  conductor,  and  laid  in  quite  good  con- 
tact with  the  earth  itself,  necessarily  assist  the  rails  and  the  rail- 
return  in  carrying  current  toward  the  power  station.  Perhaps  this  is 
as  well  told  by  saying  that  in  any  traction  system,  an  examination  of 
the  territory  covered  will  show  that  different  points  in  the  territory 
have  different  potentials  with  relation  to  the  earth  at  the  power  station; 
and  it  is  obvious  that  where  differences  of  potential  exist,  and  these 
are  connected  to  each  other  by  a  conductor,  current  will  flow.  The 
amount  of  current  which  will  flow  is  an  immediate  result  of  the  amount 
of  the  difference  of  potential  and  of  the  conductivity  of  the  connecting 
conductor,  which  in  this  case  is  the  .cable  sheath.  It  is  further  evi- 
dent that  the  difference  of  potential  depends  largely  upon  the  number 
of  cars  which  are  in  use  at  a  given  time,  upon  their  location,  and  upon 
the  amount  of  current  they  are  drawing  from  the  power  station. 

In  almost  all  underground  situations,  moisture  is  present,  and 
in  addition  has  dissolved  in  it  salts  or  acids  of  various  kinds,  which 
assist  in  the  electrolytic  process.  A  simple  experiment  of  passing 
current  from  a  piece  of  lead  through  moist  earth  to  another  conduc- 
tor will  establish  the  certainty  of  pitting  the  lead,  while  no  such  effect 
whatever  results  when  the  current  is  passed  from  a  conductor  through 
the  moist  earth  to  the  lead. 

Underground  Conditions.  Some  idea  of  the  conditions  to  which 
an  underground  cable  is  subjected  may  be  gained  from  Fig.  648,  in 
which  it  is  assumed  that,  as  is  usual,  the  negative  pole  of  the  electric- 
railway  power-station  generator  is  connected  to  the  rail  and,  there- 
fore, to  the  earth,  and  the  positive  pole  to  the  trolley  wire;  and  that 
the  telephone  cable  passes  through  the  territory  in  such  a  way  that 
one  part  of  it  is  nearer  the  power  station  than  the  other.  Indeed, 
it  is  not  essential  that  this  condition  exist,  because,  due  to  the 
existence  of  other  pipes  and  to  natural  conditions  of  the  earth,  a  cable 
may  be  laid  in  such  a  way  as  to  be  always  equally  distant  from  the 


ELECTROLYSIS  OF  UNDERGROUND  CABLES 


883 


power  station,  and  yet  be  subjected  to  the  carrying  of  return  cur- 
rent, because  it  passes  through  regions  having  different  potentials 
with  relation  to  the  power  station  and,  therefore,  to  each  other.  It  is 
evident  that,  in  a  condition  such  as  is  shown  in  Fig.  648,  no  destruc- 
tion of  the  cable  sheath  would  take  place  at  the  point  where  current 
enters,  near  the  ends  of  the  portion  shown,  but  that  its  sheath  would 
be  attacked  in  a  greater  or  less  degree  at  a  point  nearer  the  power 
station  where  the  current  is  indicated  as  leaving  the  sheath. 

An  idea  of  the  relative  conditions  of  this  destruction  may  be 
gained  from  the  statement  that  a  few  volts  difference  of  potential 
between  two  points  of  the  cable  sheath  will  cause  a  large  current  to 
flow  therein,  the  resistance  of  the  sheath  being  so  very  low.  The 


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Fig.   648.     Conditions  Causing  Electrolysis 

destruction  at  the  point  where  the  current  leaves  it  can  be  accom- 
plished with  a  difference  of  potential  between  the  sheath  and  the 
surrounding  earth  of  less  than  one  volt.  In  many  cities  there  are 
differences  of  potential  between  different  parts  of  the  city  of  as  high 
as  twenty  volts. 

.  Remedies.  As  it  has  been  accepted  that  single-trolley  systems 
may  be  used  and  that  differences  of  potential  between  points  in  cities 
must  be  expected,  the  prevention  of  cable-sheath  damage  must  evi- 
dently come  from  work  done  upon  the  cable  system  itself.  If  the 
cable  sheaths  could  be  of  some  non-conducting  material;  or  if  the 
cables  could  be  carried  in  dry  and  non-conducting  underground 
pipes;  or  if  the  current  entering  the  cables  might  be  carried  by  their 
sheaths  and  caused  to  leave  at  a  point  where  there  was  no  moisture 
contact;  or  if  some  easily  renewable  metallic  mass  could  be  attached 
to  the  cable  at  each  point  where  it  has  had  a  tendency  to  be  de- 
stroyed, the  problem  might  be  considered  solved. 

Insulating  Joints.     It  has  not  been  found  possible  at  all  as  yet 


884  TELEPHONY 

to  enclose  the  cable  wires  in  anything  but  a  metal  sheath;  nor  is  it 
generally  feasible  to  occasionally  insert  insulating  joints  in  the  cable 
sheaths,  which,  while  they  seal  the  core  satisfactorily,  interrupt  the 
continuity  of  the  metal  sheath.  This  remedy  is  being  used  with 
apparent  success  in  a  few  places,  and  if  it  proves  that  it  may  be  ap- 
plied with  practical  certainty  as  to  the  tightness  of  the  joints,  it  should 
result  in  at  least  a  very  considerable  reduction  of  electrolytic  hazard. 

Moisture-Proof  Conduits.  To  enclose  cables  having  contin- 
uous metal  sheaths  in  wholly  dry  and  insulating  conduits  has  been  the 
fond  dream  of  manufacturers  and  constructors,  but  so  far  as  proof 
has  been  given,  it  is  yet  unrealized.  Conduit  material  of  paper  or 
other  fiber,  saturated  with  asphaltic  or  bituminous  compounds,  may 
be  presumed  to  be  of  better  insulating  character  than  clay-conduit 
material;  but  when  in  the  earth,  exposed  at  manholes,  it  has  not  been 
shown  that  a  conduit  system  of  this  kind  really  and  completely  pro- 
tects the  cables  from  electrolytic  damage. 

Bonding.  To  attach  to  the  cable  sheaths  at  danger  points  some 
metal  intended  to  be  destroyed,  is  a  precaution  which  is  a  practical 
one,  with  the  simple  objection  that  such  an  attached  mass  will  itself 
be  destroyed  in  time  and  will  require  replacing.  The  ideal  cure  is, 
of  course,  one  which  is  applied  once  for  all. 

In  general,  the  method  that  is  employed  is  to  provide  dry  me- 
tallic paths  over  which  the  current  may  flow  from  sheaths  tending 
to  be  positive  to  other  things,  instead  of  flowing  from  them  through 
moisture.  Its  application  is  called  "bonding,"  as  metallic  bonds  are 
provided  as,  substitutes  for  moisture  paths.  This  is  the  method 
that  survives  in  general  practice. 

Two  steps  are  required  in  applying  a  system  of  cable  bonding  : 
First,  determine  the  areas  of  the  city  in  which  bonds  are  needed, 
and  second,  apply  them  and  test  the  result.  The  first  is  done  by  a 
system  of  observations  to  determine  the  amount  and  the  sign  of  the 
potential  of  each  cable  to  the  surrounding  earth  and  things  in  it  and 
on  it.  These  things  are,  for  example,  the  near-by  rails  of  the  trac- 
tion system,  water  pipes,  gas  pipes,  and  other  cables.  It  is  conven- 
ient to  record  these  observations  on  maps.  Usually  the  cables  in  a 
region  surrounding  each  power  station  of  the  traction  system  will 
be  found  positive  to  the  negative  bus-bar  of  that  power  station.  Out- 
side this  critical  area  surrounding  each  power  station,  the  cables 


ELECTROLYSIS  OF  UNDERGROUND  CABLES        885 

usually  will  be  found  negative  to  the  rails  and  to  other  conductors 
in  their  neighborhood. 

Cables  so  negative  are  not  in  danger  of  corrosion,  but  cables 
which  are  positive  are  in  danger  of  corrosion.  Guided  by  the  map 
record,  copper  bonds  are  attached  to  the  cable  sheaths  at  one  end  and 


Fig.  649.     Potential  Map  Surrounding  Power  Station 

at  the  other  to  the  negative  bus-bar  of  the  station  or  to  something 
leading  to  it.  Many  traction  companies  use  negative  feeders  to  sup- 
plement their  rails,  as  well  as  positive  feeders  to  supplement  their 
trolley  wires.  These  negative  feeders,  therefore  are  admirable  cir- 
cuits to  which  to  carry  cable  bonds.  Bonding  to  the  rails  themselves 
is  less  likely  to  be  permanently  useful. 


886  TELEPHONY 

Fig.  649  is  a  map  of  a  portion  of  a  city  in  which  lines  join  the 
points  of  equal  cable  potential  and  of  similar  sign.  If  copper  enough 
be  used  in  bonds  between  the  cables  of  this  area  and  the  negative 
bus-bar  of  the  power  station  of  this  area,  these  danger-indicating 
potentials  may  be  reduced  within  negligible  limits.  A  regular 
system  of  re-inspection  and  re-arrangement  is  imperative  to  keep 
them  so. 


CHAPTER  LI 
DEVELOPMENT  STUDIES 

A  development  study  is  an  examination  of  the  physical,  commer- 
cial, and  social  conditions  of  a  region  to  determine  its  present  and 
future  need  of  telephone  service.  Consciously  or  otherwise,  a  devel- 
opment study  always  is  made  before  the  construction  of  a  new  tele- 
phone system  or  part  of  a  system.  This  study  may  be  very  simple, 
confining  itself  merely  to  a  forecast  as  to  how  many  telephones  shall 
be  arranged  for  at  the  time  a  very  small  switchboard,  or  even  a  single 
telephone,  is  placed  in  a  village.  Or  the  study  may  be  very  elabo- 
rate, as  in  the  case  of  an  entire  reconstruction  of  the  central  offices, 
the  equipments,  and  the  wire  plant  of  a  large  exchange  covering  many 
districts. 

Long=Distance  or  Toll  Line.  Development  studies  address  them- 
selves to  determining  telephonic  needs  in  and  between  cities  and 
towns.  The  building  of  a  long-distance  line  between  two  points, 
known  to  have  little  communication  with  each  other  because  of  small 
population  and  little  mutual  interest,  might  be  assumed  to  be  so  sim- 
ple a  matter  as  not  to  warrant  an  investigation  deserving  the  name 
"development  study."  No  one  would  build  such  a  line,  however, 
without  giving  some  thought  as  to  whether  one  or  two  circuits  should 
be  erected  at  the  outset.  No  one  should  build  such  a  line  without 
giving  some  thought  as  to  whether  the  number  of  circuits  to  be  re- 
quired at  a  future  time  will  be  few  or  many.  For  if  the  general  growth 
of  the  region  should  indicate  that  forty  or  fifty  pairs  of  wires  would  be 
needed  on  that  line  during  the  life  of  the  poles,  good  judgment  would 
indicate  that  the  poles  chosen  should  be  large  enough  and  heavy 
enough  to  carry  that  many  pairs.  If  it  were  obvious  at  the  outset  that 
the  pole  line  never  would  need  to  carry  more  than  three  or  four  pairs 
within  its  life  of  twelve  to  fifteen  years,  it  would  be  unwise  to  use 
heavy  poles. 


888  TELEPHONY 

Long-distance  development  studies,  therefore,  are  seen  to  ad- 
dress themselves  largely  to  the  amount  of  traffic  which  may  be  ex- 
pected to  develop  in  the  future  and  to  undertake  to  make  such  fore- 
casts as  will  enable  a  conclusion  to  be  reached  as  to  the  type  of  con- 
struction most  economical  for  the  case.  If  a  development  study  in 
some  form  be  not  made,  or  if  incorrect  conclusions  be  drawn,  the 
line  may  be  built  too  light  and  have  to  be  supplemented  or  rebuilt 
while  its  poles  are  still  good;  if  it  be  built  too  heavy,  it  will  carry 
throughout  its  life  a  greater  investment  than  its  earning  warrants. 
A  money  loss  occurs  in  both  cases.  Development  studies  are  in- 
tended to  limit  these  money  losses  as  much  as  possible. 

Exchange.  If  one  should  say:  "Here  is  a  town  of  one  thou- 
sand inhabitants;  let  us  install  one  hundred  and  twenty-five  tele- 
phones, which  will  just  about  meet  present  needs,"  he  would  have 
made  a  development  study.  If  the  estimate  should  prove  to  be  cor- 
rect, he  would  have  made  a  development  study  with  little  labor  and  at 
low  cost.  If  the  nature  of  the  town  should  be  that  seventy-five 
telephones  were  all  it  could  support  at  the  outset,  the  development 
study  would  have  been  too  sanguine,  and  its  carrying-out  too  costly. 

The  construction  and  reconstruction  of  exchanges  in  cities  over 
about  twenty-five  thousand  inhabitants  involves  investments  large 
enough  to  warrant  the  expense  for  painstaking  care  in  examining  the 
conditions  as  they  are,  forecasting  conditions  as  they  will  be,  and 
reaching  conclusions  from  both.  Experience  in  such  study  is  increas- 
ing the  knowledge  and  skill  in  the  forecasting  required.  That  phase 
of  telephone  engineering  today  has  reached  as  high  a  development 
and  those  practicing  it  are  as  dextrous  as  are  found  in  transportation 
and  other  forms  of  public  service. 

Scope  of  Study.  An  exchange  development  study  undertakes 
first  to  determine  what  will  be  the  population  of  the  city  at  some 
selected  future  time;  the  distribution  of  that  population  over  the 
area  of  the  city;  how  many  telephones  it  will  require;  where  they  will 
be  located;  how  much  traffic  they  will  originate,  and  how  much  and 
what  kind  of  wire  plant,  equipment,  and  housing  will  be  necessary 
to  handle  that  traffic. 

The  future  date  for  which  population,  telephones,  and  traffic 
are  to  be  forecasted  usually  is  fifteen  or  twenty  years  from  the  time 
of  the  study.  The  further  ahead  the  forecasting  is  done,  the  less 


DEVELOPMENT  STUDIES  889 

accurate  it  will  be.  The  intention  is  to  have  the  time  long  enough 
to  include  the  useful  life  of  much  of  the  property. 

Estimates  of  future  population  are  made  by  determining  what 
the  population  has  been  during  the  past.  In  the  United  States  and 
Canada,  this  is  easier  in  the  East  than  in  the  West,  because  the  cities 
are  older,  have  more  recorded  data,  and  also  because  they  now  grow 
more  steadily. 

Ratio  of  Telephones  to  Population.  The  ratio  of  the  number  of 
telephones  to  the  population  may  be  determined  from  the  history  of 
the  exchange  as  it  exists,  although  few  elaborate  development  studies 
are  made  in  cities  having  no  telephones  whatever.  Notwithstanding 
the  fact  that  the  ratio  of  telephones  to  population  has  been  increas- 
ing in  most  cities,  a  forecast  may  be  made  to  estimate  what  that  ratio 
will  be  at  a  future  time.  The  product  of  this  ratio  and  the  esti- 
mated population  will  give  at  once  a  forecast  of  the  telephones  which 
may  be  expected  at  the  future  date. 

House  Count.  Setting  aside  this  general  forecast  for  later  ref- 
erence, a  "house  count"  is  undertaken.  A  house  count  is,  as  its 
name  implies,  a  count  of  the  buildings  in  a  city  and  is  accompanied 
by  an  estimate  of  the  telephone-using  ability  of  the  occupants.  As 
the  city  is  canvassed,  the  estimators  determine,  block  by  block,  not 
only  the  character  of  the  present  population,  but  the  probable  tendency 
of  the  future  population,  its  amount,  and  its  distribution.  As  the  study 
is  made,  it  is  customary  to  enter  on  a  map,  figures  denoting  the  present 
possibilities,  those  in  five  years,  and  those  in  fifteen  or  twenty  years. 

Ratio  of  Telephones  to  Buildings.  In  residence  areas  of  the 
best  class,  it  is  estimated  that  each  residence  will  have  one  telephone 
at  the  ultimate  time.  In  residence  areas  of  other  classes,  some  frac- 
tion of  a  telephone  per  residence  is  estimated.  In  business  areas 
the  sizes  of  office  buildings  are  recorded  and  an  estimate  of  future 
telephones  is  made  from  the  probable  number  of  ultimate  offices. 
The  ability  of  offices,  hotels,  and  apartment  houses  to  utilize  private 
exchanges  is  estimated  upon. 

Comparison  of  Estimates.  The  result  of  a  house  count  is  an 
estimate  of  the  ultimate  telephones,  and  this  now  may  be  com- 
pared with  the  first  estimate,  which  was  the  product  of  the  forecast 
of  the  population  and  the  ratio  of  telephones.  It  is  likely  that  the 
two  estimates  will  not  agree.  If  they  are  widely  at  variance,  one  or 


890  TELEPHONY 

the  other  may  be  re-studied  until  they  harmonize.  When  the  two  es- 
timates have  been  harmonized,  the  house-count  record  is  taken  as  the 
distribution  of  telephones  for  which  the  future  plant  is  to  be  designed. 
The  first  step  is  to  determine  the  proper  number  of  office  districts 
and  central  offices  within  them. 

Single-Office  District.  In  manual  practice,  one  central  office 
usually  is  chosen  for  reasonably  compact  cities  requiring  under  ten 
thousand  lines  at  the  ultimate  time.  By  "ultimate  time"  is  meant 
fifteen  or  twenty  years  hence  or  other  term,  depending  on  the  chosen 
period  of  study.  Ten  thousand  lines  can  be  handled  in  a  single 
multiple  board  without  undue  smallness  of  jacks  and  plugs. 

If  the  city,  or  a  part  of  it,  has  very  great  congestion,  two  or  more 
ten-thousand-line  manual  switchboards  may  be  placed  in  a  single 
office.  These  conditions  are  unusual  and  are  less  likely  to  be  found 
in  America  than  in  other  countries. 

Single  vs.  Multi-Office  Districts.  The  greater  the  number  of 
central  offices  in  a  city,  the  smaller  the  total  length  of  subscribers' 
lines.  For  example,  a  city  having  ten  thousand  lines  centering  in  a 
single  office  might  have  such  an  area  as  to  make  the  average  length  of 
line  two  miles;  this  will  require  a  total  of  twenty  thousand  miles  of 
subscribers'  lines,  if  these  all  are  carried  to  a  central  office.  If  the 
city  were  divided  into  fifty  districts  of  two  hundred  lines  each,  the 
average  length  of  line  might  be  one-tenth  mile,  requiring  only  one 
thousand  miles  of  subscribers'  lines.  So  far  as  subscribers'  lines  are 
concerned,  therefore,  the  subdivision  of  the  city  into  fifty  districts 
instead  of  one  has  saved  ninety-five  per  cent  of  wire. 

But  it  is  necessary  to  interconnect  the  fifty  offices  by  means  of 
trunk  lines,  and  two  kinds  of  circumstances  determine  whether 
these  trunk  lines  will  require  much  or  little  of  the  wire  which  has  been 
saved  from  subscribers'  lines  by  making  so  many  districts — (1)  the 
amount  of  traffic  originated  by  the  subscribers  and  requiring  to  be 
carried  by  the  trunks;  (2)  the  topographic  form  of  the  area  of  the 
city.  In  almost  all  practical  cases,  the  more  office  districts  there 
are,  the  fewer  total  miles  of  wire  will  be  required  for  both  subscribers' 
lines  and  trunks,  because  trunks  can  carry  so  many  more  messages 
than  subscribers'  lines  do  carry. 

Multi-Office  Districts.  With  any  arrangement  of  offices,  where 
there  are  more  than  one,  the  number  of  trunk  lines  varies  directly  as 


DEVELOPMENT  STUDIES  891 

the  number  of  calls  originated  by  the  subscribers.  While  the  varia- 
tion is  direct,  the  ratio  of  originating  calls  to  trunk  lines  is  not,  of 
course,  a  fixed  one.  As  was  shown  in  the  chapter  on  Telephone  Traffic, 
the  call-carrying  ability  of  trunks  varies  with  the  number  of  trunks 
in  a  group ;  the  number  of  calls  trunked  out  of  an  office  depends  partly 
on  its  size  relative  to  the  total  number  of  lines  in  the  city  and  partly 
on  considerations  local  to  the  district  of  that  office.  The  greater  the 
number  of  offices  in  the  city,  therefore,  the  larger  the  proportion  of 
calls  trunked  out  of  each  and  the  greater  the  number  of  trunks  which 
must  be  provided.  For  both  reasons,  the  greater  the  saving  in  sub- 
scribers' lines  by  increasing  the  number  of  districts,  the  greater  the 
cost  of  trunk  lines. 

The  cost  of  maintaining,  operating,  and  supervising  manual 
equipments  is  greater  with  many  offices  than  with  few,  the  number  of 
lines  served  being  the  same.  The  increase  of  cost,  of  maintenance, 
and  of  operation  is  less  rapid  with  automatic  equipment  when  the 
number  of  offices  is  increased. 

Number  of  Office  Districts.  The  operation  of  determining  the 
proper  number  of  central-office  districts  for  a  city  is  experimental. 
It  is  usually  done  by  plotting  the  distribution  of  ultimate  telephones 
on  a  map,  arbitrarily  dividing  the  city  into  districts,  and  successively 
comparing  the  arrangements  with  each  other  as  to  investment  costs 
(first  costs)  and  costs  of  owning,  maintaining,  and  using  the  several 
arrangements  (annual  costs'). 

As  the  number  of  districts  is  increased  in  this  experimental  study, 
the  first  costs  of  subscribers'  and  trunk  lines  fall.  Very  likely  this 
will  be  found  to  be  true  of  first  costs  of  manual  equipment  also,  due 
to  the  lessening  of  the  number  of  multiple  jacks  required.  Costs  of 
power  plants  and  other  things  which  vary  with  the  number  of  offices 
will  tend  to  increase  equipment  costs.  When  several  arrangements 
with  different  numbers  of  districts  have  been  made,  one  of  them  will 
be  found  to  have  the  lowest  annual  cost  when  all  elements  of  interest 
on  investment  and  costs  of  operating  and  maintaining  are  included. 
Perhaps  this  lowest  annual  cost  will  be  for  one  office  rather  than  for 
several.  If  two  arrangements  of  different  numbers  of  districts  are 
equal  in  annual  costs,  the  one  having  the  lower  first  cost  shall  be 
chosen  because  of  that  hazard  of  changing  styles  which  is  of  such 
strong  influence  in  telephony. 


892  TELEPHONY 

Central-Office  Locations.  The  most  economical  district  arrange- 
ment having  been  determined,  a  point  is  located  in  each  district 
where  the  least  wire  will  be  required  for  the  lines.  Taking  as  an  ex- 
ample a  rectangular  district  in  which  the  ultimate  telephones  are 
evenly  distributed,  the  proper  office  location  for  the  least  wire  would 
be  at  the  crossing  of  the  diagonals  of  the  rectangle.  • 

But  districts  are  likely  not  to  be  rectangular  in  shape,  and  it  is 
also  probable  that  the  ultimate  telephones  will  not  be  uniformly  dis- 
tributed ;  hence  this  simple  method  of  determining  the  ideal  place  for 
the  central  office  is  usually  not  feasible.  For  irregular  shapes  and 
developments  of  districts  the  ideal  location  for  the  central  office  is  de- 
termined by  dividing  the  district  by  two  straight  lines  at  right  angles 
with  each  other  in  such  manner  that  each  line  will  bisect  the  total 
number  of  stations  to  be  reached.  The  intersection  of  the  two  lines 
so  placed  is  the  ideal  location  in  that  it  results  in  the  minimum  amount 
of  wire.  In  such  a  determination  trunk  lines  leading  to  other  offices 
should  be  given  the  same  weight  as  so  many  subscribers'  stations. 

It  is  not  always  possible  to  locate  the  central  office  at  the  ideal 
district  center-  for  greatest  wire  economy — for  the  excessive  cost  of 
property  at  the  center  might  more  than  overbalance  the  added  wire 
cost  involved  in  having  the  central  office  elsewhere.  The  principal 
purpose  of  development  studies  is  to  teach  one  how  to  decide  just 
such  questions.  It  is  obvious  that  when  the  study  has  shown  where 
the  central  office  should  be,  the  next  step  is  to  learn  where  it  can  be, 
and  to  lay  out  the  conduit  plan  and  other  details  of  the  distributing 
system  in  harmony  with  the  possible  central-office  position. 

Conduit  and  Pole-Line  Routes.  Central-office  locations  being 
determined  for  each  district,  conduit  routes  are  laid  out.  Speaking 
generally,  it  is  good  practice  to  provide  a  main  or  "backbone"  con- 
duit passing  the  determined  central-office  point,  this  main  run  having 
cross-routes  extending  from  it  at  right  angles.  A  good  plan  is  to  run 
these  cross-routes  on  each  alternate  street,  causing  them  to  follow 
alleys  if  alleys  exist.  The  cross-section  of  each  conduit  run  is  then 
determined  from  the  house-count  map,  providing  ducts  enough  in 
each  run  to  accommodate  the  ultimate  subscribers'  lines  to  the  cen- 
tral office,  allowing  also  for  trunk  lines  and  such  extra  ducts  as  may 
be  required  for  municipal  purposes.  In  practice  it  is  found  that 
conduits  of  many  ducts  will  accommodate  more  wires  per  duct  in 


DEVELOPMENT  STUDIES  893 

the  average  than  conduits  of  fewer  ducts ;  and  in  forecasting  the 
number  of  ducts  a  conduit  requires,  more  allowance  has  to  be  made 
for  possible  error  in  estimating  the  smaller  runs. 

Pole  lines  then  are  laid  out  in  the  area  which  surrounds  the  un- 
derground district.  Distribution  from  conduits  within  the  under- 
ground district  is  laid  out  to  fit  the  local  requirements,  whether  they 
call  for  direct  entrances  into  buildings,  distribution  from  terminals 
on  back  walls  and  fences,  or  distribution  from  poles  in  alleys  or  on 
private  property  within  the  blocks. 

Ultimate  Sizes.  Central-office  buildings  and  conduits  when 
originally  built  usually  should  be  of  their  ultimate  sizes.  It  is  gen- 
erally not  economical  to  add  stories  to  a  central-office  building,  al- 
though in  some  very  large  cities,  where  enormous  development  is 
expected  and  where  real-estate  values  are  high,  the  buildings  are 
sometimes  planned  with  foundations  and  walls  of  sufficient  strength 
to  sustain  additional  stories  in  the  future.  In  cities  of  ordinary  size, 
it  may  be  economical  to  provide  for  extending  the  ground  area  of  the 
central-office  building,  but  it  is  not  economical  to  reopen  a  conduit 
line  in  order  to  increase  its  numbers  of  ducts.  When  a  conduit  line  is 
filled,  it  is  often  best  to  lay  its  relief  ducts  in  the  next  parallel  street. 

A  cable  plan,  made  consequent  to  the  completion  of  the  conduit 
and  pole-line  plans,  undertakes  to  show  the  general  arrangement 
of  ultimate  cables  and  the  exact  arrangement  of  immediate  cables. 
It  is  of  advantage  to  utilize  a  definite  system  of  multiple-cable  distri- 
bution. One  of  the  basic  principles  of  such  a  system  is  that  it  under- 
takes to  cause  the  number  of  wires  leaving  a  central  office  to  grow 
steadily  as  the  wire  requirements  grow,  while  the  terminal  facilities 
of  those  wires  are  more  generous  at  the  outset  and  grow  less  rapidly. 

As  a  practical  example  of  what  is  meant,  four  thousand  wires 
might  leave  a  central  office  and  by  branching  appear  in  twelve 
thousand  terminal  building  posts  or  soldering  clips.  These  twelve 
thousand  available  points  could  receive  drop  wires  or  back-wall 
wires  or  be  connected  by  jumpers  to  cables  in  buildings.  As  the 
requirements  of  the  district  increase,  additional  wires  will  be  run 
from  the  central  offices,  but  most  of  them  will  be  joined  to  the 
existing  twelve  thousand  terminal  points.  Under  ideal  conditions 
no  increase  in  the  terminal  points  might  be  required  during  all  the 
time  when  the  four  thousand  wires  were  increasing  to  twelve  thou- 


894  TELEPHONY 

sand  wires.  At  the  latter  time,  in  such  a  case,  there  would  be  no 
longer  any  multiple  distribution,  each  wire  from  the  central  office  going 
to  one,  and  only  one,  terminal  binding  post. 

Subdivision  of  Exchange  Districts.  In  automatic  practice  it 
is  becoming  possible  to  furnish  an  acceptable  telephone  service  with 
two  kinds  of  districting  in  the  same  exchange.  Assume  a  city  to  be 
divided  into  certain  major  districts,  each  containing  a  central  office 
and  a  full  complement  of  selector  switches.  Assume  each  of  these 
major  districts  to  be  divided  into  a  further  number  of  minor  districts, 
each  containing  merely  individual  line  switches  and  connector  switches. 
Each  subscriber's  line,  with  this  arrangement,  will  have  a  district- 
office  line  switch.  This  line  switch  will  have  access  to  a  plurality  of 
trunks  leading  to  the  major  office,  terminating  therein  in  a  first  se- 
lector. All  second  or  third  selectors,  as  the  case  may  be,  in  major 
offices  will  have  trunks  leading  to  connectors  in  the  minor  offices. 

By  this  arrangement  practically  all  power-plant  devices  are 
located  in  the  major  offices.  The  smaller  the  minor  districts,  the 
shorter  the  average  length  of  subscribers'  lines  will  be,  and  the  more 
minor  districts  the  greater  the  trunk  mileage  leading  from  minor  to 
major  centers.  Unless  the  character  of  the  apparatus  makes  it 
require  too  much  human  attention,  the  saving  in  wire  plant  by  this 
method  will  more  than  overcome  the  expenses  which  are  added  by 
the  greater  subdivision. 

Broadly  speaking,  there  is  no  reason  why  this  major-  and  minor- 
district  method  is  not  applicable  to  manual  practice.  A  full  utiliza- 
tipn  of  the  economics  of  minor  districting,  however,  requires  that 
some  automatic  apparatus  supplement  the  manual  apparatus.  The 
minor  district  centers,  in  such  an  arrangement,  will  have  some  coun- 
terpart of  line  switches  and  connectors;  the  major  centers  may  con- 
tain manual  switchboards  which  will  make  connections  between 
lines  as  at  present,  but  all  the  lines  will  be  trunks. 

Undoubtedly  one  of  the  most  promising  fields  of  development 
now  offered  to  the  telephone  engineer  is  in  the  greater  use  of  this  sub- 
district  plan  of  working  and  in  the  devising  of  systems  of  switching  and 
operating  adapted  fully  to  harmonize  therewith. 


CHAPTER  LII 
CARE  OF  PLANT 

Maintenance  and  Depreciation.  Whatever  be  the  cost  or  the 
value  of  a  telephone  property  at  the  outset,  its  value  at  a  later  time 
depends  on  the  care  which  is  given  to  it  as  well  as  on  the  extent  and 
kind  of  additions  which  are  made  to  it.  Circumstances  may  deter- 
mine that  the  additions  may  be  slight.  Age  affects  inanimate  tele- 
phone property  as  surely  as  it  affects  life.  Economy  and  effective- 
ness in  the  care  of  a  plant  constitute  the  largest  technical  task  in  the 
management  of  a  growing  telephone  system. 

No  element  or  general  division  of  a  telephone  plant  ever  is  as 
good  again  as  it  was  on  the  day  it  was  put  into  service.  Each  thing 
has  some  useful  life;  it  may  wear  out,  break,  or  be  destroyed  by  acci- 
dent, or  it  may  lose  its  relative  usefulness  by  the  invention  of  some- 
thing better.  The  economic  loss  due  to  the  changing  of  styles  in  the 
art  of  telephony  has  been  enormous.  Viewing  the  art  broadly,  and 
particularly  the  ultimate  good  of  the  telephone-using  public,  most 
of  the  sacrifices  in  value  by  the  adoption  of  newer  and  better  things 
and  methods  have  been  warranted. 

No  amount  of  expenditure  in  maintenance  charges  can  ever 
keep  things  new.  One  plan  of  maintenance  may  be  so  comprehen- 
sive and  so  exhaustive  as  nearly  to  do  that,  yet  it  may  cost  so 
much  as  to  be  most  unwise  to  use.  Another  plan  or  way  of  executing 
a  plan  may  spend  too  little  and  bring  sections  of  the  property  too 
soon  to  their  time  of  required  replacement.  The  problems  of  de- 
ciding upon  and  carrying  into  effect  wise  methods  of  plant-care  are 
as  grave  as  those  of  good  design,  construction,  and  management. 

Maintenance  expense,  properly  viewed,  is  a  cost  for  keeping 
property  in  as  nearly  as  may  be  its  original  condition  without  replace- 
ment or  reconstruction.  The  term  "current  repair"  expresses  the 
same  thought.  When  a  property  has  been  maintained — kept  in  a 
proper  state  of  repair  for  a  certain  length  of  time — its  condition 


896  TELEPHONY 

becomes  such  that  economy  requires  making  it  over  or  putting  some- 
thing else  in  its  place.  These  acts  are  respectively  reconstruction 
and  replacement.  Neither  is  at  all  the  same  as  maintenance  or 
current  repair. 

The  reason  a  property  is  replaced  or  reconstructed  is  because 
it  has  lessened  in  usefulness  to  a  critical  degree  and  a  more  useful  or 
usable  property  needs  to  be  put  in  its  place  or  needs  to  be  made  out 
of  it.  "Lessening  in  value"  is  only  another  way  of  saying  "deprecia- 
tion." Speaking  broadly,  all  telephone  properties  except  real  estate 
depreciate  in  value. as  time  passes.  The  price  of  copper  may  advance, 
yet  copper  wire  in  use  in  lines  usually  depreciates  in  value  as  time 
passes.  Insulated,  outdoor  copper  wire  depreciates  in  value  steadily 
from  the  time  of  its  erection,  principally  because  rubber  insulating 
materials  and  cotton  braids  are  destroyed  by  the  actions  of  air,  sun- 
light, and  moisture.  Wood  poles  may  last  fifteen  or  twenty  years 
before  requiring  to  be  replaced  by  others.  When  so  replaced,  those 
which  have  rotted  at  their  lower  ends  may  be  cut  off  and  used  again. 
Such  expense  as  was  required  to  keep  the  poles  in  serviceable  con- 
dition before  replacement  is  maintenance  cost.  Such  expense  as  was 
required  to  replace  them,  less  the  value  of  the  poles  replaced,  is  re- 
placement cost.  The  shrinking  in  values  of  properties  generally, 
due  to  the  passage  of  time,  is  a  depreciation  expense.  The  cost  of 
keeping  them  in  usable  condition  while  they  shrink  in  value  by  the 
passage  of  time  is  a  maintenance  expense. 

The  hazards  which  passing  time  brings  to  bear  upon  telephone 
properties  are  principally  those  of  decay,  wear,  and  adjustment. 
Good  design  and  construction  limit  these  actions.  Good  maintenance 
practice  repairs  damage  caused  by  decay,  replaces  parts  made  in- 
efficient by  wear,  and  restores  adjustment. 

The  quality  of  telephone  service  is  intimately  connected  with 
maintenance  methods  and  maintenance  costs.  When  an  element 
needs  repair  it  should  be  repaired  so  as  to  stay  so  and  with  as  short 
a  service-interruption  as  possible.  The  fundamental  rules  of 
maintenance  are:  Be  thorough  and  Be  prompt. 

Thoroughness  of  understanding  begets  thoroughness  of  execu- 
tion; knowledge  of  the  functions  (and  reasons  for  functions)  of 
apparatus  and  circuits  begets  a  love  of  workmanlike  handicraft. 
Workmanlike  repair  endures;  slovenly  repair  has  to  be  done  over. 


CARE  OF  PLANT  897 

The  largest  reason  for  telephone  service  is  time-saving.  Mul- 
tiple and  automatic  switchboards  are  the  outcome  of  effort  toward 
promptness  in  getting  lines  connected  and  disconnected.  Prompt- 
ness in  restoring  service  is  of  as  great  importance  as  promptness  in 
giving  it. 

The  following  are  some  general  kinds  of  troubles  and  their  reme- 
dies. 

Outside  Plant.  Supports.  Pole  lines  suffer  principally  by 
decay  of  pole  butts;  decay  of  cross-arms;  gnawing  of  poles  by  horses; 
abrasion  of  poles  by  wheel-hubs;  corrosion  of  anchors  by  electrolysis 
and  by  rust;  and,  in  general,  supports  suffer  by  accidents.  The 
preventive  measures  are  creosoting;  strip  guards  against  gnawing, 
plate  guards  against  wheels;  strain  insulators  against  electrolysis  and 
good  galvanizing  against  rust.  Corrective  measures  are  obvious. 

•  Conduits.  Underground  structures  suffer  principally  from  the 
work  of  others  than  the  owners.  The  most  effective  maintenance 
measure  is  watchfulness,  seconded  by  prompt  support  of  conduit  lines 
undermined  by  excavation.  Keep  idle  ducts  plugged;  if  otherwise, 
sand,  soil,  and  small  stones  will  wash  in.  Watch  manhole  covers. 
Replace  broken  ones  promptly.  Clean  out  scrap  left  after  cable 
splicing.  It  has  value,  and  is  a  hazard  if  left  in. 

Open  Wire.  Bare  wires  on  insulators  surfer  principally  from 
two  causes:  slack  and  foliage.  Slack  wires  cross  with  each  other  by 
wind.  The  tension  on  all  wires  of  a  span  should  be  uniform.  This 
lessens  likelihood  of  crossing.  The  allowable  tension  is  controlled 
by  the  temperatures  of  the  region.  Wires  in  foliage  have  low  insu- 
lation, making  their  lines  noisy  and  otherwise  less  efficient.  Trim 
the  trees,  insulate  those  spans,  or  put  the  wires  in  cables.  Open 
wires  suffer  from  grounding  by  contact  with  suspension  strand  and 
guys.  Clear  such  troubles  by  permanent  measures.  Twin  drop 
wires  (twisted  pairs)  under  heavy  strain,  where  wires  overlie  at 
supports,  cut  through  rubber  insulation  and  short-circuit  the  pair. 
Clear  the  cause  by  placing  the  wires  side  by  side  at  such  points  of 
support. 

Cables.  Aerial  cables  surfer  from  punctures,  bullets,  beetle- 
borings,  burns,  and  cracking.  Such  openings  in  the  sheaths  let  in 
water  from  the  air  or  from  rain.  Water  destroys  the  insulating  quality 
of  the  paper  wrapping  of  the  wires.  Such  a  fault  is  called  a  "wet 


898  TELEPHONY 

cross."  It  is  repaired  by  stripping  off  the  sheath  as  far  "as  the  core  is 
wet,  pouring  on  melted  paraffin  till  it  flows  off  without  bubbling, 
and  putting  on  a  lead  sleeve,  split  lengthwise  to  allow  it  to  be  placed 
over  the  uncut  core.  The  split  is  soldered  up  and  the  ends  wiped 
to  the  cable  sheath. 

Preventive  measures  are  few.  Arrange  aerial  cables  so  they 
do  not  bend  and  unbend  by  change  of  temperature,  or  by  swaying. 
Do  not  place  them  where  they  will  be  subject  to  vibration.  Be  sure 
cables  are  firmly  supported  where  they  enter  terminals.  Keep  them 
clear  of  contact  with  power  circuits.  Limit  the  flow  of  earth  cur- 
rents over  them. 

Underground  cables  are  subject  to  the  same  "wet  cross"  trou- 
bles when  sheaths  are  damaged.  They  are  less  exposed  to  punc- 
tures and  flexures.  They  are  more  exposed  to  electrolytic  corrosion. 
Keep  them  so  connected  to  traction  systems  of  distribution  that  little 
such  current  leaves  them  in  the  presence  of  moisture.  Make  and 
remake  observations  on  these  conditions.  Change  the  corrective 
means  as  the  conditions  change.  Watch  the  sheaths  in  danger  areas. 
Support  the  cables  in  workmanlike  fashion  in  manholes.  Let  each 
splice  be  freely  accessible.  Keep  inflammable  things  out  of  cable 
shafts,  vaults,  and  runways. 

Subscribers'  Equipment.  In  wall  sets,  receiver  cords  are  the 
perishable  element.  Test  them  at  every  visit.  In  common-battery 
sets  having  magneto  receivers  in  a  wholly  local  induction-coil  circuit, 
an  open  receiver  cord  means  the  subscriber  can  be  heard  but  can  not 
hear.  In  most  other  circuits,  it  means  impairment  or  prevention  of 
both  hearing  and  talking.  In  portable  sets,  broken  transmitter 
mouthpieces  and  receiver  shells  are  chief  troubles.  Replace  them 
promptly,  look  for  unusual  causes  of  falling  of  the  set,  and  advise  a 
suitable  support.  Inspect  other  things  at  every  such  visit.  Recom- 
mend change  of  entire  set  when  its  appearance  is  such  that  you  feel 
that  you  should  have  a  new  one  if  it  were  in  your  premises.  But 
perhaps  you  could  clean  it  somewhat  and  make  a  change  unnecessary. 
The  most  practical  sanitary  mouthpiece  is  an  ordinary  one,  fre- 
quently washed  with  soap  and  water.  A  patron  often  copies  so  good 
a  habit.  Clean  carbon  arresters  on  suspicion,  understand  thor- 
oughly the  proper  functions  and  relations  of  the  telephone  set,  and  the 
rules  of  repair  always  will  be  obvious. 


CARE  OF  PLANT 


899 


Manual  Office  Equipment.  Cords  are  a  source  of  trouble  in  cen- 
tral offices,  as  in  subscribers'  sets.  Test  them  regularly.  Tinsel 
cords  become  "scratchy"  by  breaking  gradually;  wire  and  metal- 
ribbon  cords  break  abruptly.  Breaks  in  a- switchboard  cord  usually 
occur  near  the  plug.  Cut  off  the  defective  end  and  refit  the  plug. 


Fig.  650.     Relay  Multiple-Board  Line  Circuit 

Put  a  good  cord  in  place  of  a  defective  one,  repair  the  latter  ready 
for  use,  and  keep  up  the  process.  The  key  shelf  of  a  switchboard 
is  the  operator's  workbench.  It  is  not  a  repair  shop.  Use  good 
tools  and  keep  them  in  order. 

Manual  and  automatic  central-office  equipments  are  growing 
more  alike  in  their  fundamental  circuits.  Understand  them.  All 
troubles  are  consequences  of  causes.  Know  the  causes. 

Because  of  its  wide  use,  and  because  it  is  typical  of  all  manual 
switchboards,  the  American  Telephone  and  Telegraph  Company's 


Fig.  651.     Relay  Multiple-Board  Cord  Circuit 

multiple  equipment  is  shown  in  Figs.  650  and  651.  Some  of  the 
principal  troubles  and  remedies  follow.  A  study  of  the  circuits  in 
figure  and  in  fact  will  be  of  more  value  to  the  student  than  reading 
many  chapters  in  a  book.  In  the  cases  of  trouble,  the  causes  given 
are  several  in  each  instance.  Any  one  of  them  may  be  the  cause. 
The  tests  are  to  show  which  -is  the  cause. 


900  TELEPHONY 

Line  lamp  does  not  glow  when  subscriber  calls.  Lamp  burnt  out.  Lamp 
terminals  do  not  make  contact  with  lamp  socket  springs.  Line-relay  lamp 
contact  does  not  close.  Line  relay  defective.  Line  is  open.  Test  the  line 
at  the  main  distributing  frame,  determining  whether  the  trouble  is  in  or  out. 
Test  line  relay.  Examine  lamp  and  socket. 

Line  lamp  does  not  go  out  when  operator  answers.  Cut-off  relay  defective; 
contacts  of  cut-off  relay  do  not  open.  Test-strand  of  answering  cord  is  open, 
failing  to  operate  cut-off  relay.  Try  several  cords.  Examine  the  common 
path  for  current  to  test-strands  of  that  switchboard  position.  Test  cut-off 
relay. 

Line  lamp  glows  steadily,  though  the  subscriber  is  not  calling.  Line  is 
short-circuited.  Negative  side  of  line  is  grounded.  Line  relay  is  out  of  ad- 
justment, closing  lamp  circuit  though  relay  is  not  energized.  Test  the  line 
in  and  out  at  main  distributing  frame. 

Line  lamp  glows  when  subscriber  calls  but  operator  can  not  hear  subscriber. 
Defective  subscriber's  set;  receiver  cord  broken;  defective  receiver.  Answer- 
ing jack  defective;  wiring  to  answering  jack  open.  Try  answering  in  mul- 
tiple. If  successful,  answering  jack  trouble  is  proved.  If  unsuccessful  in 
multiple,  test  from  main  distributing  frame. 

Line  lamp  glows  when  subscriber  is  being  called.  Line  relay  is  not  cut 
off  because  test-strand  of  calling  cord  is  open;  or  supervisory  lamp  of  calling 
cord  is  open;  or  cut-off  relay  is  defective. 

Supervisory  lamp  of  calling  cord  does  not  glow  when  calling  plug  is  inserted. 
Line  in  use,  busy  test  absent  or  ignored.  Line  is  short-circuited  or  negative 
wire  grounded.  Supervisory  lamp  of  calling  cord  defective.  Supervisory 
relay  of  calling  cord  has  contact  closed  though  relay  is  not  energized.  Try 
other  cords.  Test  line. 

Line  tests  busy  when  it  is  not  busy.  Improper  local  or  foreign  potential 
on  test  wire,  giving  differences  of  potential  between  test  rings  and  plug  tips 
Test  wire  is  crossed. 

Line  does  not  test  busy  when  it  is  busy.  Test  wire  grounded.  Test-strand 
of  cord  open  at  position  where  line  is  in  use.  Supervisory  lamp  open  at  that 
position. 

Line  open  in  multiple.  Test  the  line  with  regular  cords;  begin  at  section 
nearest  main  distributing  frame;  normal  operation  ceases  when  the  open  point 
is  passed.  To  test  for  open  without  using  the  line,  bridge  a  receiver  in  series 
with  battery  between  successive  points  of  wire  in  trouble.  Clicks  will  not  be 
heard  when  open  point  is  included  in  series  with  receiver  and  battery. 

Line  or  test  wires  crossed  in  multiple.  Place  tone  or  clicks  on  the  crossed 
wires  so  the  cross  will  shunt  the  interrupted  current.  Bridge  a  receiver  across 
the  crossed  wires  at  different  points.  Sound  will  be  heard  least  plainly  nearest 
the  cross.  Watch  for  solder,  wire  clippings,  and  bent  terminals  in  jacks. 

Operator  can  not  ring.  Trouble  in  ringing  current  supply  or  in  ringing 
branch  to  operator's  position,  because  it  is  open,  short-circuited,  or  grounded. 

Operator  can  not  hear  or  speak.  Defective  receiver  cord,  transmitter 
cord,  plug,  or  jack  of  operator's  set.  Listening  key  troubles.  If  confined  to 
one  cord  circuit,  the  latter.  If  common  to  all,  the  former. 

Permanent  signals.  This  is  a  colloquial  term  given  to  the  condition  where 
the  line  lamp  glows  even  though  the  subscriber  is  not  calling.  As  the  sub- 


CARE  OF  PLANT  901 

scriber's  call  is  shown  by  the  lighting  of  the  lamp,  he  can  not  call  if  current 
enough  is  passing  through  his  line  relay  to  hold  it  closed.  Permanent  signals 
may  show  when  the  line  is  otherwise  operative,  that  is,  a  subscriber  may  be 
called  and  may  talk,  though  his  line  may  be  short-circuited  through  a  resistance 
low  enough  to  operate  the  line  relay. 

In  such  cases,  partial  service  may  be  given  by  transferring  the  line  to  a 
hospital  board  or  other  arrangement  having  a  relay  with  a  lamp  on  its  back 
contact,  which  lamp  will  light  when  the  trouble  is  cleared,  thus  advising  that 
the  line  may  be  restored  to  service  on  the  multiple  board.  Calls  for  lines  so 
in  trouble  may  be  referred  to  positions  so  equipped  and  such  calls  often  can  be 
completed  when  they  would  fail  if  handled  in  the  regular  manner. 

Automatic  Systems.  Systems  of  the  Automatic  Electric  Com- 
pany vary  in  nature.  Those  requiring  successive  contacts  between 
the  line  wires  and  ground  in  order  to  operate  selectors  and  connectors 
are  called  three-wire  systems.  Those  requiring  successive  makes 
and  breaks  in  the  line  to  operate  selectors  and  connectors  are  called 
two-wire  systems.  No  ground  connections  at  subscribers'  stations 
are  required  in  the  two-wire  systems. 

In  early  types  of  automatic  systems,  each  subscriber's  line  is  con- 
nected permanently  to  an  individual  first  selector.  Second  and  third 
selectors  and  connectors  are  common  devices,  being  chosen  for  use 
when  required.  In  later  types  of  automatic  systems,  individual 
line  switches  less  elaborate  than  first  selectors  are  permanently  con- 
nected to  subscribers'  lines,  and  selectors  and  connectors  are  chosen 
when  required.  All  selectors  and  connectors  of  the  Automatic  Elec- 
tric Company's  systems,  either  two-wire  or  three-wire,  with  or  with- 
out individual  line  switches,  are  of  one  general  type  as  to  mechanical 
motions.  In  all  of  them  the  connection  between  the  calling  and 
called  portions  of  the  line  is  made  by  metal  wipers  (seeking  contact- 
pieces),  which  pass  over  sets  of  punchings  (waiting  contact-pieces), 
stopping  on  a  desired  or  an  available  set. 

The  mechanical  motions  of  selectors  and  connectors  are  two: 
vertical  and  rotary.  The  wipers  make  no  connection  with  contacts 
during  vertical  motions;  they  sweep  over  contacts  during  rotary  mo- 
tions. Most  selectors  and  connectors  change  their  functions  several 
times  during  a  set  of  operations  of  connecting  and  releasing.  These 
changes  principally  are  caused  by  motions  of  a  side  switch  or  its 
equivalent.  A  side  switch  is  a  device  adapted  to  repeal  one  set  of 
electrical  conditions  and  establish  another.  Each  position  of  a  side 
switch  represents  the  conditions  of  one  particular  set  of  functions. 


902  TELEPHONY 

Wipers  carried  by  the  shaft  of  a  selector  or  connector  are  either  four 
or  six  in  number.     Always  they  represent  three  conductors. 

The  analogy  is  close  between  line  circuits  in  automatic  systems 
and  in  multiple  manual  systems.  Each  has  two  line  wires  and  a 
test  wire.  The  latter  in  automatic  systems  is  called  the  private  wire 
and  like  the  test  wire  in  manual  systems,  it  is  local  to  the  office.  Its 
principal  duty  is  to  guard  a  line  in  use  against  intrusion  by  another 
line  calling  it.  In  some  automatic  systems  it  is  called  a  guard  wire. 
The  bank  of  private  (guard,  test)  wires  usually  is  mounted  above  the 
bank  or  banks  of  line  wires.  Line  wires  may  be  in  one  bank  of  100 
pairs  or  two  banks  of  50  pairs  each. 

Opens,  crosses,  and  grounds  in  automatic  multiples  may  be 
tested  for  the  same  as  in  manual  multiples.  These  problems  usually 
are  simpler  in  automatic  systems,  as  the  multiples  are  limited  to  100 
lines  (300  wires). 

Selector  and  connector  switches  require  their  shafts  to  be  kept 
clean  and  lubricated.  Wipers  are  required  to  be  kept  adjusted  so  as 
to  engage  and  disengage  contacts  freely.  Side  switches  require  ad- 
justment tight  enough  to  ensure  good  contacts  in  each  position,  yet 
weak  enough  to  move  freely;  these  two  requirements  being  opposed, 
a  compromise  is  necessary.  Better  side  switches  or  equivalents  would 
avoid  this  compromise.  Fine  graphite  on  side  switches  is  a  great 
help  and  causes  no  trouble. 

Automatic  equipments  require  that  switches  be  kept  warm,  dry, 
and  clean.    If  the  switches  be  cooler  than  the  air  around  them,  they 
will  condense  water  from  the  air  for  the  same  reason  that  dew  forms 
Moist  switches  operate  less  freely  than  dry  ones.     Freedom  from 
dust  is  essential  in  all  telephone  equipment. 

Soldering.  Soldering  is  the  most  important  handicraft  of  the 
telephone  repairman.  The  first  rule  in  using  soldering  tools  is  that 
the  tip  of  the  copper,  or  "iron"  as  it  is  called,  shall  be  kept  clean  and 
well  tinned.  By  the  latter  is  meant  that  the  surface  of  the  copper- 
shall  be  kept  evenly  coated  with  solder  thoroughly  united  with  it. 
Good  work  is  impossible  unless  this  tinning  exists,  and  it  cannot  exist 
unless  the  iron  be  kept  reasonably  smooth  and  free  from  corroded 
spots.  From  time  to  time  the  iron  must  be  dressed  up  by  filing,  and 
occasionally  during  use  should  either  be  dipped  in  sal  ammoniac 
solution  for  an  instant  or  wiped  on  a  pad  of  cotton  waste  which  is 


CARE  OF  PLANT  903 

saturated  wtih  sal  ammoniac  solution.  This  tends  to  clean  off  the 
copper  oxides  which  gather  on  the  metal  and  thus  "insulate"  it  from 
tinning.  The  copper  must  be  hot  when  dipped  in  or  wiped  on  the 
sal  ammoniac  solution,  and  there  should  be  some  solder  on  the  iron 
at  the  time;  it  will  be  found  to  spread  easily,  with  a  little  rubbing, 
all  over  the  surface  of  the  tip. 

With  a  clean,  hot  copper  it  is  easy  to  solder  together  two  pieces 
of  metal  already  coated  with  solder,  unless  they  be  so  large  that  they 
cannot  be  well  heated  by  the  copper.  If  both  parts  are  tinned  or 
coated  with  solder,  then  the  only  flux  required  to  make  the  applied 
solder  flow  is  rosin,  and  in  telephone  work  most  soldering  is  done  with 
solder  already  provided  with  powdered  rosin  in  it.  Good  results 
cannot  be  expected  in  soldering  untinned  metals  together  with  rosin 
only,  so  that  it  is  unwise  to  use  untinned  copper  wire  in  connection 
with  switchboard  apparatus.  The  reason  for  using  no  other  flux 
than  rosin  about  apparatus  is  that  none  has  been  discovered  which 
will  serve  its  purpose  as  a  soldering  flux  and  leave  the  insulating 
parts  in  any  sufficient  state  of  insulating  quality  after  the  work  is 
done.  Many  fluxes  also  tend  to  corrode  the  wires  and  apparatus 
parts  after  the  soldering  has  been  done. 

The  code  of  the  National  Board  of  Fire  Underwriters  specifies  a 
flux  composed  of  chloride  of  zinc,  alcohol,  glycerine,  and  water.  This 
is  a  good  material  for  soldering  untinned  surfaces,  as  it  is  easily  ap- 
plied and  remains  in  place,  causes  the  solder  to  flow  freely,  and  is  not 
the  most  corrosive  of  chemical  fluxes  after  the  soldering  is  done. 
But  useful  as  this  flux  may  be  in  such  work  as  mending  tinware  and 
patching  up  mechanical  devices,  it  is  absolutely  to  be  prohibited  in  the 
soldering  of  conductors  to  each  other  and  to  apparatus  in  telephone 
systems.  Equal  condemnation  must  be  given  to  all  forms  of  paste 
and  soldering  sticks,  none  of  which  is  more  suitable  in  these  particu- 
lars than  is  the  chloride  of  zinc  solution. 

If  parts  of  apparatus  can  be  soldered  together  and  then  washed 
before  assembling,  there  is  no  more  hurtfulness  in  using  acid  or 
alkaline  soldering  fluxes  than  in  the  use  of  chemical  solutions  in  the 
process  of  plating.  But  one  must  appreciate  exactness  of  division 
between  a  rosin  flux  on  the  one  hand,  and  all  other  fluxes  on  the  other, 
as  being  suitable  and  unsuitable,  respectively,  for  use  in  soldering 
conductors  in  the  assembling  and  repairing  of  telephone  apparatus. 


CHAPTER  LIII 

f 
TESTING 

Electrical  tests  are  used  to  determine  the  qualities  of  working 
lines  and  apparatus,  the  same  fundamental  principles  being  applied 
also  to  the  latter  during  the  process  of  manufacture. 

Implements.  Tests  of  lines  are  made  by  voltmeters  and  gal- 
vanometers. In  both,  one  magnetic  field  is  created  by  the  current 
and  another  by  a  permanent  magnet.  In  the  D'Arsonval  galvanom- 
eter and  in  the  commercial  direct-current  voltmeters  of  the  same 
type  a  suspended  or  pivoted  coil  moves  in  the  field  of  a  permanent 
magnet.  In  the  Thomson  type  of  galvanometer,  however,  a  sus- 
pended permanent  magnet  moves  in  the  field  of  a  coil.  In  all  three 
cases,  the  current  in  the  coil  moves  an  index  pointer  which,  in  volt- 
meters, is  a  flattened  aluminum  wire,  while  in  galvanometers,  it  is 
such  a  flattened  aluminum  wire  or  it  is  a  beam  of  light  reflected  upon 
a  scale  or  into  a  telescope.  Voltmeters  have  scales  graduated  in  volts 
and  fractions  of  volts  while  galvanometers  have  scales  arbitrarily 
graduated. 

Although  magnetic  voltmeters  do  not  measure  true  differ- 
ences of  potential,  they  do  measure  currents.  But  currents  are 
consequences  of  differences  of  potential,  so  a  magnetic  voltmeter 
measures  volts  within  close  limits  of  error  if  the  resistance  of  the 
voltmeter  coil  is  high,  relative  to  the  resistance  of  the  circuit  exter- 
nal to  it.  Electrostatic  voltmeters  measure  true  differences  of  po- 
tential, but  they  are  not  commercial  for  low  pressures. 

Faults.  Voltmeter  tests  of  telephone  lines  cover  all  necessary 
measurements  except  accurate  location  of  faults,  and  tests  for  accept- 
ance of  cables  and  line  materials.  They  are  adapted  for  the  use  of 
the  wire  chief  of  an  office,  as  they  are  more  rapid  than  galvanometer 
tests.  Wire  chief's  tests  cover: 

Insulation  resistance  (grounds) 
Continuity  (breaks;  opens) 
Crosses 


TESTING  905 

Conductor  resistance 

Discharge  capacity 

Foreign  currents  (earth  potentials) 

Talking,  ringing,  and  adjustments 

These  tests  usually  are  made  from  the  wire  chief's  desk,  which 
has  voltmeter,  relay,  sounder,  and  key  circuits,  and,  in  automatic 
equipments,  has  test  switches  for  observing  the  performance  of  im- 
pulse-sending mechanisms  (calling  devices). 

Galvanometer  tests  cover: 

Conductor  resistance 

Insulation  resistance 

Capacity 

Opens 

Grounds 

Crosses 

The  last  three  are  "faults."     Finding  them  is  "fault  location." 

Continuity.  Figs.  652  and  653  are  tests  for  continuity.  If  the 
wire  under  test  is  continuous,  the  voltmeter  will  show  almost  the 
full  voltage  of  the  test  battery.  Consider  the  pair  tested  in  Fig.  652 


Fig.  652.     Test  for  Continuity 

to  be  open  at  the  distant  end;  the  test  then  is  for  insulation  between 
the  upper  wire  and  all  grounds.  Its  insulation  is  inversely  as  the 
deflection — high  if  the  deflection  be  small,  low  if  the  deflection  be 
large.  If  the  deflection  be  half  the  full  voltage  of  the  test  battery, 
the  insulation  of  the  tested  wire  is  the  same  as  the  resistance  of  the 
voltmeter.  In  the  case  cited,  if  the  test  battery  is  of  60  volts,  the  de- 
flection through  the  wire  30  volts,  and  the  voltmeter's  resistance  10,000 
ohms,  the  insulation  resistance  of  the  wire  is  10,000  ohms. 

Insulation.  Tests  for  insulation  and  continuity  are  made  also 
by  means  of  a  relay  and  sounder.  With  a  fixed  voltage  and  adjust- 
ment of  relay,  the  sounder  of  Fig.  654  will  respond  for  insulations 
below  a  certain  amount. 


906 


TELEPHONY 


Foreign  Potentials.     Foreign  potentials  are  read  on  the  volt- 
meter by  the  circuit  of  Fig.  655.     Be  sure  the  potential  to  be  read  is 


Fig.  653.     Test  for  Continuity 


not  greatly  in   excess  of  the  voltmeter's  scale.     A  lamp  of  higher 
voltage  is  a  convenient  device  for  a  first  test,  for  if  it  is  damaged,  the 


L/rtE 


Fig.  654.     Sounder  Test  for  Insulation 


loss  is  small.     The  key  of  Fig.  655  enables  the  voltmeter's  connec- 
tion to  the  line  to  be  reversed  if  the  foreign  potential  requires  it. 


L//YE 


Fig.  655.     Test  for  Foreign  Potentials 


Capacity.  Capacities  of  lines  usually  are  measured  by  galvanom- 
eter methods.  Often  it  is  useful,  however,  to  check  the  condition 
of  a  line  by  taking  a  discharge  reading  on  a  voltmeter.  The  con- 
denser in  the  subscriber's  telephone,  if  the  line  is  in  good  condition, 


TESTING  907 

will  give  a  large  deflection  on  the  voltmeter.  If  the  line  is  not  in  good 
condition,  the  deflection  will  be  less.  The  key  of  Fig.  656,  when 
depressed,  allows  the  battery  to  charge  the  line  and  the  condenser. 


<xP 


Discharge  Test  for  Capacity 

When  the  key  is  released,  the  condenser  and  the  line  discharge 
through  the  voltmeter,  the  degree  of  deflection  indicating  the  capac- 
ity. This  method  is  used  for  measuring  the  capacities  of  conden- 
sers as  they  are  made. 

Opens.  Open  wires  in  cables  may  be  measured  in  the  same 
manner  as  are  capacities.  A  good  wire  being  available,  the  discharges 
from  the  open  and  the  good  wire  may  be  compared,  their  lengths 
being  as  their  discharges.  Fig.  657  shows  a  simple  method  using 


L/HE 


Fig.  657.     Telephone  Test  for  Capacity 

a  telephone  receiver.  The  larger  the  capacity,  the  louder  the  click 
when  the  key  is  released.  Expert  workmen  can  estimate  the  dis- 
tance to  a  break  by  this  simple  test. 

Resistance.  Resistance  of  a  wire  or  loop  is  measurable  by 
voltmeter  methods.  The  process  is:  Read  test-battery  voltage  E. 
Read  voltage  with  line  in  series  D,  as  in  Fig.  658.  Let  R  be  the  line 
resistance  desired  and  V  the  voltmeter's  resistance.  Then 


908  TELEPHONY 

which  in  words  is:  Deduct  voltage  with  line  in  series  from  full 
test-battery  voltage;  multiply  this  remainder  by  the  resistance  of  the 
voltmeter;  divide  by  the  voltage  with  line  in  series.  The  result  is 
the  line  resistance.  In  Fig.  658  with  the  voltmeter  resistance  10,000, 


} 

i 


Fig.  658.     Test  for  Kesistance 

consider  test-battery  voltage  to  be  60;  deflection  through  line,  25. 
Then  the  line  resistance  is  14,000  ohms. 

Low  resistances  are  more  accurately  measured  with  low-resistance 
voltmeters.  That  is,  it  is  obvious  that  the  full  test-battery  voltage 
and  that  through,  say,  300  ohms  of  line  wire,  would  give  similar  de- 
flections on  a  10,000-ohm  voltmeter,  but  when  the  voltmeter  is 
shunted  so  as  to  give  it  an  effective  resistance  of  the  desired  amount, 
this  difficulty  disappears. 

For  example,  in  Fig.  659,  a  shunt  of  416.66  ohms  is  placed  across 
a  10,000-ohm  meter,  making  the  joint  resistance  400  ohms  Using  this 


Fig.  659.     Shunted  Voltmeter 


as  in  Fig.  660,  a  lower  resistance  may  be  measured  with  greater  accu- 
racy than  with  the  unshunted  instrument.  Assume  the  test-battery 
deflection  to  be  40  and  the  deflection  through  the  line  to  be  25;  then, 
by  the  formula  given,  the  line  resistance  is  six-tenths  of  400,  or  240. 
With  the  unshunted  voltmeter  the  deflections  would  have  been  much 


TESTING 


909 


closer  together,  viz,  40  and  39.06,  making  the  difference — .94  volt — 
less  easy  to  read  accurately  than  in  the  case  of  Fig.  660  for  the  same 
line  resistance. 


Fig.  660.     Loop  Resistance  Test 

Crosses.  Tests  for  crosses,  as  in  Fig.  661,  are  made  by  observ- 
ing one  of  the  suspected  wires  while  ground  (or  the  pole  of  the  battery 
not  joined  to  the  voltmeter)  is  applied  to  the  other  suspected  wire. 
If  the  ends  of  the  crossed  wires  are  in  offices  equipped  for  testing  in 
this  way,  as  in  the  case  of  trunks,  both  can  measure  resistance  in- 


L/ME 


Fig.  661.     Test  for  Cross 

eluding  the  cross.  The  loop  resistance  of  a  good  pair  being  known 
accurately  from  records  or  actual  test,  it  is  easy  to  know  just  where 
the  cross  lies.  For  instance,  assume  that  the  loop  resistance  of  the 
wire  is  160  ohms.  One  wire  chief  measures  the  loop  through  the 
cross  to  be  1,195  ohms;  the  other  finds  it  1,125  ohms.  Each  has  in- 
cluded all  the  wire  from  his  office  to  the  cross,  and  the  cross  itself 
also.  Each  has  measured  a  different  part  of  the  loop  but  the  same 


910  TELEPHONY 

cross.  The  difference  between  the  resistances  observed  by  the  two 
wire  chiefs  is  70  ohms.  This  is  the  difference  between  the  two  parts  of 
the  loop.  The  sum  of  the  two  parts  is  160  ohms,  for  that  is  the  known 
whole  loop  resistance.  The  sum  being  160  and  the  difference  70, 
the  two  parts  must  be,  respectively,  half  of  (160  plus  70}  and  half  of 
(160  less  70),  or  115  and  45.  These  are  the  loop  resistances  to  the 
cross  from  the  two  offices. 

Wire-Chiefs'  Desks.  Facilities  for  the  tests  illustrated  in  Figs. 
652  to  661  are  combined  in  modern  wire-chiefs'  desks.  The  volt- 
meter, sounder,  and  telephone  set,  in  such  equipments,  are  associated 
with  cords,  plugs,  lamps,  and  keys  so  that  all  the  tests  are  available 
quickly. 

Tone  Methods.  Mutual  inductive  action  enables  grounds  to  be 
located  with  great  accuracy  by  means  of  the  simple  apparatus  of  Fig. 
662.  This  method  was  first  used  by  power  companies  for  the  loca- 


Fig.  662.     Fault  Location  with  "Jigger" 

tion  of  such  faults  in  underground  cables.  It  has  been  re-invented 
in  several  forms  for  telephone  purposes,  marketed  as  a  trade  secret, 
and  published  by  at  least  three  persons — L.  R.  Hoffmann,  William 
Maver,  Jr.,  and  Donald  McNicol.  The  exploring  device  is  called 
by  many  a  "jigger  " 

The  apparatus  consists  of  two  parts:  a  tone-producer— -indi- 
cated in  the  figure  as  a  commutator  interrupting  a  battery  circuit — 
and  a  receiver  joined  to  an  exploring  coil.  The  latter  is  held  with  its 
turns  parallel  to  the  cable.  The  tone  on  the  grounded  wire  can  not 
be  heard  in  the  receiver  beyond  the  fault,  but  can  be  heard  between 
the  fault  and  the  end  where  the  tone  is  applied. 

Identification.  Identifying  the  conductors  of  a  cable  is  the  kind 
of  testing  most  often  necessary  in  construction  and  installation.  The 
methods  are  simple.  In  testing  an  aerial  or  underground  cable  for 


TESTING  911 

the  identity  of  its  conductors,  connect  the  wires  to  the  office  terminal 
first.  Identify  the  various  groups  of  pairs  and  connect  them  in  a 
recorded  order.  These  groups  are  the  fixed  parts  into  which  the 
whole  number  of  pairs  is  divided,  and  their  existence  greatly  simpli- 
fies the  work  of  testing  out.  A  telephone  receiver,  a  buzzer,  a  vi- 
brating bell,  or  a  polarized  ringer,  may  be  used  to  identify  the  wire 
upon  which  the  testing  current  is  placed;  the  latter  naturally  should 
be  that  of  a  battery  or  generator,  depending  on  which  of  the  detecting 
devices  is  chosen.  As  pair  by  pair  of  the  cable  is  identified,  it  is  placed 
in  position  on  the  terminal  to  which  it  is  to  be  fixed,  and  when  a  given 
group  or  the  whole  cable  has  been  tested  out,  the  whole  test  should  be 
repeated  rapidly  to  discover  any  error  which  may  have  crept  in. 
In  order  to  make  the  identification  of  pairs  rapid  and  easy,  switch- 
board cables  are  laid  up  of  pairs  in  which  one  wire  of  each  differs  in 
color  or  marking  from  the  rest. 

Cable  Testing.  When  an  aerial  or  underground  cable  is  placed 
in  position  and  terminated,  the  tests  for  identifying  the  conductors 
will  have  shown  whether  any  of  the  wires  are  open  and  whether  any 
of  them  are  crossed  with  each  other.  The  fact  that  they  are  open  or 
grounded  on  the  sheath  will  be  shown  by  a  failure  to  get  the  testing 
potential  from  the  distant  end.  The  fact  that  they  are  crossed 
together  will  be  shown  in  the  first  or  final  test  for  identification  by 
the  fact  that  current  is  received  over  more  than  one  wire  when  it  is 
sent  from  the  distant  end  on  only  one.  For  this  reason  the  final  test 
should  be  made,  running  over  the  whole  group  in  checking  the  iden- 
tification of  each  conductor. 

Cable  Quality.  When  this  part  of  the  work  has  been  completed, 
it  is  necessary  to  know  whether  the  conductors  are  in  good  condition 
and  whether  they  comply  with  the  guarantee  of  the  manufacturer. 
This  guarantee,  or  requirement  of  the  specifications  under  which 
the  cable  was  made,  provides  that  it  shall  have  an  insulation  resist- 
ance not  below  a  certain  number  of  megohms  per  mile  and  an 
electrostatic  capacity  not  above  a  certain  fraction  of  a  microfarad 
per  mile.  The  insulation  resistance  per  mile  is  simply  the  re- 
sistance of  the  insulating  material,  and  with  reference  to  testing  and 
the  quality  of  lines  is  often  referred  to  as  "insulation."  Because 
it  is  simpler,  insulation  resistance  is  usually  expressed  in  meg- 
ohms. The  insulation  resistance  ordinarily  required  by  specifica- 


912  TELEPHONY 

tions  and  usually  guaranteed  by  manufacturers,  is  500  megohms  per 
mile  or  more,  and  it  is  not  difficult  to  make  and  install  cables 
which  will  have  much  more  than  this  degree  of  insulation  resistance. 

The  method  of  finding  out  the  amount  of  insulation  resistance 
of  a  cable  is,  in  principle,  simpler  than  that  of  finding  the  resistance 
of  a  wire  and  simply  involves  the  allowing  of  the  current  to  flow 
through  a  galvanometer  into  the  wire  under  test,  from  which  it  will 
then  flow  through  the  insulation  resistance  to  the  earth  or  to  the 
cable  sheath.  In  all  the  accompanying  drawings  of  testing  methods 
a  ground  is  shown.  It  is  to  be  understood  that  the  sheath  of 
the  cable  may  be  considered  to  be  that  ground  in  case  it  is  the  test- 
ing of  cable  conductors  which  is  considered  with  reference  to  the 
drawing. 

Thomson  Galvanometer.  Because  the  insulation  resistance  of 
cable  conductors  in  reasonably  short  lengths  is  very  high,  and  be- 
cause the  potentials  to  be  used  in  testing  must  be  kept  within  reason- 
able limits  for  conditions  of  safety,  the  galvanometer  for  insulation- 
resistance  tests  must  be  of  high  sensibility.  The  first  successful  form 
of  high  sensibility  galvanometer  was  that  designed  by  Lord  Kelvin 
and  was  used  for  receiving  signals  over  the  early  Atlantic  cables. 
He  changed  the  form  of  earlier  galvanometers  having  a  compass 
needle  and  a  coil,  by  making  the  needle  very  light,  suspending  it  by 
a  single  fiber  of  unspun  silk,  attaching  a  very  light  silvered  glass  mir- 
ror to  the  needle,  and  surrounding  it  with  a  large  coil  of  very  fine 
wire.  This  arrangement  makes  it  possible  for  very  feeble  currents 
in  the  wire  to  move  the  needle  and  the  mirror  in  some  degree.  In 
addition  to  these  advantages  the  mirror  reflected  a  beam  of  light  upon 
a  scale,  thus  multiplying  the  mirror's  movement.  As  a  beam  of  light 
so  used  as  a  pointer  is  always  straight,  as  it  can  be  of  considerable 
length,  as  it  has  no  weight,  and  as  the  angle  of  the  mirror's  move- 
ment is  always  doubled,  the  Thomson  reflecting  galvanometer  was 
a  remarkable  step  forward.  It  enabled  things  to  be  done  which 
could  not  be  done  before — and  this  is  the  measure  of  the  value  of 
invention. 

D'Arsonval  Galvanometer.  For  two  reasons  the  Thomson 
galvanometer  is  a  particularly  annoying  device.  The  silk-fiber  sus- 
pension of  the  mirror  system  is  so  delicate  that  it  often  is  found  to  be 
broken  at  the  critical  moment  of  use,  also  the  instrument  is  sensitive 


TESTING  913 

to  mechanical  vibration  and  to  external  magnetism.  D'Arsonval 
reversed  the  arrangement  of  elements  of  the  Thomson  instrument,  and 
used  a  light  movable  coil  and  a  powerful  fixed  magnet.  The  coil  can 
be  suspended  in  different  ways,  but  always  by  means  of  wires  or  other 
conductors,  so  that  current  may  be  carried  to  the  coil  even  though  it 
is  movable.  A  mirror  carried  by  the  coil  throws  a  beam  of  light  upon 
a  scale,  or  allows  the  use  of  a  telescope,  which  is  pointed  at  the  mir- 
ror, and  receives  the  reflection  of  a  scale,  usually  attached  to  the  tele- 
scope stand.  The  telescope  forms  a  magnified,  inverted  image  and 
hence  the  scale  is  marked  with  reversed  figures  and  is  placed  in  an 
inverted  position  on  the  stand.  What  the  observer  sees,  therefore, 
is  some  portion  of  the  scale,  erect  and  non-reversed.  The  sensi- 
bility of  this  type  of  galvanometer  is  not  as  great  as  that  of  the 
Thomson  form,  but  its  many  advantages  make  it  the  best  existing 
form. 

Galvanometer  Shunts.  For  all  purposes  of  measuring  insulation 
resistance,  the  galvanometer  must  have  a  means  of  reducing  the  cur- 
rent passing  through  it;  this  is  accomplished  in  the  use  of  a  shunt. 
Commercial  galvanometer  shunts  are  small  resistance  boxes  con- 
taining coils,  of  resistances  proportional  to  that  of  the  galvanometer. 
A  common  form  has  four  coils  in  one  box,  with  arrangements  for 
selecting  any  one  of  the  four,  the  resistances  of  the  coils  being  re- 
spectively 1/9,  1/99,  1/999,  and  1/9999,  of  the  galvanometer  resist- 
ance. When  the  galvanometer  is  shunted  by  one  or  the  other  of 
these  coils,  1/10,  1/100,  1/1000,  or  1/10000  of  the  current  will 
pass  through  the  galvanometer  coil,  and  the  remainder  through  the 
shunt. 

Insulation  Resistance.  In  addition  to  the  device  already  men- 
tioned, other  essential  things  for  insulation-resistance  tests  are,  a 
key  which  normally  short-circuits  the  galvanometer,  some  kind  of  a 
switch,  a  battery  or  other  source  of  potential,  and  a  standard  high 
resistance.  The  arrangement  of  these  parts  is  shown  in  Fig.  663, 
and  the  method  of  using  such  an  arrangement  is  as  follows:  First 
find  the  deflection  which  the  galvanometer  will  give  through  the 
standard  high  resistance  of  known  amount;  then  the  deflection  with 
the  same  battery  through  the  insulation  resistance  to  be  measured. 
If  the  standard  high  resistance  were  of  many  megohms,  this  compar- 
ison would  be  simple.  However,  as  a  standard  resistance  of  many 


914 


TELEPHONY 


megohms  would  be  expensive  and  bulky,  it  is  customary  to  have  the 
standard  resistance  100,000  ohms,  which  is  1/10  megohm,  and  to 
make  a  preliminary  test-reading  through  that  resistance  with  the 
assistance  of  the  galvanometer  shunts.  Making  such  a  test-reading 
to  determine  conditions  for  testing  the  unknown  insulation  resistance 
is  called  "getting  a  constant." 

To  find  this  constant,  place  the  switch  in  Fig.  663  in  the  proper 
position,  use  the  largest  shunt,  press  the  short-circuit  key  and  observe 
the  deflection;  if  it  is  very  small,  release  the  short-circuit  key  and 
choose  the  next  lower  shunt;  again  press  the  short-circuit  key  and 
observe  the  deflection;  if  the  deflection  is  on  the  scale,  and  is  fairly 
large,  note  it  as  soon  as  it  is  well  settled;  if  not,  take  a  lower  shunt 


J/%7/?7-  C/RCV/T//V6  KEY , 

O 

MOOOO*        GA^A»QMETEX 


/ — WsAr 


TO  TEST) 


W/ftE  UNDER 
TEST 


TO  TAKE 


Fig.  663.     Insulation  Test 

and  when  a  satisfactorily  large  reading  is  secured  multiply  it  by  the 
multiplying  value  of  the  shunt  and  by  the  resistance  in  megohms  of 
the  standard.  For  example,  suppose  the  following  conditions  to  exit: 

Shunt,  1/999 

This  will  let  only  1  /1000  of  the  current  through  the  galvanometer;  the 
multiplying  value  of  this  shunt  is  1000. 

Standard  resistance,  100,000  ohms  (1/10  megohm) 
Deflection,  85  scale  divisions 

The  constant  is,  therefore,  85X1000X1/10  =  8,500 

That  is,  the  insulation  resistance  through  which  the  same  battery 
would  give  one  scale  division  deflection  is  8,500  megohms.  The 
result  of  this  preliminary  operation  is  to  give  a  constant  number 
which  may  be  used  through  a  series  of  tests  and  which  only  needs 
to  be  re-secured  at  intervals  to  make  sure  the  conditions  have  not 
changed. 


TESTING  915 

The  constant  secured,  throw  the  switch  shown  in  Fig.  603  to 
the  position  required  for  testing,  select  a  shunt,  depress  the  short- 
circuit  key,  hold  it  down  one  minute,  note  the  deflection,  and  if  neces- 
sary repeat  the  process,  selecting  lower  and  lower  shunts  or  cutting 
out  the  shunt  altogether.  In  most  cases  the  first  trial  will  tell  whether 
any  shunt  is  necessary.  Suppose  that  with  no  shunt  a  reading  of  five 
divisions  is  secured.  The  insulation  resistance  of  the  wire  under  test 
will  then  be  found  at  once  by  dividing  the  constant  8,500  by  the 
observed  deflection  5,  giving  1,700  megohms  as  the  actual  insulation 
resistance  of  the  wire  under  test.  Similarly,  if  the  deflection  with  no 
shunt  had  been  found  to  be  10  scale  divisions,  the  insulation  resistance 
of  the  wire  would  have  been  850  megohms.  If  the  wire  had  been  in 
such  condition  as  to  give  15  scale  divisions  writh  the  -^  shunt  on  the 
galvanometer,  the  multiplying  power  of  that  shunt  tells  us  that  150 
scale  divisions  would  have  been  given  with  no  shunt.  Therefore, 
the  insulation  resistance  in  such  a  case  would  be  8,500  divided  by 
150,  which  equals  56f  megohms,  the  insulation  resistance  of  that 
conductor. 

In  all  these  cases  the  observer  was  getting  the  insulation  resistance 
of  the  wire  irrespective  of  its  length,  and  it  is  necessary  to  know  the 
result  in  megohms  per  mile,  as  the  guarantee  and  the  specifications 
were  written  in  such  terms.  When  it  is  remembered  that  the  reason 
the  insulation  resistance  is  not  immeasurably  high  is  that  current  is 
leaking  from  the  conductor  through  its  insulating  material,  it  will 
be  seen  that  with  other  conditions  equal,  the  longer  the  wire  the 
lower  its  insulation  resistance;  so  that  the  insulation  resistance  of  one 
mile  of  such  wire  will  be  greater  than  that  of  two  miles  or  less  than 
that  of  a  fraction  of  one  mile.  To  determine  the  insulation  resistance 
per  mile,  therefore,  of  any  conductor,  learn  its  actual  insulation  re- 
sistance as  described,  and  multiply  this  result  by  the  length  of  the 
conductor  in  miles.  For  example,  in  the  case  first  cited,  having  an 
actual  insulation  resistance  of  1,700  megohms,  suppose  the  length 
to  be  f  mile.  Multiply  1,700  megohms  by  f  and  the  result,  637.5 
megohms,  is  the  insulation  resistance  of  that  conductor  per  mile. 

In  case  short  lengths  of  cable,  found  by  preliminary  tests  to  have 
high  insulation  resistance,  are  to  be  tested  merely  to  discover  whether 
the  manufacture  and  erection  can  be  passed,  a  number  of  wires  can 
be  tested  at  once,  thus  shortening  the  operation  greatly.  Taking 


916 


TELEPHONY 


the  case  of  the  cable  which  had  a  conductor  testing  1,700  megonms 
actual  insulation  resistance,  if  50  pairs  (or  100  conductors)  had  been 
tested  at  once,  the  result  would  have  been  17  megohms  instead  of 
1,700  megohms,  assuming  the  character  of  all  the  wires  to  be  the 
same. 

In  all  measurements  of  cables  for  insulation  resistance  it  is 
customary  to  test  each  wire  against  all  the  rest  grounded  with  the 
sheath.  If  the  wires  are  tested  in  groups,  all  except  the  group  under 
test  should  be  connected  with  the  sheath  and  to  ground.  Measure- 
ment of  insulation  resistance  by  means  of  galvanometers  and  volt- 
meters is  the  most  usual  way. 

The  "Megger": — A  device  known  as  a  "megger"  nas  come  into 
some  use  and  deserves  more.  It  consists  of  a  hand-driven  generator, 
a  coil  moving  in  a  permanent  field,  a  pointer,  and  certain  stationary 
coils,  not  required  to  be  adjusted.  The  circuit  whose  insulation  is 


Fig.  664.     The  Megger 

required  is  connected  to  the  external  terminals  of  Fig.  664,  and  the 
generator  driven,  when  the  resistance  is  read  directly  on  the  scale. 
The  megger  reads  directly  in  terms  of  ohms  or  megohms;  its  pointer 
is  dead-beat;  the  handle  may  be  turned  as  fast  as  wished,  a  friction 
clutch  slipping  beyond  a  critical  speed.  The  results  given  by  the 
megger  require  no  calculation  whatever  to  know  the  resistance  of 
the  circuit  connected  to  the  test  terminals. 

Capacity.  The  second  thing  required  to  be  known  in  testing 
cables  for  general  condition  is  the  electrostatic  capacity  of  each  wire. 
Specifications  usually  call  for  an  average  mutual  capacity  of  about 
.065  microfarad  per  mile  more  or  less.  As  it  is  of  advantage  to  have  the 
insulation  resistance  as  high  as  possible,  it  is  also  of  advantage  to  have 


TESTING 


917 


the  capacity  as  low  as  possible.  A  statement  of  the  mutual  capacity 
of  a  wire  and  its  mate  is  merely  a  statement  of  how  large  a  condenser 
the  two  wires  form.  As  two  wires  with  the  insulating  material  be- 
tween them  do  form  a  condenser,  although  one  of  small  capacity, 
the  effect  is  to  short-circuit  the  voice  currents  in  the  line  to  just  that 
extent,  acting  as  if  the  wires  had  no  such  quality,  but  that  a  con- 
denser— small  perhaps,  but  none  the  less  a  condenser — had  been 
bridged  across  the  pair.  If  the  pairs  were  used  not  for  talking  but 
for  telegraphy,  or  some  other  electrical  use  with  direct  current,  the 
capacity  would  have  a  less  harmful  effect,  as  alternating  currents 
pass  through  condensers  with  much  freedom,  and  the  voice  currents 
sent  over  the  line  will  pass  through  the  condenser  formed  by  the  wires 
and  thus  be  lost  from  usefulness  at  the  distant  end. 


9&tM6E 

MEY 
•v? 

GAL  I/A/YOMETEK 

TO  TEST 

•SHUMT 

Qj3  C±D  Qp  QrD 

A 
f 

1         ^5s  /c'  ^A^ 

Ml  STAMDAftD 
*r  CAPACITY 

Fig.  665.     Capacity  Test 

In  Fig.  G65  are  shown  the  conditions  for  capacity  tests,  using 
the  principal  items  shown  in  Fig.  663  for  insulation  tests.  The  dis- 
charge key  is  one  which  has  two  contacts,  one  above  and  one  below, 
and  is  usually  arranged  so  that  it  may  be  locked  in  the  lower  position 
and  released  quickly  when  desired.  The  condenser  is  one  of  accu- 
rately known  capacity  and  is  usually  either  of  |  microfarad  or  is 
adjustable  in  small  fractions  up  to  about  \  microfarad.  For  tests 
such  as  we  are  now  considering,  the  ^  microfarad  standard  is  as  good 
as  any  and  is  cheaper  than  the  adjustable  form.  The  procedure  for 
capacity  measurements  is  as  follows: 

Place  the  switch  in  the  position  to  take  the  constant;  depress  the 
discharge  key,  holding  or  leaving  it  down  for  a  few  seconds,  thus 
causing  the  battery  to  charge  the  condenser.  Release  the  discharge  key 


918  TELEPHOMY 

and  note  the  galvanometer  deflection.  If  but  two  or  three  cells  of  dry 
battery  are  used,  the  deflection  will  be  on  the  scale  with  no  shunt, 
and  the  galvanometer  movement  simply  makes  one  steady  swing, 
then  stopping  and  returning  to  zero.  The  figure  thus  found  is  the 
deflection  which  ^  microfarad  will  give  with  no  shunt  and  with  the 
battery  used.  Throw  the  switch  to  the  testing  position,  depress  the 
discharge  key  for  a  few  seconds,  and  release  as  before,  noting  the 
deflection  now  given.  The  capacity  of  the  wire  under  test  is  now 
found  by  multiplying  ^  microfarad  by  the  last  deflection  and  dividing 
by  the  first. 

For  example:  Suppose  the  deflection  to  get  the  constant  were 
50  scale  divisions  with  the  ^  shunt;  as  this  shunt  has  a  multiplying 
power  of  ten,  the  deflection  is  equal  to  500  scale  divisions  with  no 
shunt;  suppose  the  test  deflection  were  two  divisions  with  no  shunt; 
then  ^  microfarad  times  2,  divided  by  500,  equals  .00133  microfarad, 
which  is  the  capacity  of  the  wire  under  test. 

As  a  condenser  has  greater  capacity  when  the  conducting  sur- 
face is  increased,  so  a  pair  of  wires  has  greater  capacity  as  its  length 
is  greater,  if  other  things  are  equal.  Insulation  and  separation 
being  the  same,  large  wires  have  greater  capacity  per  length  of  pair 
than  small  ones;  size,  length,  and  separation  of  the  wires  of  a  pair 
being  the  same,  the  capacity  is  lower  or  higher,  depending  on  the 
kind  of  insulating  material  used.  Dry  paper  is  the  standard  insula- 
ting material  for  telephone  cables,  because  lower  capacity  is  secured 
with  it  than  with  any  other  convenient  material. 

Fig.  665  shows  conditions  for  capacity  between  a  wire  and  earth. 
To  measure  capacity  between  two  wires  (mutual  capacity),  connect 
one  wire  as  shown  and  the  other,  instead  of  to  the  ground,  to  all 
three  of  the  points  shown  grounded  in  the  figure. 

It  often  is  necessary  to  practice  insulation  tests  on  lines  which 
are  not  yet  brought  into  a  building  and  as  one  may  have  to  go  from 
town  to  town  on  such  work,  it  is  of  advantage  to  have  the  testing  out- 
fit light  enough  to  be  portable,  and  strong  enough  to  stand  handling 
and  shipment.  Improvement  is  still  going  on  in  testing  apparatus, 
although  it  was  reasonably  well  developed  long  before  the  invention 
of  the  telephone.  Much  instruction  may  be  had  from  the  catalogues 
of  instrument  makers,  and  the  student  is  recommended  to  study 
them. 


TESTING 


919 


Testing  Sets.  A  typical  form  of  insulation  testing  sets  is  that 
made  by  Nalder  Brothers,  London.  The  arrangement  of  the  parts 
and  the  circuit  are  shown  in  Fig.  666.  A  notable  feature  is  the  ab- 
sence of  keys.  The  only  parts  requiring  to  be  moved  by  hand  are 
two  switches,  one  controlling  the  taking  of  the  constant  and  the  ac- 
tual testing,  and  the  other  applying  the  galvanometer  shunt  in  its 
varying  degrees.  The  shunt  is  subdivided  in  smaller  units  than 
usual,  and  the  switch  method  enables  it  to  be  used  with  much  greater 
speed  than  if  the  plug  form  were  chosen.  With  such  a  set,  the  oper- 
ation is  to  take  the  constant  by  throwing  the  main  switch  to  the  left 
and  moving  the  shunt  switch  until  the  largest  deflection  which  will 
be  on  the  scale,  is  found.  Suppose  that  this  deflection  is  320;  sup- 


Fig.   666.     Insulation  Test 


pose  the  shunt  to  be  on  the  1/1000  point  at  that  time;  then  the  con- 
stant will  be  32,000,  because  the  resistance  through  which  the  con- 
stant was  taken  was  1/10  megohm  (320X1000 X.I  =  32,000); 
throwing  the  switch  to  the  position  marked  "test",  deflections  may 
be  taken  on  the  various  wires  and  their  insulation  found  at  once  by 
dividing  32,000  by  the  observed  deflection,  multiplied  by  the  shunt 
multiplier  used.  For  instance,  if  after  taking  the  constant  as  above, 
one  were  to  test  a  wire  upon  which  the  shunt  switch  stopped  at  the 
1/10  point,  giving  a  deflection  of  64  divisions,  640  would  represent 
the  deflection  which  would  have  been  given  with  no  shunt,  and  this 
is  contained  in  32,000,  fifty  times;  therefore,  the  insulation  of  the  wire 
under  test  is  50  megohms.  For  convenience  in  other  tests  the  gal- 
vanometer terminals  are  brought  out  to  binding  posts,  as  well  as 
being  permanently  wired  into  the  circuit  of  the  set. 


920  TELEPHONY 

A  minor,  but  attractive  feature  of  this  set  is  its  compactness; 
the  reading  of  the  galvanometer  deflection  is  by  means  of  a  tele- 
scope, through  which  the  worker  sees  the  image  on  a  scale  rather  than 
by  watching  the  movement  of  a  spot  of  light  on  a  scale.  The  tele- 
scope is  small — only  about  3|  inches  long — and  the  scale  is  short  and 
finely  graduated,  however,  the  high  power  of  the  telescope  enables 
the  divisions  to  be  clearly  read. 

In  any  of  the  reflecting  types,  the  use  of  the  apparatus  requires 
reasonably  intelligent  care  not  to  pass  large  currents  through  the 
windings.  With  such  precautions,  pressures  of  several  hundred  volts 
may  be  used  with  safety. 

In  Fig.  663,  the  100,000-ohm  standard  resistance  is  left  in  the 
circuit  during  the  test  as  well  as  when  taking  the  constant.  This 
prevents  injury  to  the  apparatus  by  getting  on  a  very  poorly  insu- 
lated or  grounded  wire,  and  it  is  customary  to  arrange  sets  in  this 
way  for  that  reason.  Where  it  is  necessary  to  get  very  accurate 
results,  1/10  megohm  must  be  deducted  from  the  result  when  the  test 
is  completed. 

Loss=of=Charge  Test.  In  the  foregoing  method  of  testing  insula- 
tion resistance,  the  principle  is  that  of  sending  current  to  the  conduc- 
tor and  through  the  insulation,  and  noting  the  effect  of  that  current 
on  some  movable  thing  under  its  influence.  In  a  method  described 
by  Carhart  and  Patterson  in  "Electrical  Measurements,"  and  by 
Gray  in  "Absolute  Measurements  in  Electricity  and  Magnetism," 
another  principle  is  involved.  It  is  that  of  loss  of  charge,  and  is 
practiced  by  charging  the  conductor  under  test  to  a  definite  potential, 
allowing  a  known  time  to  elapse,  then  testing  to  see  what  potential 
remains.  An  insulated  conductor  must  form  a  condenser  with  ref- 
erence to  something,  even  if  it  hangs  in  air  and  the  other  condenser 
conductor  is  the  earth.  But  if  the  conductor  under  test  is  not  per- 
fectly insulated,  the  potential  to  which  it  is  charged  must  diminish 
by  the  leaking  away  of  the  charge,  so  that  after  a  known  time  the  loss 
may  be  known  by  observing  how  much  is  left.  A  calculation  then 
will  tell  what  insulation  resistance  exists  to  allow  that  loss  with  the 
known  time  and  charge.  The  method  has  the  practical  advantage 
that  a  sensitive  millivoltmeter  is  a  suitable  instrument  for  the  pur- 
pose and  is  readily  portable.  A  direct-current  power-source  may 
be  used  for  the  charge.  If  much  work  is  done  with  the  method 


TESTING  921 

tables  or  curves  may  be  laid  out,  thus  enabling  the  observer  to  omit 
calculations  and  to  read  insulations  at  once. 

Varley  Loop  Test  for  Grounds.  Two  of  the  three  things  which 
may  happen  to  working  wires  are — becoming  grounded  and  becoming 
crossed  with  other  wires.  Both  of  these  troubles  may  be  located  by 
means  of  the  Varley  loop  test,  the  circuit  of  which  is  shown  in  Fig.  667. 
There  are  many  methods  of  making  ground  locations,  but  of  them 
all  the  Varley  test  and  the  similar  one  designed  by  Murray  are  those 
best  adapted  for  general  use,  principally  because  they  are  free  from 
errors  which  might  result  from  the  existence  of  earth  potentials.  The 


1 


Fig.  667.     Varley  Loop  Test  for  Grounds 

pieces  of  apparatus  used  in  the  Varley  test  are  the  usual  Wheatstone 
bridge,  rheostat,  and  galvanometer.  It  is  not  necessary  that  the 
latter  be  of  the  reflecting  type,  although  in  some  cases  it  may  be 
found  convenient  to  use  one  of  that  sort. 

Referring  to  Fig.  667,  showing  the  Varley  tests  for  grounds, 
the  arrangement  is  of  the  Wheatstone-bridge  type,  in  which  it  is  in- 
tended that  the  current  from  the  battery  shall  flow  through  two  paths, 
and  at  the  time  of  completing  the  test  shall  cause  no  deflection  of  the 
galvanometer.  In  the  figure,  A  and  B  are  the  two  bridge  resistances; 
R  is  the  rheostat  (the  resistance  of  which  may  be  varied  at  will  by 
plugs  or  switches);  the  battery,  galvanometer,  and  ground  can  be 
recognized  by  form.  The  test  requires  that  in  addition  to  the  wire 
having  the  ground  fault  on  it,  an  extra  wire  shall  be  provided  which 
is  clear  of  faults.  Fortunately,  in  telephone  work,  wires  generally 
go  in  pairs,  and  one  often  is  good  when  the  other  is  bad.  The 
two  wires  must  be  directly  connected  at  the  distant  end. 

If  the  two  wires  are  alike,  and  if  the  resistance  of  the  bridge  arms 
A  and  B  are  equal  to  each  other,  the  galvanometer  will  show  no  de- 


922  TELEPHONY 

flection  when  the  resistance  in  the  rheostat  R  is  equal  to  twice  the 
resistance  from  the  ground  to  the  distant  end.  This  is  the  simplest 
form  of  the  test,  and  on  a  long  line  may  give  valuable  help  in  know- 
ing just  about  where  to  find  the  ground.  The  rheostat  being  adjust- 
able in  ohms,  however,  one  cannot  get  results  to  fractions  of  an  ohm 
unless  the  bridge  arms  are  in  another  ratio  than  equal.  And  as  the 
result,  in  integral  ohms,  may  be  half  an  ohm  wrong,  with  No.  10  B. 
&  S.  gauge  wire  the  location  may  be  wrong  by  500  feet  or  so.  If 
this  will  do  well  enough,  it  is  simplest  to  remember  the  test  by  the 
following  rule,  suitable  when  the  two  wires  are  alike: 

Use  equal  bridge  arms;  adjust  the  rheostat  till  no  deflection,  or  the 
least  deflection,  exists;  divide  the  resistance  cut  in  at  the  rheostat  by 
two;  the  result  is  the  resistance  from  the  ground  to  the  distant  end  of 
the  line. 

No.  10  B.  &  S.  gauge  and  No.  12  N.  B.  S.  gauge  hard-drawn 
copper  wire  have  a  resistance,  roughly,  of  5  ohms  per  mile  of  wire, 
or  10  ohms  per  mile  of  metallic  circuit.  For  such  lines,  which  are 
common  for  toll  service,  this  riile  is  quick  and  good: 

With  equal  bridge  arms,  balance  as  usual;  divide  the  resulting 
rheostat  resistance  by  10  (or  point  off  the  right-hand  figure');  the  result 
is  the  distance  in  miles  from  the  ground  to  the  further  end  of  the  line. 

Varley  Loop  Test  for  Crosses.  The  Varley  test  for  crosses, 
Fig.  668,  is  the  same  as  for  grounds,  except  for  a  slight  difference  in 


Pig.  668.     Varley  Loop  Test  for  Crosses 

the  arrangement  of  the  wires  under  test.  The  battery  is  connected 
to  one  of  the  crossed  conductors  3;  the  rheostat  is  connected  to  the 
other,  2;  a  good  wire  1  is  connected  to  the  set  as  in  the  test  for  ground, 
and  is  looped  to  the  crossed  wire  2  at  the  distant  end.  The  further 
operations  are  by  the  rules  given  for  grounds. 


TESTING  923 

The  Varley  test  with  bridge  arms  of  a  ratio  other  than  equality 
will  give  results  to  such  a  fraction  of  an  ohm  as  may  be  desired.  As 
the  apparatus  is  usually  made,  bridge-arm  relations  may  be  se- 
lected so  that  A  is  to  B  as  10  is  to  1,  or  100  to  1,  or  1,000  to  1,  or 
the  reverse.  Suppose  the  arrangements  are  such  that  B  is  greater 
than  A,  say,  10  to  100  times,  and  the  two  wires  in  the  test  are  of  equal 
resistance.  Then,  having  adjusted  the  resistance  in  the  rheostat 
until  there  is  no  deflection  in  the  galvanometer,  the  equation  of  the 
resistance  from  the  observing  point  to  the  ground  is 

2BC-AR 
A+B 

in  which  C  is  the  resistance  of  one  wire  to  the  distant  end.  Stated  in 
other  terms,  the  resistance  to  the  fault  may  be  determined  thus: 

Multiply  the  resistance  of  bridge  arm  B  by  that  of  the  good 
wire,  and  double  the  result;  subtract  from  this  the  product  of  the  re- 
sistances of  arm  A  and  of  the  rheostat;  divide  the  remainder  by  the  sum 
of  arms  A  and  B. 

As  a  practical  example :  A  line  35  miles  long  is  made  of  two  copper 
wires  of  175  ohms  each,  and  one  has  a  ground  on  it  at  some  place 
unknown;  the  bridge  arms,  in  testing,  are  10  for  A  and  100  for  B; 
a  balance  is  reached  with  2,015  ohms  in  the  rheostat.  Then,  by  the 
rule,  100  times  175  times  2  =  35,000;  subtracting  10  times  2,015 
leaves  14,850;  dividing  by  100  plus  10  equals  135;  this  is  the  resist- 
ance in  ohms  to  the  fault.  In  the  case  of  a  cross,  the  result  is  fig- 
ured in  the  same  way. 

If  the  records  tell  the  resistance  of  all  principal  lines  under  good 
conditions,  the  location  of  the  fault  can  be  accurately  figured.  It 
is  hardly  enough  to  know  the  theoretical  resistance  per  mile  of  the 
size  of  wire  used.  The  actual  resistance  ought  to  be  known  and 
used. 

Murray  Loop  Tests  for  Grounds.  The  Murray  loop  test  is  simi- 
lar to  the  Varley  loop  test  in  its  freedom  from  error  due  to  earth 
currents.  It  differs  from  it  in  that  the  bridge  ratio  is  varied  instead 
of  the  rheostat  being  varied  with  fixed  bridge  ratio. 

A  dial  form  of  bridge  is  most  convenient  for  the  Murray  loop  test, 
one  type  of  which  is  illustrated  in  Fig.  669.  The  dial,  the  arm  of 
which  swings  one  end  of  the  testing  battery,  has  101  points  in  the 


924  TELEPHONY 

circle;  a  resistance  coil  of  1  ohm  is  connected  across  each  gap  between 
points;  there  are  thus  100  such  coils.  The  coil  A  is  of  the  same  re- 
sistance, 100  ohms,  as  the  total  of  the  100  coils  in  the  circle.  Then 
with  a  loop  of  one  good  wire  and  the  faulty  one,  the  dial  lever  is 
turned  till  the  galvanometer  deflection  is  nothing.  If  the  ground  is 
one  ohm  or  more  from  the  extreme  further  end  of  the  looped  wires, 
at  least  one  step  from  zero  will  be  required  to  balance  the  needle. 
The  amount  the  switch  is  turned  from  zero  is  marked  C  in  the  figure; 
the  remainder  of  the  circle  is  marked  B;  the  loop  resistance  may  be 
called  L.  Then  the  equation  is 


B  +  C  +  A 

but  B  +  C  4-  A  equals  the  sum  of  all  resistance  in  the  set,  or,  in 
this  case,  200  ohms.  Hence 

Multiply  the  loop  resistance  by  the  steps  between  the  switch  and 
the  end;  divide  by  200.  The  result  is  the  resistance  in  ohms  from 
the  point  of  testing  to  the  fault. 

To  trace  the  following  case  with  the  aid  of  Fig.  669  will  aid  in 
clinching  the  principle  in  one's  mind:  Loop  resistance,  84  ohms; 


Fig.  669.     Dial  Set  for  Murray  Loop  Test  for  Grounds 

distance  switch  lacks  of  reaching  the  end,  68  coils;  then  the  resist- 
ance to  the  fault  is  68  times  84  ohms  divided  by  200,  equaling  28.56 
ohms.  From  this  the  distance  can  be  computed  in  feet  or  miles  as 
desired. 

Capacity  Tests  for  Opens.  Besides  being  grounded  and  crossed 
with  other  wires,  lines  may  become  open.  A  break  in  a  wire,  if  it  is 
a  bare  aerial  one  on  poles,  may  result  in  one  or  both  ends  becoming 
grounded.  In  some  cases,  the  location  of  the  break  can  be  located 
by  a  method  for  grounds.  In  some  other  cases,  one  of  the  ends  may 
be  crossed  with  another  wire,  and  the  method  for  locating  a  cross 


TESTING  925 

may  enable  the  location  to  be  made.  In  cables,  however,  opens  are 
often  merely  breaks  with  no  ground  or  cross  to  assist  the  test,  in  which 
case  the  test  must  be  based  on  a  comparison  of  electrostatic  capacities. 
By  applying  a  capacity  test  to  the  broken  wire,  and  also  to  a  good 
wire  of  the  same  size,  route,  and  of  similar  surroundings,  a  compar- 
ison of  the  two  results  will  give  the  length  of  the  defective  piece. 
The  good  wire  must  be  open  at  the  distant  end.  Suppose  that  a  toll 
line  45  miles  long,  of  two  No.  10  B.  &  S.  gauge  wires,  has  one  wire 
broken;  this  wire  is  found  by  test  to  be  clear  of  ground  at  the  break. 


Ooooo0o 


Fig.  670.     Dial  Set  for  Tests  for  Opens 

A  test  for  capacity  on  the  good  wire,  opened  at  the  distant  end,  gives 
.2925  microfarads  for  the  whole  piece;  a  test  on  the  broken  wire  gives 
.1172  microfarads;  then  the  distance  to  the  break,  in  miles,  is  45  times 
.1172,  divided  by  .2925,  equaling  18  miles  and  a  little  over — to  be 
exact,  18  miles,  163  feet.  In  a  word,  the  rule  for  the  distance  to  the 
break  is:  Multiply  the  length  of  the  good  wire  by  the  ratio  between 
the  bad  wire  and  good  wire  capacities. 

Another  method  for  locating  a  break  in  a  conductor,  particularly 
adaptable  to  conductors  in  cables,  utilizes  the  apparatus  of  Fig.  670. 
For  use  in  locating  breaks,  the  set  is  associated  with  a  telephone  re- 
ceiver instead  of  a  galvanometer.  The  current  from  the  battery  is 
supplied  to  the  conductors  under  test  in  the  form  of  an  alternating 
current  which  is  generated  by  means  of  an  induction  coil  7  and  a 
buzzer  B.  The  resistance  R  is  a  fixed  non-inductive  one,  and  the 
dial  is  composed  of  100  equal  coils  connected  between  the  steps, 
the  total  dial  resistance  being  the  same  as  that  of  the  fixed  re- 
sistance. The  cable  conductor  which  is  broken  is  joined  with 
its  mate  at  one  of  the  terminals  of  the  set,  and  two  wires  of  another 
pair  of  the  same  size  and  length  are  connected  to  the  set  at  the  ter- 
minals of  the  secondary  of  the  induction  coil.  Listening  in  the  re- 


926 


TELEPHONY 


ceiver,  the  dial  lever  is  turned  until  silence  is  reached,  or  as  near 
silence  as  is  found  possible.  Then  the  distance  from  the  point  of 
observation  to  the  break  is  found  by  multiplying  the  total  length  of 
the  cable  under  test  by  the  number  of  ohms  between  the  switch  lever 


c  row 


QU/ET  SHOftT  C/RCU/T          /VO/-5Y 


(Ground) 


C/tfCV/T    _QWET 


_QU/ET  SHORT   C/^CU/T QU/ET 


Fig.  671.     Short-Circuiting  Tests 

and  the  end,  dividing  by  200.  In  this,  as  in  any  capacity  test  for 
the  location  of  a  break,  the  wires  involved  in  the  test  must  be  open  at 
the  distant  end. 

Listening  Tests.     When  both  wires  of  a  long-distance  line  are 
equal  in  resistance,  insulation,  and  capacity  of  each  to  the  earth  and 


TESTING  927 

other  things,  the  line  is  quiet.  When  one  or  more  of  these  condi- 
tions is  changed  from  the  equality,  the  line  is  noisy.  Only  very  short 
lines  are  quiet  when  unbalanced,  unless  in  regions  where  no  power 
circuits  exist. 

If  one  wire  of  a  long  line  is  open,  grounded,  or  crossed  with  an- 
other wire,  even  if  the  latter  is  open  at  both  ends,  the  long  line  is  un- 
balanced. Long-distance  wire  chiefs  become  very  expert  in  de- 
tecting the  causes  and  locations  of  faults  by  the  sounds  they  hear 
when  lines  are  noisy,  and  by  the  behavior  of  the  noises  under  changes 
in  the  line's  connections. 

In  general,  make  listening  tests  while  the  line  is  short-circuited 
at  various  places.  The  fault,  if  an  open,  a  ground,  or  a  cross,  is 
beyond  the  point  short-circuited,  if  doing  so  makes  it  quiet.  The 
Middleton  Brothers  describe  the  facts  graphically,  as  in  Fig.  671. 
Four  cases  are  shown,  C,  D,  E,  and  F.  In  case  C,  placing  a  short 
circuit  at  the  point  indicated  quieted  the  line  for  observer  A;  that 
is,  when  the  ground  lay  beyond  the  short  circuit,  the  line  was  quiet. 
In  case  D,  the  line  is  quiet  for  B,  noisy  for  A.  In  case  E,  the  ground 
and  short  circuit  are  at  the  same  point;  the  line  is  quiet  for  both  A 
and  B.  In  case  F,  short  circuit  at  2  quiets  for  A;  short  circuit  at 
4  quiets  for  B;  short  circuit  at  3  will  quiet  for  neither.  A  double 
fault  exists,  a  difficult  condition  for  bridge  location,  but  easy  for 
listening  tests. 

Quiet  local  lines  may  be  connected  to  quiet  long-distance  lines 
making  a  noisy  connection.  A  fault  on  the  local  line  is  indicated. 
Inserting  a  repeating  coil  quiets  the  connection,  but  impairs  trans- 
mission. Short-circuiting  the  local  line  at  successive  points,  further 
and  further  from  the  connecting  point,  will  quiet  the  long-distance 
line,  until  the  fault  is  passed. 


INDEX 


Acousticon  transmitter 
Acoustics 

characteristics  of  sound 
loudness 
pitch 
timbre 

human  ear 

human  voice 

propagation  of  sound 
/iir-gap  vs.  fuse  arresters 
Amalgamated  zincs 
Arrester  separators 
Audible  signals 

magneto  bell 

telegraph  sounder 

telephone  receiver 

vibrating  bell 
Automanual  system 


Page 


67 

9 

9 

10 

10 

11 

13 

12 

9 

294 
87 

286 
29 
30 
29 
33 
30 

584 


automatic  distribution  of  calls  589 
automatic  switching  equipment  588 
building  up  a  connection  590 

characteristics  of  584 

operation  585 

operator's  equipment  586 

setting  up  a  connection  590 

speed  in  handling  calls  590 

subscriber's  apparatus  585 

Automatic  desk  stand  524 

Automatic  Electric  Company's  tele- 
phone system  515 


Page 

Automatic  Electric  Company's  tele- 
phone system 

automatic  sub-offices  567 

connector  551 

function  of  551 

location  of  552 

operation  of  552 

first  selector  operation  545 

line  switch                              519,  529 

bridge  cut-off  539 

circuit  operations  533 

function  of  518 

guarding  functions  539 

line  and  trunk  contacts  530 

locking  segment  538 

master  switch  537 

relation  of,  to  connectors  540 

structure  of  532 

summary  of  operation  540 

trunk  ratio  531 

trunk  selection  531 

multi-office  system  562 

party  lines  568 

release  after  conversation  562 

rotary  connector  568 

second  selector  operation  548 

selecting  switches  519,  541 

release  mechanism  544 

side  switch  541 

subdivision  of  subscribers'  lines  518 

subscribers'  station  apparatus  524 

operation  526 


930 


INDEX 


Page 

Automatic  Electric  Company's  tele- 
phone system 

subscribers'  station  apparatus 
operation 

bell  and  transmitter 

springs  526 
ground  springs  526 
impulse  springs  527 
release  springs  529 
ringing  springs  529 
trunking  520 
connector  action  523 
first  selector  action  523 
line  switch  action  520 
second  selector  action  522 
two-wire  automatic  systems  569 
two-wire    and    three-wire    sys- 
tems 523 
underlying  feature  of  trunking 

system  519 
Automatic  shunt  114 
Automatic  telephone  systems  501 
arguments  against  501 
attitude  of  public  507 
complexity  502 
expense  506 
flexibility  506 
subscriber's  station  equip- 
ment 508 
automatic  vs  manual  509 
comparative  costs  508 
definition  501 
methods  of  operation  509 
fundamental  idea  513 
grouping  of  subscribers  511 
local  and  inter-office  trunks  514 
Lorimer  system  510 
magnet  vs.  power-driven 

switches  510 

multiple  vs.  trunking  511 


Automatic  telephone  systems 
methods  of  operation 

outline  of  action 

Strowger  system 

testing 

trunking  between  groups 
Automatic  wall  set 


Page 


512 
509 
514 
511 
524 


B 

Bar  electromagnet  138 

Battery  bell  105 

Battery  symbols  104 

Biased  bell  120 

Blake  single  electrode  54 

Blocking  sets  721 

Broken-back  ringer  270 
Broken-line  method  of  selective 

signaling  ,251,  265 

Busy  test  414 

busy-test  faults  416 

potential  of  test  thimbles  415 

principle  415 


Cable  color  code  500 

Cable  splicing  828 

lead  sleeves,  sizes  of  834 

necessity  for  dryness  828 

pot-heads  834 

central-office  837 

filling  836 

splicing,  general  method  of  829 

straight-splice  829 

enclosing  the  splice  832 

final  boiling  out  832 

joining  the  wires  831 

preparing  the  conductors       830 

top  splice  833 

Y-splice  834 


INDEX 


931 


Page 

Cables  736 

capacity  739 

effect  of  temperature  on        740 

mutual  and  regular  739 

diameters  and  weights  741 

dry  paper  736 

manufacture  736 

alloyed  sheath  739 

conductors  736 

core  737 

drying  738 

forming  lead  sheath       738 

pairs  737 

early  types  736 

insulation  740 

Submarine  cables  741 

loading  743 

paper  741 

armor  741 

rubber  and  gutta-percha       743 

Capacity  reactance  45 

Carbon  21 

adaptability  21 

limitations  23 

preparation  of  65 

superiority  22 

Carbon  air-gap  arrester  290 

Carbon-block  arrester  286 

Care  of  plant  895 

automatic  systems  901 

maintenance  and  depreciation      895 

manual-office  equipment  899 

outside  plant  897 

cables  897 

conduits  897 

open  wire  897 

supports  897 

soldering  902 

subscriber's  equipment  898 

Carrying  capacity  of  transmitter  66 


Page 

Central-office  protectors  300 
Characteristics  of  sound  9 
loudness  10 
pitch  10 
timbre  11 
Chloride  of  silver  cell  103 
Circuits  687 
applications  698 
composite  692 
phantom  687 
transmission  over  690 
transpositions  689 
railway  composite  693 
ringing  693 
simplex  690 
Closed-circuit  cells  96 
Closed-circuit  impedance  coil  154 
Common-battery    multiple    switch- 
board 435 
assembly  472 
Dean  multiple  board  459 
cord  circuit  460 
line  circuit  459 
listening  key  460 
ringing  keys  460 
test  460 
Kellogg  two-wire  multiple  board  450 
battery  feed  454 
busy  test  456 
complete  cord  and  line  cir- 
cuit 454 
cord  circuit  452 
line  circuit  451 
summary  of  operation  457 
supervisory  signals  453 
wiring  of  line  circuit  458 
multiple      switchboard      appa- 
ratus 463 
jacks  465 
lamp  jacks  466 


932 


INDEX 


Page 

Common-battery    multiple    switch- 
board 
multiple  switchboard  apparatus 

relays  467 
Stromberg  -  Carlson      multiple 

board  462 

cord  -circuit  462 

supervisory  signals  463 

test  463 

Western   Electric  No.    1   relay 

board  435 

capacity  range  446 

cord  circuit  437 
functions    of    distributing 

frames  443 

line  circuit  435 

modified  relay  windings  445 

operation  438 

operator's  circuit  detail  441 

order-wire  circuits  442 

pilot  signals  445 

relay  mounting  446 

testing-called  line  busy  441 

testing-called  line  idle  440 

wiring  of  line  circuit  442 

Western  Electric  No.  10  board  446 

circuits  447 

economy  450 

operation  449 

test  449 

Common-battery  switchboard  377 

advantages  of  operation  377 

common  battery  vs.  magnets  378 

cord  circuit  386 

battery  supply  386 

complete  circuit  387 

supervisory  signals               -  387 

cycle  of  operations  389 

jacks  396 

lamps  390 


Common-battery  switchboard 
lamps 

mounting 
line  signals 

direct-line  lamp 
direct-line  lamp  with 

last 

line  lamp  with  relay 
pilot  signals 
mechanical  signals 
Kellogg 
Monarch 
Western  Electric 
relays 

switchboard  assembly 
Common-battery  telephone  sets 
Composite  circuits 
Condensers 
capacity 
charge 

conventional  symbols 
definition  of 
dielectric  materials 
functions 

means  for  assorting  current 
sizes 
theory 

Conductivity  of  conductors 
Connector 

Conventional  symbols 
Cook 

air-gap  arrester 

arrester 

arrester   for   magneto 

tions 
Cord  circuits 

battery  supply 
complete  circuit 
supervisory  signals 
Cord-rack  connectors 


Page 


391 
380 
380 


bal- 


sta- 


288 
301 

303 

386 
386 

387 
387 
432 


INDEX 


933 


Crowfoot  cell 
Cummings-Wray  selector 
Current  supply  to  transmitters 
common  battery 
advantages 


bell    substation    arrange- 
ment 

bridging  battery  with  im- 
pedance coils 

bridging  battery  with  re- 
peating coil 

current    supply  from  dis- 
tant point 

current  supply  over  limbs 
of  line  in  parallel 

Dean  substation  arrange- 
ment 

double  battery  with  im- 
pedance coil 

Kellogg     substation     ar- 
rangement 

North  Electric  Company 
system 

series  battery 

series  substation  arrange- 
ment 

Stromberg-Carlson  system 

supply   many   lines   from 
common  source 
repeating  coil 
retardation  coil 
local  battery 

D 

Dean 

drop  and  jack 

multiple  board 

receiver 

wall  telephone  hook 


•Page 

96      Desk  stand  hooks 
708  Kellogg 

176  Western  Electric 
178      Development  studies 

178  exchange 

central-office  locations 
182  comparison  of  estimates 

conduit      and      pole-line 

184  routes 
house  count 

181  multi-office  districts 

number  of  office  districts 
194  ratio     of     telephones     to 

buildings 
190  ratio     of    telephones    to 

population 

187  scope  of  study 

single  vs.  multi-office  dis- 

185  tricts 
single-office  districts 

186  subdivision    of    exchange 

districts 
189  ultimate  sizes 

179  long-distance  or  toll  line 
Dielectric 

180  Dielectric  materials 

188  dry  paper 
mica 

192      Differential  electromagnets 
192      Direct-current  receiver 
194      Dispatchers'  keys 

177  Dispatching  on  electric  railways 
Drainage  coils 

Dry  paper 


Page 
128 
130 
128 

887 
888 
892 
889 

892 
889 
890 
891 

889 
889 


890 
890 

894 
893 
887 
170 
172 
172 
172 
141 
74 
705 
722 
305 
736 


334 

459 

78      Electric  lamp  signal 
127      Electrical  hazards 


E 


34 
277 


934 


INDEX 


Page 

Electrical  reproduction  of  speech  14 
carbon  21 
conversion  from  sound  waves 
to   vibration   of  dia- 
phragm 14 
conversion  from  vibration  to 

voice  currents  14 
conversion  from  voice  currents 

to  vibration  15 
cycle  of  conversation  16 
detrimental  effects  of  capacity  26 
early  conceptions  18 
electrostatic  telephone  20 
induction  coil  24 
limitations  of  magneto  trans- 
mitter 19 
loose  contact  principle  17 
magneto  telephone  16 
measurements  of  telephone 

currents  26 
variation  of  electrical  pressure  20 
variation  of  resistance  20 
Electrical  signals  29 
audible  29 
magneto-bell  30 
telegraph  sounder  29 
telephone  receiver  33 
vibrating  bell  30 
visible  33 
electric  lamp  signal  34 
electromagnetic  signal  33 
Electrodes  65 
arrangement  of  53 
carbon  preparation  65 
multiple  55 
single  54 
Electrolysis  305 
Electromagnetic  method  of  measur- 
ing    telephone     cur- 
rents 27 


Electromagnetic  signal 
Electromagnets      and 
coils 


Page 
33 
inductive 

133,  161 


conventional  symbols  161 

differential  electromagnet  141 

direction  of  armature  motion       141 

direction  of  lines  of  force  136 

electromagnets  133 

low-resistance  circuits  137 

horseshoe  form  137 

iron-clad  form  139 

special  horseshoe  form  140 

impedance  coils  153 

kind  of  iron  154 

number  of  turns  153 

types 

closed-circuit  154 

open-circuit  154 

toroidal  155 

induction  coil  155 

current  and  voltage  ratios     156 

design  156 

functions  156 

use  and  advantage  157 

magnet  wire  143 

enamel  146 

silk  and  cotton  insulation     145 

space  utilization  146 

wire  gauges  144 

magnetic  flux  134 

magnetization  curves  135 

magnetizing  force  133 

mechanical  details  142 

permeability  134 

reluctance  137 

repeating  coil  158 

winding  methods  148 

winding  calculations  152 

winding  data  149 

winding  terminals  149 


Electrostatic  capacity 

unit  of 

Electrostatic  telephone 
Enamel 


Five-bar  generator 
Fuller  cell 

G 

Galvani 

Generator  armature 
Generator  cut-in  switch 
Generator  shunt  switch 
Generator  symbols 
Gill  selector 
Granular  carbon 
Gravity  cell 


Hand  receivers 

Harmonic  method  of  selective  sig- 
naling 

advantages 

circuits 

in-time  system 

limitations 

principles 

tuning 

under-tune  system 
Head  receivers 
Heat  coil 

Holtzer-Cabot  arrester 
Hook  switch 

automatic  operation 

contact  material 

design 


INDEX 

935 

'age 

Page 

40 

Hook  switch 

42 

desk  stand  hooks 

128 

20 

Kellogg 

130 

146 

Western  Electric 

128 

purpose 

122 

symbols 

131 

wall  telephone  hooks 

125 

113 

Dean 

127 

99 

Kellogg 

125 

Western  Electric 

126 

Horseshoe  electromagnet 

138 

Housing  central-office  equipment 

615 

82 

arrangement   of  apparatus   in 

109 

small  manual  offices 

618 

115 

combined  main  and  inter- 

114 

mediate  frames 

619 

117 

floor  plans  for 

618 

707 

types  of  line  circuits 

621 

56 

automatic  offices 

633 

96 

typical  automatic  office 

636 

central-office  building 

615 

fire  hazard 

615 

provision    for    cable   run- 

80 

ways 

617 

provision  for  employes 

617 

238 

size  of  building 

616 

248 

strength  of  building 

616 

247 

large  manual  office 

622 

243 

Human  ear 

13 

248 

Human  voice 

12 

238 

240 

I 

240 

Impedance  coils 

153 

80 

kind  of  iron 

154 

296 

number  of  turns 

153 

290 

symbols  of 

155 

122 

types 

123 

closed-circuit 

154 

125 

open-circuit 

154 

124 

toroidal 

155 

936 


INDEX 


Page 

Inductance  vs.  capacity  46 

Induction  coil  .  24,  155 

current  and  voltage  ratios  156 

design  156 

functions  156 

use  and  advantage  157 

Inductive  neutrality  164 

Inductive  reactance  45 

Insulated  open  wire  731 

Insulation  of  conductors  46 

Insulation-resistance  740 

Intercommunicating  systems  648 

common-battery  systems  649 

Kellogg  slug  type  650 

Kellogg  push-button  type  651 

Monarch  system  653 

Western  Electric  system  651 

definition  648 

limitations  648 

for  private-branch  exchanges  656 

simple  magneto  system  648 

Introduction  to  telephony  1 

Iron-clad  electromagnet  139 

Iron  wire  ballast  167 


Jacks  396 


Kellogg 

air-gap  arrester  289 

desk  stand  hook  130 

drop  and  jack  332 

mechanical  signal  394 

receiver  73 

ringer  119 

transmitter  60 

trunk  circuits  491 


315, 


Kellogg 

two-wire  multiple  board 
wall  telephone  hook 

Keyboard  wiring 


Lalande  cell 

Lamp  filament 

Lamp  mounting 

Lamps 

Le  dandie"  cell 

Lenz  law 

Line  signals 

direct-line  lamp 

direct-line  lamp  with  ballast 

line  lamp  with  relay 

pilot  signals 
Line  switch 

Lines  of  force,  direction  of 
Loading  coils 
Lock-out  party-line  systems 

broken-line  method 

operation 
Long-distance  switching 

definitions 

center-checking 

operators'  orders 
by  call  circuits 
by  telegraph 

particular  party  calls 

switching  through  local  board 

ticket  passing 

trunking 

high-voltage  toll  trunks 
through  ringing 

two-number  calls 

use  of  repeating  coil 

way  stations 
Lorimer  automatic  system 


510, 


Page 

450 
125 
433 


102 
166 
391 
390 
89 
44 
380 
380 
381 
383 
383 
529 
136 
49 
253 
265 
272 
659 
659 
663 
660 
660 
660 
661 
659 
662 
661 
661 
661 
660 
659 
663 
571 


INDEX 


937 


Page 
Lorimer  automatic  system 

central-office  apparatus  574 
connective  division  576 
sectional  apparatus  575 
switches  579 
interconnector  580 
interconnector   selec- 
tor 580 
primary  connector  579 
rotary  switch  579 
secondary  connector  580 
signal     transmitter 

controller  580 

operation  581 

subscriber's  station  equipment  572 

Loudness  of  sound  10 

Low-reluctance  circuits  137 

horseshoe  form  137 

iron-clad  form  139 

step-by-step  system  257 

Poole  system  254 


M 

Magnetic  flux 
Magnetization  curves 
Magnetizing  force 
Magneto  bell 


134 
135 
133 

30,  105 


Magneto  multiple  switchboard  419 


424 


branch  -  terminal    multiple 

board 

arrangement  of  apparatus  427 

magnet  windings  427 

operation  426 

field  of  utility  419 

modern   magneto   m  u  1 1  i  p  le 

board  429 

assembly  432 

cord  circuit  430 

test  430 


Magneto  multiple  switchboard 
series-multiple  board 
defects 
operation 
Magneto  operator 
Magneto  signaling  apparatus 
armature 
automatic  shunt 
battery  bell 
generator  symbols 
magneto  bell 
magneto  generator 
method  of  signaling 
polarized  ringer 
pulsating  current 
ringer  symbols 
theory 
Magneto  switchboard 

automatic  restoration 
mechanical 
Dean  type 
Kellogg  type 
Monarch  type 
Western  Electric  type 
circuits    of    complete    switch- 
boards 

code  signaling 
commercial  types  of  drops  and 

jacks 

early  drops 
jack  mounting 
manual  vs.  automatic  res- 
toration 

methods  of  associating 
night  alarm 
tubular  drops 
component  parts 
jacks  and  plugs 
keys 
line  and  cord  equipments 


420 
423 
422 
106 
105 
109 
114 
105 
117 
105 
106 
105 
118 
116 
121 
107 
314 
331 
331 
334 
332 
335 
333 

346 
336 

325 
325 
328 

330 
329 
328 
326 
315 
315 
317 
318 


938 


INDEX 


Page 

Magneto  switchboard 
component  parts 

line  signal  315 

operators'  equipment  318 

cord-circuit  considerations  354 

double  clearing-out  type  358 

lamp-signal  type  361 

non-ring  through  type  356 

series  drop  type  356 

simple  bridging  drop  type  354 

definitions  314 

electrical  restoration  337 

grounded  and  metallic-circuit 

lines  350 

mode  of  operation  314 

night-alarm  circuits  348 

operation  in  detail  318 

clearing  out  325 

essentials  of  operation  325 

normal  condition  of  line  319 

operator  answering  320 

operator  calling  322 

subscriber  calling  320 

subscribers  conversing  323 

operator's    telephone 

equipment  354 
cut-in  jack  345 
ringing  and  listening  keys  341 
horizontal  spring  type  341 
party-line  ringing  keys  344 
self-indicating  keys  344 
vertical  spring  type  342 
switchboard  assembly  363 
functions  of  cabinet  363 
sectional  switchboards  374 
upright    type    of    switch- 
board 366 
wall  type  switchboard  363 
switchboard  cords  340 
concentric  conductors  340 


Page 

Magneto  switchboard 
switchboard  cords 

parallel  tinsel  conductors      341 

steel  spiral  conductors  340 

switchboard  plugs  338 

Magneto  telephone  16 

Magneto  telephone  sets  199 

Measured  service  676 

local  service  682 

meter  method  682 

prepayment  method  684 

ticket  method  682 

rates  676 

toll  service  677 

long  haul  677 

short  haul  677 

timing  toll  connections  678 

units  of  charging  677 

Mechanical  signals  393 

Kellogg  394 

Monarch         .  394 

Western  Electric  393 

Mercury-arc  rectifier  circuits  603 

Mica  card  resistance  164 

Mica  slip  fuse  292 

Microtelephone  set  113 

Monarch 

drop  and  jack  335 

receiver  76 

transmitter  63 

visual  signal  394 

Multi-office  exchanges,  necessity  for    475 

Multiple  electrode  55 

Multiple  switchboard  409 

busy  test  414 

cord  circuits  412 

diagram  showing  principle  of       413 

double  connections  412 

field  of  each  operator  417 

field  of  utility  409 


INDEX 


939 


Multiple  switchboard 
influence  of  traffic 
line  signals 
multiple  feature 

Mutual  induction 


N 


Non-inductive  resistance  devices 
inductive  neutrality 
provisions  against  heating 
temperature  coefficient 
types 

differentially-wound  unit 

iron  wire  ballast 

lamp  filament 

mica  card  unit 

Non-selective  party-line  systems 
bridging 
limitations 
series 
signal  code 


Page  Page 
Open  wires  728 
418              copper  729 
411              copper  vs.  iron  729 
409              copper-clad  steel  729 
44                      characteristics  730 
Monnot  process  730 
uses  730 
copper-clad  wire,  characteris- 
tics of  735 
162              designation  of  sizes  732 
164                      wire  gauges  733 

164  insulated  open  wire  731 
162                     braiding  732 

drops  731 

165  wall  and  fence  wire  732 
167              iron 

166  galvanizing  728 
164                       mile-ohm  729 
217                      strength  729 
220              iron  wire,  characteristics  of  735 
225      Open-circuit  cells  89 
219       Open-circuit  impedance  coil  154 
225      Operator's  receiver  80 


O 


Office  terminal  cables  838 

aerial  cable  entrance  838 
underground  cable  entrance  for 

small  plants  838 
underground  entrance  for  larg- 
er plants  840 
cable  runs  844 
pot-head   method  of  ter- 
minating 843 
subdivision  into  small  ter- 
minal cables  844 
treatment  of  cable  ends  841 
wool  cable  ends  842 


Packing  of  transmitters  65 

Permeability  134 

Phantom  circuit  687 

Pilot  signals  383 

Pitch  10 

Doppler's  principle  10 

vibration  of  diaphragms  11 

Plug-seat  switch  404 

Polarity  method  of  selective  signal- 

ng  229 

Polarized  ringer  31,  118 

brazed  bell  120 

Kellogg  119 


940 


INDEX 


Page 

Page 

Polarized  ringer 

Poles  and  pole  fittings 

Western  Electric 

118 

pole  equipment 

Pole  changers  for  harmonic   ring- 

hardware 

ing 

597 

hardware   require- 

Poles  and  pole  fittings 

745 

ments 

753 

city  exchange  lines 

775 

pole  steps 

753 

cable 

787 

through  bolts 

751 

erection 

787 

pole  setting 

755 

hangers 

788 

poles 

745 

splices 

789 

cedar 

745 

supporting  splices 

789 

chestnut 

745 

messengers 

779 

cutting 

746 

attaching 

781 

gaining 

749 

pole  shims 

785 

Northwestern  Cedar 

sag  and  strains 

782 

men's    Specifica- 

sizes and  grades 

779 

tions 

747 

splices 

786 

roofing 

750 

poles 

775 

sizes 

746 

alley  arms 

779 

trimming 

747 

crossings 

777 

treating 

747 

distributing 

778 

weights 

749 

grading 

777 

rural  lines 

774 

locating 

776 

toll  lines 

756 

mechanica.      protec- 

distributing poles 

758 

tion 

778 

equipping  poles 

759 

stepping 

776 

grading 

758 

terminals 

791 

guying  and  bracing 

759 

pole  seats 

798 

locating  poles 

756 

position 

797 

route 

756 

protected 

795 

scheme  of  transposition 

771 

unprotected 

793 

setting  poles 

759 

pole  equipment 

745 

sizes  of  poles 

756 

cross-arms 

751 

stringing  wire 

766 

pins 

751 

dead  ending 

770 

equipped  pole 

755 

joints 

768 

hardware 

751 

sag 

768 

braces 

752 

test  points 

771 

carriage  bolts  and  lag 

transpositions 

767 

screws 

753 

tying 

767 

galvanizing 

755 

tree  trimming 

765 

INDEX 


941 


Page 

Poole  lock-out  system  254 

Power  plants  593 

auxiliary  signaling  currents  599 

currents  employed  593 

alternating  current  593 

direct  current  593 

operator's  transmitter  supply  594 

power  plant  circuit  614 

power  switchboard  612 

meters  612 

protective  devices  614 

switches  614 

primary  sources  600 
charging  from  direct-cur- 
rent mains  600 
charging  dynamos  601 
mercury-arc  rectifiers  602 
rotary  converters  600 

provision  against  breakdown  603 
capacity  of  power  units  604 
duplicate  charging  ma- 
chines 604 
duplicate  primary  sources  604 
duplicate  ringing  machines  604 

ringing-current  supply  595 

magneto  generators  595 

pole  changers  595 

ringing  dynamos  598 

storage  battery  605 

initial  charge  607 

installation  606 

low  cells  610 

operation  608 

overcharge  609 

pilot  cell  609 

regular  charge  610 

replacing  batteries  611 

sediment  611 

types  593 

common-battery  systems  594 


Page 

Power  plants 
types 

magneto  systems  594 

Power  switchboard  612 

Primary  cells  82 

conventional  symbol  104 

series  and  multiple  connections      88 

simple  voltaic  82 

types  of 

closed-circuit  96 
Fuller  99 
gravity  96 
Lalande  102 
prevention  of  creep- 
ing 96 
setting  up  98 
open-circuit  89 
Le  ClandiS  89 
standard  103 
chloride  of  silver  103 
Private-branch  exchange  637 
with  automatic  offices  644 
secrecy  645 
battery  supply  645 
circuits,  key-type  board  642 
definitions  637 
desirable  features  647 
functions  of  the  private-branch 

exchange  operator  638 

marking  of  apparatus  647 

private-branch  switchboards  639 

common-battery  type  639 

cord  type  641 

key  type  641 

magneto  type  639 

ringing  current  646 

supervision  of  private-branch 

connections  643 

Propagation  of  sound  9 

Protective  means  284 


942 


INDEX 


Page 
Protective  means 

against  high  potentials  284 
air-gap  arrester  285 
advantages  of  carbon  286 
commercial  types  287 
continuous  arcs  290 
discharge  across  gaps  285 
dust  between  carbons  286 
introduction    of    im- 
pedance 289 
metallic  electrodes  290 
vacuum  arresters  289 
against  sneak  currents  294 
heat  coil  296 
sneak-current  arresters  295 
against  strong  currents  291 
fuses  291 
enclosed  292 
mica  291 
proper  functions  291 
central-office  protectors  300 
self-soldering  heat  coils  300 
sneak-current  and  air-gap 

arrester  300 

city  exchange  requirements  304 

complete  line  protection  296 

electrolysis  305 
subscribers'  station  protectors 

ribbon  fuses  303 

Pulsating-current  commutator  117 


R 


Receivers 
Dean 

direct-current 
early 
Kellogg 
modern 


70 
78 
74 
70 
73 
71 


Page 
Receivers 

Monarch  76 

operator's  80 

single-pole  71 

symbols  81 

Western  Electric  72 

Relays  394 

Reluctance  137 

Repeating  coil  158 

Ribbon  fuses  303 

Ringer  symbols  121 

Ringing  and  listening  key  317 

Robert's  latching  relay  266 

Robert's  self-cleansing  arrester  287 

Rolled  condenser  172 

Rotary  connector  568 


Saw-tooth  arrester  286 

Selecting  switches  541 

Selective  party-line  systems  228 

broken-line  method  251 

classification 

broken-line  systems  229 

harmonic  systems  228 

polarity  systems  228 

step-by-step  systems  229 

harmonic  method  238 

polarity  method  229 

step-by-step  method  249 

Selector  541 

Self-induction  44 

Service  connections  848 

from  bare  wire  lines  848 

from  cable  lines  848 

distribution  from  underground 

terminals  in  buildings     861 

drop-wire  distribution  848 


INDEX 


943 


Service  connections 

drop-wire  distribution 

circle-top  distribution  854 

splicing  drop  wire  854 

stringing  drop  wires  849 

rear-wall  or  fence  distribution     855 

aerial  conduit  858 

fence  wiring  859 

wall-ring  wiring  856 

Signal  code  225 

Signaling,  method  of  105 

Silk  and  cotton  insulation  145 

Simplex  circuits  690 

Single  electrode  54 

Single-pole  receiver  71 

Sneak-current  arresters  295 

Solid-back  transmitter  56 

Sound 

characteristics  of  9 

loudness  10 

pitch  10 

timbre  1 1 

Standard  cell  103 

Step-by-step  lock-out  system  257 

Step-by-step   method   of   selective 

signaling  249 

Storage  battery  605 

Storage  cell  606 

Stromberg-Carlson  multiple  board     462 

Strowger  automatic  system  509 

Subscribers'  board  625,  627 

Subscribers'  station  protectors  302 

Subscribers'  station  wiring  862 

apartment  houses  874 

conduits  and  outlets  875 

main  distributing  terminal    875 

typical  arrangement  876 

flats  877 

distributing  point  877 

location  of  outlets  877 


Page 

Subscribers'  station  wiring 
flats 

special  facilities  877 

general  conditions  862 

hotels  870 

arrangement  of  apparatus     871 

distributing  cables  872 

location  of  outlets  872 

riser  cables  873 

typical  arrangement  874 

office  buildings  864 

cable  in  elevator  shaft  868 

cable  in  vent  shafts  869 

ceiling  conduits  865 

combined     pipe-and-wire 

shaft  868 
hall  terminal  boxes  866 
main   distributing   termi- 
nals 870 
raceway  moldings  864 
riser-cable  shafts  867 
typical  arrangement  870 
private  dwellings  877 
Switchboard  assembly  397 
Switchboard  cords  339 
Switchboard  plugs  338 
Switchboard  transmitter  69 
Submarine  cables  741 
Symbols 

battery  104 

condenser  174 

generator  117 

hook  switch  131 

impedance  coil  155 

induction  coil  161 

receiver  81 

repeating  coil  161 

ringer  121 

ringing  and  listening  key  317 

transmitter  69 


944 


INDEX 


Page 
T 
Table 

aerial   and  underground   tele- 
phone cable  742 
automanual  system  time  data  591 
automatic   systems,    messages 

per  trunk  in  671 

calling  rates  668 

capacity,  corrective  factors  of  740 

cedar  poles  750 

color  code  for  cables,  standard  500 

condenser  data  174 
copper  wire                             144,  733 

copper-clad  wire  data  734 

drawing  in  stress  on  cable  825 
factor  for  strain  on  suspension 

strand  784 
German  silver  wire  —  18  per 

cent  164 
German  silver  wire  —  30  per 

cent  165 

insulation,  corrective  factors  of  741 

iron  wire,  data  735 

lead  sleeves  835 
long-distance  groups,  messages 

per  trunk  in  671 

loop  in  marline  hangers  792 

manholes,  inside  dimensions  of  815 
manual  system,  messages  per 

trunk  in  670 
messengers,  size  and  breaking 

weight  780 
metals,  behavior  of,  in  different 

electrolysis  85 
IP 

minimum  sag  in  regular  span  783 

out-trunking,  effect  of,  on  oper- 
ator's capacity  669 
pole  setting  data  755 
pole  step  data'  776 
sag  at  time  of  erecting  770 


Page 
Table 

sags  for  various  spans  784 

signal  code  225 

specific  inductive  capacities  171 
strain   at    center   of    100-foot 

span  785 

subscribers'  waiting  time  592 

temperature  coefficients  162 

transmission  distances,  limiting  47 
winding    data    for    insulating 

wires  150 

Tandem  differential  electromagnet  142 

Telegraph  sounder  29 
Telephone  currents,  measurements 

of  26 

electromagnetic  method  27 

thermal  method  27 

Telephone  exchange,  features  of  307 

districts  308 

subscribers'  lines  308 

switchboards  309 

toll  lines  308 

trunk  lines  308 

Telephone  lines  37 

conductivity  of  conductors  39 

electrostatic  capacity  40 

inductance  vs.  capacity  46 

insulation  of  conductors  46 

transmission  50 

types  of  725 

Telephone  sets  197 

classification  of  198 

common-battery  telephone  198 

magneto  telephone  198 

wall  and  desk  telephones  198 

common-battery  207 

desk  210 

hotel  209 

wall  209 

magneto  199 


INDEX 


945 


Telephone  sets 
magneto 

circuits  of 
bridging 
series 
desk 
wall 
Telephone  traffic 

importance  of  traffic  study 


205 
202 
201 
199 
664 
666 


methods  of  traffic  study  667 

observation  of  service  674 

quality  of  service  671 

accuracy  and  promptness  673 

answering  time  672 

busy    and    don't    answer 

calls  673 

courtesy  and  form  673 

disconnecting  time  672 

enunciation  674 

team  work  674 

rates  of  calling  666 

representative  traffic  data  668 

rfalling  rates  668 

operators'  loads  668 

toll  traffic  670 

trunk  efficiency  669 

trunking  factor  669 

traffic  variations  664 

busy  hour  ratio  665 

unit  of  traffic  664 

Telephone  train  dispatching  699 

advantages  701 

apparatus  704 

Cummings-Wray  selector  708 

dispatcher's  transmitter  709 

Gill  selector  707 

portable  train  sets  711 

siding  telephones  711 

waystation  telephones  710 

Western  Electric  selector  704 


Page 
Telephone  train  dispatching 

blocking  sets  721 

causes  of  its  introduction  700 

Cummings-Wray  circuits  716 

on  electric  railways  722 

Gill  circuits  715 

railroad  conditions  703 

rapid  growth  699 

test  boards  719 

transmitting  orders  703 

waystation  circuits  714 

Western  Electric  circuits  713 
Telephone  train-dispatching  circuit 

Cummings-Wray  716 

Gill  715 

waystation  714 

Western  Electric  713 
Telephony 

cable  splicing  828 

cables  736 

care  of  plant  895 

development  studies  887 

office  terminal  cables  838 

open  wires  728 

poles  and  pole  fittings  745 

service  connections  848 

subscriber's  station  wiring  862 

testing  904 

underground  cables  878 

underground  construction  799 

Temperature  coefficients  162 

Test  boards  719 

Testing  904 

cable  testing  911 

cable  quality  911 

capacity  916 

D'Arsonval  galvanometer     912 

galvanometer  shunts  913 

insulation  resistance  913 

megger  916 


946 


INDEX 


Page 

Testing 

cable  testing 

insulation  resistance 

Thomson  galvanome- 
ter 912 
capacity  test  for  open  924 
faults 

capacity  906 
continuity  905 
crosses  909 
foreign  potentials  906 
insulation  905 
opens  907 
resistance  907 
tone  methods  910 
wire-chief's  desks  910 
identification  910 
implements  904 
listening  tests  926 
loss-of-charge  test  920 
Murray     loop      tests  for 
.  /                    grounds  923 
testing  sets  919 
Varley  loop  test  for  crosses  922 
Varley  loop  test  for  grounds         921 
Thermal  method  of  measuring  tele- 
phone currents  27 
Timbre  11 
Toll  lines  756 
Toroidal  impedance  coil  155 
Toroidal  repeating  coil  160 
Transfer  switchboard  400 
field  of  usefulness  407 
handling  transfers  404 
limitations  406 
plug-seat  switch  404 
transfer  lines  401 
jack-ended  trunk  401 
plug-ended  trunk  403 
Transmission,  ways  of  improving         50 


Page 

Transmitters  53 

acousticon  67 

Automatic  Electric  Company  62 

carrying  capacity  66 

conventional  diagram  69 

electrode  65 

arrangement  of  53 

multiple  55 

single  54 

granular  carbon  56 

Kellogg  60 

materials  53 

Monarch  63 

packing  65 

sensitiveness  67 

switchboard  69 

symbols  69 

variable  resistance  53 

Western  Electric  solid-back  56 

Trunking  in  multi-office  systems  475 

classification  478 

one-way  trunks  469 

two-way  trunks  478 

Kellogg  trunk  circuits  491 

necessity  for  exchanges  475 

Western  Electric  trunk  circuits  482 


U 


Underground  cables,  electrolysis  of    878 

early  controversy  878 

court  decisions  880 

electrolysis  troubles  881 

causes  881 

underground  conditions  882 

remedies  883 

bonding  884 

insulating  joints  883 

moisture-proof  conduits  884 


INDEX 


947 


Page 

Underground  cables,  electrolysis  of 

McCluer  system  880 

metallic  circuits  and  cables  881 

Underground  conduit  800 

Underground  construction  799 

buried  cable  800 

conduit  cross-sections  809 

conduit  work,  general  features 

of  809 
main  line  and  lateral  con- 
duit 811 
manholes  812 
location  of  815 
shape  of  813 
sizes  of  815 
construction  of  conduit  816 
cable  supports  819 
concrete  820 
curves  in  conduit  line  817 
grading  817 
lateral  risers  819 
lateral  runs  817 
mortar  821 
safeguards  821 
test  holes  816 
trenching  816 
duct  material  801 
fiber-pipe  conduit  805 
laying  fiber  ducts  806 
iron  conduit  809 
vitrified-clay  conduit  802 
multiple-duct  tile  803 
single-duct  tile  802 
use  of  concrete  with 

clay  tile  804 

wooden  conduit  808 
installing  underground  cables     821 
arrangements  of  cables  in 

vaults  827 

drawing  in  822 


Page 


Underground  construction 

installing  underground  cables 
long  conduit  sections 
lubrication 
rodding 

speed  of  drawing  in 
placing     wires     underground, 

reasons  for 
interests  of  the  operating 

company 

interests  of  the  public 
underground  conduit 
Under-tuned  ringer 


Vacuum  arrester 
Variable  resistance 
Vibrating  bell 
Visible  signals 

electric  lamp 

electromagnetic 
Volta 
Voltaic  cell 

amalgamated  zincs 

difference  of  potential 

local  action 

polarization 

theory 


W 

Wall  telephone  hooks 

Dean 

Kellogg 

Western  Electric 
Warner  pole  changer 


826 
824 
821 

825 

799 

800 
799 
800 

242 


289 
53 
30 
33 
34 
33 
82 
82 
87 
84 
86 
86 
83 


125 
127 
125 
126 
596 


948 


INDEX 


Waystation  telephones 

Western  Electric 
air-gap  arrester 
desk  stand  hook 
drop  and  jack 
mechanical  signal 
receiver 
ringer 


Page  Page 

710  Western  Electric 

selector  704 

288              solid-back  transmitter  56 

128              station  arrester  303 

333              trunk  circuits  482 

393               wall  telephone  hook  126 

72  White  transmitter  56 

118  Wire  gauges  144 


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